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Abstract: The evolution of exploitative specificity can be influenced by environmental variability in space and time and the intensity of trade-offs. Coevolution, the process of reciprocal adaptation in two or more species, can produce variability in host exploitation and as such potentially drive patterns in host and parasite specificity. We employed the bacterium Pseudomonas fluorescens SBW25 and its DNA phage Phi2 to investigate the role of coevolution in the evolution of phage infectivity range and its relation with phage growth rate. At the phage population level, coevolution led to the evolution of broader infectivity range, but without an associated decrease in phage growth rate relative to the ancestor, whereas phage evolution in the absence of bacterial evolution led to an increased growth rate but no increase in infectivity range. In contrast, both selection regimes led to phage adaptation (in terms of growth rates) to their respective bacterial hosts. At the level of individual phage genotypes, coevolution resulted in within-population diversification in generalist and specialist infectivity range types. This pattern was consistent with a multilocus gene-for-gene interaction, further confirmed by an observed cost of broad infectivity range for individual phage. Moreover, coevolution led to the emergence of bacterial genotype by phage genotype interactions in the reduction of bacterial growth rate by phage. Our study demonstrates that the strong reciprocal selective pressures underlying the process of coevolution lead to the emergence and coexistence of different strategies within populations and to specialization between selective environments.

Abstract: Positive relationships between species diversity and productivity have been reported for a number of ecosystems. Theoretical and experimental studies have attempted to determine the mechanisms that generate this pattern over short timescales, but little attention has been given to the problem of understanding how diversity and productivity are linked over evolutionary timescales. Here, we investigate the role of dispersal in determining both diversity and productivity over evolutionary timescales, using experimental metacommunities of the bacterium Pseudomonas fluorescens assembled by divergent natural selection. We show that both regional diversity and productivity peak at an intermediate dispersal rate. Moreover, we demonstrate that these two patterns are linked: selection at intermediate rates of dispersal leads to high niche differentiation between genotypes, allowing greater coverage of the heterogeneous environment and a higher regional productivity. We argue that processes that operate over both ecological and evolutionary timescales should be jointly considered when attempting to understand the emergence of ecosystem-level properties such as diversity-function relationships.

Abstract: Research on the interactions between evolutionary and ecological dynamics has largely focused on local spatial scales and on relatively simple ecological communities. However, recent work demonstrates that dispersal can drastically alter the interplay between ecological and evolutionary dynamics, often in unexpected ways. We argue that a dispersal-centered synthesis of metacommunity ecology and evolution is necessary to make further progress in this important area of research. We demonstrate that such an approach generates several novel outcomes and substantially enhances understanding of both ecological and evolutionary phenomena in three core research areas at the interface of ecology and evolution.

Abstract: BACKGROUND: Recent work on the complexity of life highlights the roles played by evolutionary forces at different levels of individuality. One of the central puzzles in explaining transitions in individuality for entities ranging from complex cells, to multicellular organisms and societies, is how different autonomous units relinquish control over their functions to others in the group. In addition to the necessity of reducing conflict over effecting specialized tasks, differentiating groups must control the exploitation of the commons, or else be out-competed by more fit groups. RESULTS: We propose that two forms of conflict - access to resources within groups and representation in germ line - may be resolved in tandem through individual and group-level selective effects. Specifically, we employ an optimization model to show the conditions under which different within-group social behaviors (cooperators producing a public good or cheaters exploiting the public good) may be selected to disperse, thereby not affecting the commons and functioning as germ line. We find that partial or complete dispersal specialization of cheaters is a general outcome. The propensity for cheaters to disperse is highest with intermediate benefit:cost ratios of cooperative acts and with high relatedness. An examination of a range of real biological systems tends to support our theory, although additional study is required to provide robust tests. CONCLUSION: We suggest that trait linkage between dispersal and cheating should be operative regardless of whether groups ever achieve higher levels of individuality, because individual selection will always tend to increase exploitation, and stronger group structure will tend to increase overall cooperation through kin selected benefits. Cheater specialization as dispersers offers simultaneous solutions to the evolution of cooperation in social groups and the origin of specialization of germ and soma in multicellular organisms.

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Abstract: Recent advances in our knowledge of parasitic and mutualistic associations have confirmed the central role of coevolutionary interactions in population and community ecology. Here, we discuss the potential coevolutionary interdependence of the strength and specificity of symbiotic interactions with the complexity and productivity of their environment. We predict that interactions become less beneficial with increasing environmental quality and that the association of productivity with symbiont specificity depends on the relative strengths of tradeoffs between host range and other life-history parameters. However, as biotic complexity increases, pathogen specificity is predicted to decline, whereas mutualist specificity will increase. Testing these predictions on a geographical scale would contribute significantly to the predictive science of coevolution, and to our ability to manage biological interactions embedded in increasingly fragmented landscapes.

Abstract: Caterpillars of the butterfly Maculinea rebeli develop as parasites inside ant colonies. In intensively studied French populations, about 25% of caterpillars mature within 1 year (fast-developing larvae [FDL]) and the others after 2 years (slow-developing larvae [SDL]); all available evidence indicates that this ratio is under the control of egg-laying females. We present an analytical model to predict the evolutionarily stable fraction of FDL (pESS). The model accounts for added winter mortality of SDL, general and kin competition among caterpillars, a competitive advantage of SDL over newly entering FDL (priority effect), and the avoidance of renewed infection of ant nests by butterflies in the coming season (segregation). We come to the following conclusions: (1) all factors listed above can promote the evolution of delayed development; (2) kin competition and segregation stabilize pESS near 0.5; and (3) a priority effect is the only mechanism potentially selecting for pESS < 0.5. However, given the empirical data, pESS is predicted to fall closer to 0.5 than to the 0.25 that has been observed. In this particular system, bet hedging cannot explain why more than 50% of larvae postpone growth. Presumably, other fitness benefits for SDL, for example, higher fertility or longevity, also contribute to the evolution of delayed development. The model presented here may be of general applicability for systems where maturing individuals compete in small subgroups.

Abstract: BACKGROUND: Social scientists have suggested that cultural diversity in a nation leads to societal instability. However, societal instability may be affected not only by within-nation or alpha diversity, but also diversity between a nation and its neighbours or beta diversity. It is also necessary to distinguish different domains of diversity, namely linguistic, ethnic and religious, and to distinguish between the direct effects of diversity on societal instability, and effects that are mediated by economic conditions. METHODOLOGY/PRINCIPAL FINDINGS: We assembled a large cross-national dataset with information on alpha and beta cultural diversity, economic conditions, and indices of societal instability. Structural equation modeling was used to evaluate the direct and indirect effects of cultural diversity on economics and societal stability. Results show that different types and domains of diversity have interacting effects. As previously documented, linguistic alpha diversity has a negative effect on economic performance, and we show that it is largely through this economic mechanism that it affects societal instability. For beta diversity, the higher the linguistic diversity among nations in a region, the less stable the nation. But, religious beta diversity has the opposite effect, reducing instability, particularly in the presence of high linguistic diversity. CONCLUSIONS: Within-nation linguistic diversity is associated with reduced economic performance, which, in turn, increases societal instability. Nations which differ linguistically from their neighbors are also less stable. However, religious diversity between neighboring nations has the opposite effect, decreasing societal instability.

Abstract: Natural populations of hosts and parasites are often subdivided and patchily distributed such that some regions of a host species' range will be free from a given parasite. Host migration from parasite-free to parasite-containing patches is expected to alter coevolutionary dynamics by changing the evolutionary potential of antagonists. Specifically, host immigration can favor parasites by increasing transmission opportunities, or hosts by introducing genetic variation. We tested these predictions in coevolving populations of Pseudomonas fluorescens and phage Phi2 that received immigrants from phage-free populations. We observed a negative quadratic relationship between sympatric resistance to phage and host immigration rate (highest at intermediate immigration) but a positive quadratic relationship between coevolution rate and host immigration rate (lowest at intermediate immigration). These results indicate that for a wide range of rates, host immigration from parasite-free patches can increase the evolutionary potential of parasites, and increase the coevolutionary rate if parasite adaptation is limiting in the absence of immigration.

Abstract: Altruism presents a challenge to evolutionary theory because selection should favor selfish over caring strategies. Greenbeard altruism resolves this paradox by allowing cooperators to identify individuals carrying similar alleles producing a form of genic selection. In side-blotched lizards, genetically similar but unrelated blue male morphs settle on adjacent territories and cooperate. Here we show that payoffs of cooperation depend on asymmetric costs of orange neighbors. One blue male experiences low fitness and buffers his unrelated partner from aggressive orange males despite the potential benefits of defection. We show that recognition behavior is highly heritable in nature, and we map genetic factors underlying color and self-recognition behavior of genetic similarity in both sexes. Recognition and cooperation arise from genome-wide factors based on our mapping study of the location of genes responsible for self-recognition behavior, recognition of blue color, and the color locus. Our results provide an example of greenbeard interactions in a vertebrate that are typified by cycles of greenbeard mutualism interspersed with phases of transient true altruism. Such cycles provide a mechanism encouraging the origin and stability of true altruism.

Abstract: Resource competition within a group of cooperators is expected to decrease selection for cooperative behavior but can also result in diversifying selection for the use of different resources, which in turn could retard the breakdown of cooperation. Diverse groups are likely to be less susceptible to invasion by noncooperating social cheats: First, competition repression resulting from character displacement may provide less of a selective advantage to cheating; second, cheats may trade off the ability to exploit cooperators that specialize in one type of resource against cooperators that specialize in another ; third, diverse communities of any kind may have higher invasion resistance because there are fewer resources available for an invader to use . Furthermore, diverse groups are likely to be more productive than clonal groups if a wider range of total resources are being used . We addressed these issues by using the cooperative trait of biofilm formation in Pseudomonas fluorescens. Character displacement through resource competition evolved within biofilms; productivity increased with increasing character displacement, and diverse biofilms were less susceptible to invasion by cheats. These results demonstrate that diversification into different ecological niches can minimize selection against cooperation in the face of local resource competition.

Abstract: The evolution of resistance in insect pests will imperil the efficiency of transgenic insect-resistant crops. The currently advised strategy to delay resistance evolution is to plant non-toxic crops (refuges) in close proximity to plants engineered to express the toxic protein of the bacterium Bacillus thuringiensis (Bt). We seek answers to the question of how to induce growers to plant non-toxic crops. A first strategy, applied in the United States, is to require Bt growers to plant non-Bt refuges and control their compliance with requirements. We suggest that an alternative strategy is to make Bt seed more expensive by instituting a user fee, and we compare both strategies by integrating economic processes into a spatially explicit, population genetics model. Our results indicate that although both strategies may allow the sustainable management of the common pool of Bt-susceptibility alleles in pest populations, for the European corn borer (Ostrinia nubilalis) one of the most serious pests in the US corn belt, the fee strategy is less efficient than refuge requirements.

Abstract: The fitness consequences of deleterious mutations are sometimes greater when individuals are parasitized, hence parasites may result in the more rapid purging of deleterious mutations from host populations. The significance of host deleterious mutations when hosts and parasites antagonistically coevolve (reciprocal evolution of host resistance and parasite infectivity) has not previously been experimentally investigated. We addressed this by coevolving the bacterium Pseudomonas fluorescens and a parasitic bacteriophage in laboratory microcosms, using bacteria with high and low mutation loads. Directional coevolution between bacterial resistance and phage infectivity occurred in all populations. Bacterial population fitness, as measured by competition experiments with ancestral genotypes in the absence of phage, declined with time spent coevolving. However, this decline was significantly more rapid in bacteria with high mutation loads, suggesting the cost of bacterial resistance to phage was greater in the presence of deleterious mutations (synergistic epistasis). As such, resistance to phage was more costly to evolve in the presence of a high mutation load. Consistent with these data, bacteria with high mutation loads underwent less rapid directional coevolution with their phage populations, and showed lower levels of resistance to their coevolving phage populations. These data suggest that coevolution with parasites increases the rate at which deleterious mutations are purged from host populations.

Abstract: Models of virulence evolution generally consider the outcome of competition between resident and mutant parasite strains at or near endemic equilibrium. Less studied is what happens during the initial phases of invasion and adaptation. Understanding initial adaptive dynamics is particularly important in the context of emerging diseases in wildlife and humans, for which rapid and accurate intervention may be of the essence. To address the question of virulence evolution in emerging diseases, we employ a simple stochastic modeling framework. As is intuitive, the pathogen strains most likely to emerge are those with the highest net reproductive rates (R-0). We find, however, that stochastic events shape the properties of emerging pathogens in sometimes unexpected ways. First, the mean virulence of emerging pathogens is expected to be larger in dense host populations and/or when transmission is high, due to less restrictive conditions for the spread of the pathogen. Second, a positive correlation between average virulence and transmissibility emerges due to a combination of drift and selection. We conclude that at least in the initial phases of adaptation, special assumptions about constraints need not be invoked to explain some virulence-transmission correlations and that virulence management practices should consider how residual variation in transmission and virulence can be selected to reduce the prevalence and/or virulence of emerging infectious diseases.

Abstract: BACKGROUND: Confronted with well-defended, novel hosts, should an enemy invest in avoidance of these hosts (behavioral adaptation), neutralization of the defensive innovation (physiological adaptation) or both? Although simultaneous investment in both adaptations may first appear to be redundant, several empirical studies have suggested a reinforcement of physiological resistance to host defenses with additional avoidance behaviors. To explain this paradox, we develop a mathematical model describing the joint evolution of behavioral and physiological adaptations on the part of natural enemies to their host defenses. Our specific goals are (i) to derive the conditions that may favor the simultaneous investment in avoidance and physiological resistance and (ii) to study the factors that govern the relative investment in each adaptation mode. RESULTS: Our results show that (i) a simultaneous investment may be optimal if the fitness costs of the adaptive traits are accelerating and the probability of encountering defended hosts is low. When (i) holds, we find that (ii) the more that defended hosts are rare and/or spatially aggregated, the more behavioral adaptation is favored. CONCLUSION: Despite their interference, physiological resistance to host defensive innovations and avoidance of these same defenses are two strategies in which it may be optimal for an enemy to invest in simultaneously. The relative allocation to each strategy greatly depends on host spatial structure. We discuss the implications of our findings for the management of invasive plant species and the management of pest resistance to new crop protectants or varieties.

Abstract: We employ an empirically motivated "case model" approach to investigate the theoretical foundations for the conservation of the endangered butterfly Maculinea arion. Maculinea butterflies have highly specialized larvae that sequentially exploit a plant and an ant species. Our study establishes that M. arion's specialized life cycle, including scramble competition for limiting resources, and the spatially discrete nature of its resources, make it more sensitive to environmental variation and more prone to local extinction than other univoltine phytophagous species. We find that the number and spatial distribution of the butterfly's resources are key factors in their population dynamics, especially for M. arion populations in habitats associated with high larval survival and high adult fecundity. Factors that increase juvenile competition have first a positive effect on adult population size, but beyond a threshold this effect becomes negative. In general, oscillatory dynamics emerge for high potential growth rates and spatially homogeneous juvenile competition. We discuss the relevance of our results to population management, investigate the consequences of environmental variation, and consider different scenarios of conservation. Our model, although based on the Maculinea genus, should apply to a broad range of species for which the form of competitive interactions changes predictably at distinct points in the life cycle. Complex life cycles can lead to negative feedbacks involving parameters that are usually thought to optimize population size. We suggest that conservation strategies are neither generalizable for the Maculinea genus nor for disparate populations of each species of Maculinea, and rather that management should be conducted on a case-by-case basis.

Abstract: We develop a "case model" approach to investigate how conservation measures may affect the ecology of a community module, defined as a small number of tightly interacting species. The community module consists of a parasitic butterfly and its two hosts, a plant and an ant. The butterfly Maculinea alcon and its host plant Gentiana pneumnonanthe have long been used as indicators of high-quality Palearctic heath and moist grassland ecosystems, and both species have been targeted for ecological research and specific conservation management. We constructed a mechanistic model of this community module, including dynamics for the three species, and conducted simulation studies of different conservation strategies (burning, sod cutting, mowing, and grazing). We identified several key parameters for the conservation of Maculinea alcon and its host plant as well as the most efficient conservation strategies for their dual long-term persistence. Our results show that the conditions that optimize the size of the butterfly and the plant populations differ, suggesting that choices must be made in adopting conservation measures. Despite the potential for apparent competition between the ant, Myrmica scabrinodis, and the plant via the butterfly, realized apparent competition is asymmetric (ants are more affected than plants) and occurs only at intermediate successional stages. Our study provides an example whereby an endangered species (the plant) and its endangered specialist natural enemy (the butterfly) are adversely affected by successional dynamics via direct (for the plant) and indirect (for the butterfly) effects. We argue that different field situations will necessitate particular conservation solutions.

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Abstract: The inevitable escape of transgenic pollen from cultivated fields will lead to the emergence of transgenic crop-wild plant hybrids in natural patches of wild plants. The fate of these hybrids and that of the transgene depend on their ability to compete with their wild relatives. Here we study ecological factors that may enhance the fitness of genetically modified hybrids relative to wild plants for a Bacillus thuringiensis ( Bt) transgene conferring resistance to insects. Mixed stands of wild plants and first-generation hybrids were grown under different conditions of herbivore pressure and density, with Bt oilseed rape ( Brassica napus) as the crop and B. rapa as the wild recipient. Biomass and fitness components were measured from plant germination to the germination of their offspring. The frequency of transgenic seedlings in the offspring generation was estimated using the green fluorescent protein marker. The biomass of F(1) Bt-transgenic hybrids relative to that of wild-type plants was found to be sensitive to both plant density and herbivore pressure, but herbivore pressure appeared as the major factor enhancing their relative fitnesses. In the absence of herbivore pressure, Bt hybrids produced 6.2-fold fewer seeds than their wild neighbors, and Bt plant frequency fell from 50% to 16% within a single generation. Under high herbivore pressure, Bt hybrids produced 1.4-fold more seeds, and Bt plant frequency was 42% in the offspring generation. We conclude that high-density patches of highly damaged wild plants are the most vulnerable to Bt-transgene invasion. They should be monitored early to detect potential transgene spread.

Abstract: Identifying the factors underlying the origin and maintenance of the latitudinal diversity gradient is a central problem in ecology, but no consensus has emerged on which processes might generate this broad pattern. Interestingly, the vast majority of studies exploring the gradient have focused on free-living organisms, ignoring parasitic and infectious disease (PID) species. Here, we address the influence of environmental factors on the biological diversity of human pathogens and their global spatial organization. Using generalized linear multivariate models and Monte Carlo simulations, we conducted a series of comparative analyses to test the hypothesis that human PIDs exhibit the same global patterns of distribution as other taxonomic groups. We found a significant negative relationship between latitude and PID species richness, and a nested spatial organization, i.e., the accumulation of PID species with latitude, over large spatial scales. Additionally, our results show that climatic factors are of primary importance in explaining the link between latitude and the spatial pattern of human pathogens. Based on our findings, we propose that the global latitudinal species diversity gradient might be generated in large part by biotic interactions, providing strong support for the idea that current estimates of species diversity are substantially underestimated. When parasites and pathogens are included, estimates of total species diversity may increase by more than an order of magnitude.

Abstract: Cultivating non-toxic conventional crops (refuges) in the proximity to transgenic crops that produce Bacillus thuringienesis (Bt) toxins is widely recommended to delay pest adaptation to these toxins. Using a spatially structured model of resistance evolution, Vacher and co-workers (Vacher, C., Bourguet, D., Rousset, F., Chevillon, C. & Hochberg, M.E. 2003. J. Evol. Biol.16: 378-387.) show that the percentage of refuge fields required for the sustainable control of pests can be reduced through intermediate levels of refuge field aggregation and by lowering the toxin dose produced by Bt plants. Tabashnik, B.E., Gould, F. & Carriere, Y. (2004 J. Evol. Biol doi: 10.1111/j1420-9101.2004.00695.x) call into question the results of Vacher et al. (2003) concerning the effect of toxin dose. They argue that these results arise from invalid assumptions about larval concentration-mortality responses for the insect considered, the cotton pest Heliothis virescens. We show here that the models presented by Vacher et al. (2003) and Tabashnik et al. (2004) both show inaccuracies in their definitions of genotypic fitness. The level of dominance estimated by Tabashnik et al. (2004) from larval mortality rates data is irrelevant to resistance evolution, and the fitness cost of resistance evolution, and the fitness cost of resistance is inaccurately integrated into their framework. Neverthless, the comments of Tabashnik et al. (2004) are very helpful in elucidating the definitions of genotypic fitness used in Vacher et al. (2003) and in pointing out the essential factors in predicting the evolution of insect resistance to Bt transgenic crops, namely, accurate estimations of the fitness cost of resistance, of the dominance level of this cost, and of the variations in the dominance level of the advantage conferred by the resistance with Bt toxin dose.

Abstract: Many cultural attributes such as adornment, language slang, mannerisms and rituals are thought to have little or no influence on individual survival and reproduction, functioning rather as markers of cultural identity that promote group cohesion. Here, I show that if cultural markers are under weak selection and subject to loss or substitution, then the breakdown of cultural cohesiveness may proceed without stabilizing reactions until many or most of a culture's identifiers are forever lost. This may culminate in a 'cultural meltdown', whereby the culture is caught in a vortex of ever-decreasing membership and insufficient selection against the accumulation of unfamiliar markers. In progressively altering the topology of communication from diffusion to broadcasting, globalization may be both accelerating the erosion of cultural identities and amplifying dominance behaviours above their normal adaptive levels.

Abstract: Many cultural attributes such as adornment, language slang, mannerisms and rituals are thought to have little or no influence on individual survival and reproduction, functioning rather as markers of cultural identity that promote group cohesion. Here, I show that if cultural markers are under weak selection and subject to loss or substitution, then the breakdown of cultural cohesiveness may proceed without stabilizing reactions until many or most of a culture's identifiers are forever lost. This may culminate in a 'cultural meltdown', whereby the culture is caught in a vortex of ever-decreasing membership and insufficient selection against the accumulation of unfamiliar markers. In progressively altering the topology of communication from diffusion to broadcasting, globalization may be both accelerating the erosion of cultural identities and amplifying dominance behaviours above their normal adaptive levels.

Abstract: We develop and analyse a model of inducible defence where two traits-defence and its inducibility-jointly evolve. Inducibility reduces costs of defence in the absence of enemies thereby permitting higher defence levels when attacked. If the cost of inducibility is low, then inducibility and defence may reinforce one another, resulting in a runaway leading to a highly inducible and highly effective defence. When inducibility is more costly, a new joint-equilibrium in defence/inducibility emerges displaying intermediate levels of both traits, and the prior 'run-away' scenario (high defence, high inducibility) may disappear. In contrast to the cost of inducibility, the cost of defence has mixed effects. An increase in costs of defence generally diminishes the level of both defence and inducibility at the intermediate locally stable equilibrium, but can favour the existence of the 'run-away' scenario of high defence-high inducibility. The enemy encounter-rate also has mixed effects. At high encounter rates an increase in encounters can lead to a higher/maximal defence and a lower level of inducibility (defence being almost always useful), but at low rates, an increase in encounters can lead to both higher defence and higher inducibility. We finally consider potential enemy responses to defensive change, and illustrate that herd immunity (reduction of encounter rates due to population-level defence) can affect both individual defence and induction that can be, depending on conditions, increased or decreased. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: Although the truffle beetle, Leiodes cinnamomea, inflicts substantial damage to the ripe stage of fruiting bodies of the economically important black truffle ( Tuber melanosporum), it is not attracted by ripe truffle odours. Rather, male beetles are attracted to infested truffles only in the presence of female beetles, suggesting that the former employ a pheromone to locate truffles over short distances. In contrast, female beetles show no attraction to infested or uninfested truffles, suggesting that they employ other cues, possibly linked to odours emitted by truffles prior to the ripe stage. We hypothesize that the chemical composition of truffle volatiles changes over the life of the truffle fruiting body, being attractive to insects early on and to mammals just prior to decomposition.

Abstract: We employ a simple model to show that social selection can lead to prezygotic reproductive isolation. The evolution of social discrimination causes the congealing of phenotypically similar individuals into different, spatially distinct tribes. However, tribal formation is only obtained for certain types of social behavior: altruistic and selfish acts can produce tribes, whereas spiteful and mutualistic behaviors never do. Moreover, reduced hybrid fitness at tribal borders leads to the selection of mating preferences, which then spread to the core areas of the respective tribes. Unlike models of resource competition, our model generates reproductive isolation in an ecologically homogeneous environment. We elaborate on how altruistic acts can lead to reproductive isolation, but also predict that certain types of competition can lead to the speciation effect. Our theory provides a framework for how individual-level interactions mold lineage diversification, with parapatric speciation as a possible end product.

Abstract: Although very common under natural conditions, the consequences of multiple enemies (parasites, predators, herbivores, or even 'chemical' enemies like insecticides) on investment in defence has scarcely been investigated. In this paper, we present a simple model of the joint evolution of two defences targeted against two enemies. We illustrate how the respective level of each defence can be influenced by the presence of the two enemies. Furthermore, we investigate the influences of direct interference and synergy between defences. We show that, depending on certain conditions (costs, interference or synergy between defences), an increase in selection pressure by one enemy can have dramatic effects on defence against another enemy. It is generally admitted that increasing the encounter rate with a second natural enemy can decrease investment in defence against a first enemy, but our results indicate that it may sometimes favour resistance against the first enemy. Moreover, we illustrate that the global defence against one enemy can be lower when only this enemy is present: this has important implications for experimental measures of resistance, and for organisms that invade an area with less enemies or whose community of enemies is reduced. We discuss possible implications of the existence of multiple enemies for conservation biology, biological control and chemical control.

Abstract: The 'high-dose-refuge' (HDR) strategy is widely recommended by the biotechnology industry and regulatory authorities to delay pest adaptation to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. This involves cultivating nontoxic plants (refuges) in close proximity to crops producing a high dose of Bt toxin. The principal cost associated with this strategy is due to yield losses suffered by farmers growing unprotected, refuge plants. Using a population genetic model of selection in a spatially heterogeneous environment, we show the existence of an optimal spatial configuration of refuges that could prevent the evolution of resistance whilst reducing the use of costly refuges. In particular, the sustainable control of pests is achievable with the use of more aggregated distributions of nontransgenic plants and transgenic plants producing lower doses of toxin. The HDR strategy is thus suboptimal within the context of sustainable agricultural development.

Abstract: The biocontrol of insect pests may pose a risk to native insects if the biocontrol agent attacks nontarget species. Potential biocontrol agents are screened before release to determine their acceptance of nontarget species and the suitability of nontarget species for their development. Here we show that, even though a biocontrol agent has very low acceptance of a nontarget species, it may nonetheless have a large impact on the nontarget population. This impact does not require the nontarget species to be a suitable prey capable of supporting the biocontrol agent population, but instead may be a transient impact that occurs soon after the agent is released. Because the population of biocontrol agents is likely to increase rapidly in response to the high density of its target pest, the resulting high density of the agent population may dominate its low acceptance of the nontarget species, causing a strong decline or even local extirpation of the nontarget. We demonstrate this possibility using models of host-parasitoid dynamics that incorporate a broad range of assumptions about the life histories of hosts and parasitoids, and that demonstrate how various common aspects of host-parasitoid biology are likely to reduce this risk considerably. The predictions of the models are reasonably approximated with a simple formula, which potentially provides a simple method for assessing the risk of transient impacts, but which should only be applied loosely (in a qualitative manner) and in the context of a fuller understanding of other factors affecting risk in the system in question.

Abstract: Classical models of virulence evolution conclude that the increased competition favoured by multiple infection will select for increasing consumption and deterioration of the host resource, or 'virulence'. However, recent empirical and theoretical studies suggest that this view of virulence has some shortcomings. Here, we argue that the evolutionary consequences of multiple infection depend critically on whether the exploitation rate of an individual parasite is governed directly by the behaviour of the individual, or whether it is limited by the collective behaviour of the coinfecting group. We illustrate that, depending on the mechanistic details of exploitation, multiple infection can select for reduced virulence.

Abstract: The colonization dynamics of the black truffle in an artificial field were assessed through analyses of microsatellite and RAPD markers. The truffle field was composed of three tree species and mycelial inoculum of three different origins, and was monitored for the first three years of truffle production. We found very low levels of genetic diversity. Isolation by distance was detected only at the between-tree level. This could be interpreted as local colonization around each tree facilitated by the presence of the tree root system. At the larger spatial scale of the European range, the absence of isolation by distance corroborates the hypothesis of an impact of glaciation on genetic variation, followed by rapid postglaciation demographic expansion. In addition, genetic variation of harvested truffles was explained by neither inoculation origin, nor tree species. Our study questions the real impact of man-made inoculation of tree root systems with fungal mycelia.

Abstract: We consider an explicit mutation-selection process to investigate the dynamics underlying the coevolution of parasite's virulence and host's prereproductive life span in a system with discrete generations. Conforming with earlier models, our model predicts that virulence generally increases with natural mortality of the host, and that a moderate increase in virulence selects for lower ages at reproduction. However, the epidemiological feedback in our model also gives rise to unusual and unexpected patterns. in particular, if virulence is sufficiently high the model can lead to a bifurcation pattern, where two strategies coexist in the host population. The first is to develop rapidly to reproduce before being infected. Individuals following this strategy suffer, however, from reduced fecundity. The second strategy is to develop much more slowly. Because of the high virulence, the effective period of transmission is short, so that a few slowly developing individuals escape infection. These individuals, although choosing a risky strategy, benefit from high fecundity.

Abstract: Wolbachia are bacteria that live in the cells of various invertebrate species to which they cause a wide range of effects on physiology and reproduction. We investigated the effect of Wolbachia infection in the parasitic wasp, Asobara tabida Nees (Hymenoptera, Braconidae). In the 13 populations tested, all individuals proved to be infected by Wolbachia. The removal of Wolbachia by antibiotic treatment had a totally unexpected effect-aposymbiotic female wasps were completely incapable of producing mature oocytes and therefore could not reproduce. In contrast, oogenesis was not affected in treated Asobara citri, a closely related species that does not harbor Wolbachia. No difference between natural symbiotic and cured individuals was found for other adult traits including male fertility, locomotor activity, and size, indicating that the effect on oogenesis is highly specific. We argue that indirect effects of the treatments used in our study (antibiotic toxicity or production of toxic agents) are very unlikely to explain the sterility of females, and we present results showing a direct relationship between oocyte production and Wolbachia density in females. We conclude that Wolbachia is necessary for oogenesis in these A. tabida strains, and this association would seem to be the first example of a transition from facultative to obligatory symbiosis in arthropod-Wolbachia associations.

Abstract: Indirect interactions are almost inevitable in any multi-species community. Understanding the implications of such interactions is a challenging task, in light of the very large number of ways species can be tied together in complex food webs. One approach to this complexity is to focus on strong interactions among a relatively small number (e.g. 3-6) of species interacting in defined configurations: community modules. In recent years, the discipline of community ecology has developed a substantial body of theory focused on such modules. Modules often clearly describe the basic features of empirical systems, particularly in simplified anthropogenic landscapes, and also help to isolate and characterize key processes driving the dynamics of more complex communities. In this chapter, we draw out a number of insights from ecological studies of modules which we believe are relevant to biological control. We emphasize in particular the module of 'shared predation', where a natural enemy attacks two or more species of prey. Theoretical studies suggest a number of 'rules of thumb', including: (i) the greatest risk to non-targets may occur from control agents that are only moderately effective on the target; (ii) targets with a high reproductive capacity can indirectly endanger non-targets; (iii) there can be transient phases of extinction risk for non-targets during the establishment phase of control agents, particularly for species with high attack rates; (iv) at a landscape scale, mobile agents can endanger the fate of non-targets at sites other than the area of control; (v) using specialist natural enemies can pose risks to non-targets, if there are generalist resident predators/ parasitoids which can exploit these introduced agents. The theoretical models help to highlight circumstances when these effects should be particularly strong.

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Abstract: The existence of parasitic constraints on the evolution of life-history traits in free-living organisms has been demonstrated in several plant and animal species. However, the association between different diseases and human traits is virtually unknown. We conducted a comparative analysis on a global scale to test whether the diversity of human diseases, some of them responsible for high incidences of morbidity and mortality, were associated with host life-history characteristics. After controlling for direct confounding effects exerted by historical, spatial, economic, and population patterns and their interactions, our findings show that human fertility increases with the diversity and structure of disease types. Thus, disease control may not only lower the costs associated with morbidity, but could also contribute directly or indirectly to reductions in human population growth.

Abstract: Employing a mathematical model we show how insularity, genotypic interactions and victim life-history/demography can influence adaptation in a simple enemy-victim interaction where genotypes migrate between a large source and a smaller, initially unoccupied, isolated habitat. We find that when there are explicit costs to heightened enemy virulence and victim resistance, large/close islands resemble their immigration sources, whereas small and/or distant islands tend to be occupied only by the least defended victims and least virulent enemies. In a model with no explicit cost to genotypic identity, frequencies do not differ on average between source and island. Despite these trends in genotype frequencies, for a range of realistic conditions, both cost and cost-free genotypic interactions yield an increase in the frequency of resistant encounters as a function of isolation. Moreover, in models with explicit costs, maximal island to island variation in genotypic frequencies is found on islands of intermediate distance from the source. In contrast, the model without explicit costs produces more variable communities, attaining maximum variability in genotypic frequencies at the most isolated islands. We hypothesize that adaptive patterns in mainland-island comparisons may differ substantially from those generated by centre-periphery comparisons in continental systems.

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Abstract: We constructed a model to investigate conditions under which intraspecific competition amplifies or diminishes the selective advantage to resistance. The growth trajectories of competing individual plants were depicted by logistic difference equations that incorporated basic costs (lowered growth rate) and benefits (lowered damage) of defense. Analytical results showed that when competition is absent, resistance is favored by high damage, low cost, and slow growth rate. Competition makes selection more complex. When herbivore damage reduces the size of a susceptible plant, resistant neighbors can usurp its resources and thus suppress its regrowth. This competitive interaction amplifies the relative fitness of the resistants. Numerical simulations explored a broader range of conditions. Three factors were varied: competition mode (symmetric vs. asymmetric), resistance type (damage avoidance vs, damage reduction), and timing of attack (early, mid, or late season). We found that competition mode had drastic effects on outcomes. Under symmetric competition, increased plant density intensified the selective advantage of resistance, damage avoidance was more strongly favored than damage reduction, and resistance to late attack was more favored than to early attack. Asymmetric competition had opposite effects: selection acted against resistance at high density, damage reduction was more strongly favored, and resistance against early attack was more favored. Interestingly, the two competition modes induced opposite patterns of density-dependent selection. The difference between the symmetric and asymmetric cases is explained by the fact that resistance costs during the preattack phase are more strongly amplified by asymmetric competition. When resistance is induced, so that pre-attack costs of resistance are zero, asymmetric competition more strongly amplified the benefits during the postattack phase. The prediction that selection on resistance will he plant density-dependent has complex implications for the evolutionary dynamics of defense evolution.

Abstract: Plants are known to maintain fitness despite herbivore attack by a variety of damage-induced mechanisms. These mechanisms are said to confer tolerance, which can be measured as the slope of fitness over the proportion of plant biomass removed by herbivore damage. It was recently supposed by Stowe et al. (2000) that another plant property, general vigor, has little effect on tolerance. We developed simple models of annual monocarpic plants to determine if a genetic change in components of growth vigor will also change the fitness reaction to damage. We examined the impact of intrinsic growth rate on the tolerance reaction norm slope assuming plants grow geometrically, i.e., without self-limitation. In this case an increase in intrinsic growth rate decreases tolerance (the reaction norm slope becomes more negative). A logistic growth model was used to examine the impact of self-limiting growth on the relationship between intrinsic growth rate and the tolerance reaction norm slope. With self-limitation, the relationship is sensitive to the timing of attack. When attack is early and there is time for regrowth, increasing growth rate increases tolerance (slope becomes less negative). The time limitations imposed by late attack prevent appreciable regrowth and induce a negative relationship between growth rate and tolerance. In neither of these simple cases will the correlation between vigor and tolerance constrain selection on either trait. However, a positive correlation between growth rate and self-limitation will favor fast growth/strong self-limitation in a high-damage environment, but slow growth/weak self-limitation in a low-damage environment. Thus, fundamental growth rules that determine vigor have constitutive effects on tolerance. The net costs and benefits of damage-induced tolerance mechanisms will thus be influenced by the background imposed by fundamental growth rules.

Abstract: Using a population model of selection on an obligate symbiont and its host, we evaluate how demographic differences across geographical landscapes can produce selection mosaics in interacting species. The model assumes that the host populations vary geographically from demographic sources to sinks in the absence of effects by the symbionts, and that a virulent and a relatively avirulent form of the symbiont compete with one another across all habitats. Our results indicate that productivity gradients can create selection mosaics across habitats, resulting in complex fitness landscapes over which evolution occurs. We find that relatively virulent symbionts only persist if they have an advantage over avirulent strains or species in terms of interference (i.e. competition, and/or cross-transmission) interactions. When such a trade-off exists, we predict that the more virulent symbiont is most likely to be found in habitats where host population growth is highest, whereas the more avirulent symbiont should tend to persist in more marginal habitats or even habitat sinks for symbiont-free hosts. Demographic sinks may be the habitats most likely to favour the origin of new mutualisms. Very productive mutualisms can be exploited by hyperparasites or cheaters. We discuss our findings in terms of geographical scenarios for the emergence of mutualisms, and the long-standing debate about geographical patterns in the maintenance of sex.

Abstract: Species interactions commonly coevolve as complex geographic mosaics of populations shaped by differences in local selection and gene flow. We use a haploid matching-alleles model for coevolution to evaluate how a pair of species coevolves when fitness interactions are reciprocal in some locations ("hot spots") but not in others ("cold spots"). Our analyses consider mutualistic and antagonistic interspecific interactions and a variety of gene flow patterns between hot and cold spots. We found that hot and cold spots together with gene flow influence coevolutionary dynamics in four important ways. First, hot spots need not be ubiquitous to have a global influence on evolution, although rare hot spots will not have a disproportionate impact unless selection is relatively strong there. Second, asymmetries in gene flow can influence local adaptation, sometimes creating stable equilibria at which species experience minimal fitness in hot spots and maximal fitness in cold spots, or vice versa. Third, asymmetries in gene flow are no more important than asymmetries in population regulation for determining the maintenance of local polymorphisms through coevolution. Fourth, intraspecific allele frequency differences among hot and cold spot populations evolve under some, but not all, conditions. That is, selection mosaics are indeed capable of producing spatially variable coevolutionary outcomes across the landscapes over which species interact. Altogether, our analyses indicate that coevolutionary trajectories can be strongly shaped by the geographic distribution of coevolutionary hot and cold spots, and by the pattern of gene flow among populations.

Abstract: We develop a simple mathematical model to investigate the question as to whether a specialised consumer san be responsible for creating a range limit in the population of its dynamic resource. The model is most attuned for parasitoid-host relationships, but the central results should apply to a broad range of systems. Specifically, at the beginning of each simulation host and parasitoid populations are distributed at random along a string of patches. In each discrete generation and for each patch, host and parasitoid populations grow and interact, and then a constant fraction of those remaining disperses one sr more patch distances in either direction according to a geometric distribution. We iterate the model 200 generations, and in any generation for any patch, either host and/or parasitoid can go locally extinct if its population falls below a threshold density. We find that a specialised parasitoid can enforce a limit, and it is even more likely to fragment its host population. The two most important conditions for parasitoid-enforced range limits are: 1) the theoretical host equilibrium density in the presence of the parasitoid be very small at sites eliminated from the host's range, and 2) the parasitoid disperses at high rates. We close by discussing our findings for specialist and generalist natural enemies, and the relevance of our study to the wealth of investigations on the causes of geographical range limits.

Abstract: We present a dynamic model of the evolution of host resistance to avian brood parasites, when the latter can retaliate against hosts that reject parasitic eggs. In a verbal model, Zahavi (1979, American Naturalist, 113, 157-159) suggested that retaliatory cuckoos might prevent the evolution of host resistance by reducing the reproductive success of rejecter hosts (i.e. by destroying their eggs or nestlings). Here we develop a model based on the association between the great spotted cuckoo, Clamator glandarius, and its main host, the European magpie, Pica pica, because this is the only system that has provided supportive evidence, to date, for the existence of retaliatory behaviour. Our aims were (1) to derive the conditions for invasion of the retaliation strategy in a nonretaliatory parasite population and (2) to investigate the consequences of retaliation for the evolution of host defence. If we assume a cost of discrimination for rejecter hosts in the absence of parasitism, and a cost paid by a retaliator for monitoring nests, our model shows cyclical dynamics. There is no evolutionarily stable strategy, and populations of both hosts and parasites will cycle indefinitely, the period of the cycles depending on mutation and/or migration rate. A stable polymorphism of acceptors and rejecters occurs only when parasites are nonretaliators. The spread of retaliator parasites drives rejecter hosts to extinction. Copyright 1999 The Association for the Study of Animal Behaviour.

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Abstract: Hyperparasitism is a widespread interaction in natural communities, but has to date received little attention in the theoretical literature. In this paper, we compared canonical models for food chains (resource-prey-predator systems) and host-parasite-hyperparasite interactions. We focus on microparasites, so the dynamical variables are the abundances of host individuals in different classes (e.g. with or without a particular parasite), and assume that the parasite is the only factor regulating a host population. Analysis of a "donor-controlled" model in which the primary parasite regulates host population growth, but with no additional demographic impact of the hyperparasite, suggests that intrinsic growth rate r of the host population is a fundamental parameter governing persistence of the hyperparasite. We then examine a model in which the hyperparasite can affect host births, deaths, and rate of recovery from the primary parasite. A wide range of outcomes are possible. For instance, hyperparasites can stabilize inherently unstable host-parasite systems, or destabilize stable systems. Persistence at a stable equilibrium often requires that the host intrinsic growth rate r lie within defined bounds; at low r, the hyperparasite may not be able to persist (in stable systems), whereas at high r the system is unstable and the host population grows in an unbounded fashion. We conclude by sketching directions for future work, and suggesting some possible practical implications of our results.Copyright 1998 Academic Press

Abstract: We investigate the influence of seven explanatory variables based on individual characteristics of galls on parasitism in the oak-galler, Neuroterus quercusbaccarum (sexual generation). The community consists of three species of parasitoid and one species of inquiline (which is lethal to the galler). Our analysis shows that there is considerable spatial heterogeneity in parasitism from site to site and from tree to tree within sites. With regard to the placement of galls on tree organs, galls on catkins are less parasitised than those on leaves. Gall size does not explain this difference because the external diameter of catkin galls is not significantly different from those on leaves. We hypothesise that the precocious abscission of catkin galls prevents their exploitation by parasitoid species with long developmental times. Moreover, there is a distinct sequence of parasitism, reflected by a partitioning in the sizes of galls attacked by each parasite species. However, the growth dynamics of the galls themselves show that just external diameter is not the only size parameter affecting the niches of the different parasites. Even though Synergus spp. is one of the earliest acting parasites, it attacks galls with thicker walls relative to external diameter than occurring in unparasitised galls from which the galler emerges. A delay between gall growth and feeding activity of the gallmaker would induce non-linear growth of gallwall thickness, with major consequences for the accessibility of the host larva to the parasitoid.

Abstract: For many host-parasite interactions, virulence is necessarily affected by population densities, transmission biology of the parasite, and proliferation of the parasite at the expense of its host. Attempts to experimentally demonstrate genetic correlations involving virulence therefore need to employ protocols controlling for variation in the number of successful infections (i.e., the end-point of transmissibility). If protocols are not controlled, then correlations may be spurious, as appears to be the case in recent experimental studies by Ebert (1994) and Ebert and Magnin (1997). There is a need to explore the modes of the evolution of each of the many sequential steps in nonsymbiotic and symbiotic phases of host-parasite associations and the implication of such evolution for overall virulence. I argue that it is the interdependence of these sequential steps (and not overall virulence) that should be at the center of attempts to establish genetic correlations.

Abstract: Previous studies have synthesized life-table data from herbivore species to identify general trends in the demography of herbivorous insects. Frequency-based analyses were used to ascertain which of five mortality sources (enemies, plant factors, competition, weather, intrinsic developmental failure) and which of five ecological characteristics of herbivores (feeding biology, invasion status of the herbivore, latitude, cultivation, and successional status of the habitat) had important influences on mortality patterns. Here these results are reinforced with a quantitative analysis that relies on actual numbers of herbivores killed at different developmental stages by each of the five mortality sources in different ecological settings. We also examine the relationship between taxonomic category (Coleoptera, Diptera, Lepidoptera, and Hymenoptera) and mortality. The analysis identified developmental changes of herbivores as having an important influence on sources of mortality; feeding biology, latitude, and cultivation status also influenced the distribution of mortality sources. Other aspects of the herbivores' ecology and taxonomy had limited effects. Natural enemies were identified as the most important mortality source overall, and their importance increased from the early larval stages to the pupal stages. They also kill more exophytic insects than endophytic insects, and kill a higher proportion of insects in cultivated habitats than in natural habitats. Weather kills more temperate-zone immatures than tropical/subtropical immatures. The results of the quantitative analysis generally confirm the earlier frequency-based tests. Several predictions that can serve as the foundation of an empirically-based theory of herbivore demography are offered: (1) natural enemies are the dominant cause of mortality in exophytic herbivore populations and may compete more intensely than on endophytics; (2) plant factors and enemies play a more balanced role in endophytic populations; (3) exophytic species should be particularly susceptible to top-down effects, especially in agroecosystems; (4) plant defences will often have sublethal effects, but when they are lethal they will be most important as the hatchling larva is just getting established on the plant. These predictions should be viewed as a challenge to engage in a broader way of thinking about herbivore demography.

Abstract: ABSTRACT This study addresses the question of how spatial heterogeneity in prey productivity and migration act to determine geographic patterns in antagonistic coevolution with a predator. We develop and analyze a quantitative coevolutionary model for a predator-prey interaction. If the model is modified appropriately, the results could broadly apply to multispecies communities and to herbivore-plant, parasite-host, and parasitoid-host associations. Model populations are distributed over a gradient in prey birth rate (as a measure of productivity). Each population, in each patch, is made up of a suite of strains. Each strain of the predator has a certain ability to successfully attack each strain of the prey. We consider scenarios of isolated patches, global migration, and stepping-stone (i.e., local) migration over a linear string of patches. The most pervasive patterns are the following: investments in predator offense and prey defense are both maximal in the patches of highest prey productivity; when there are no constraints on maximal investment, mean predation evolves to highest levels in the most productive patches; similarly, the predator has a greater impact (measured as the percentage reduction in prey density) on the prey population in high productivity patches as compared with low productivity ones-in spite (even after evolution) of prey abundance being highest in the most productive patches; and migration has the net effect of shunting relatively offensive and defensive strains from productive patches to nonproductive ones, potentially resulting in the elimination of otherwise rare, low-investment clones. A modification of the model to gene-for-gene type interactions predicts that generalist strains (in terms of the range of strains the predator can exploit or the prey can fend off) dominate in productive areas of the prey, whereas specialists prevail in marginal habitats. Assuming a wide range of productivities over the prey's geographical distribution, the greatest strain diversity should be found in habitats of intermediate productivity. We discuss the implications of our study for adaptation and conservation. Empirical studies are in broad accord with our findings.

Abstract: The evolution of resistance by insect and weed pests to chemical pesticides is a problem of increasing importance in applied ecology. It is striking that the evolution of resistance by target pest species in biological control is much less frequently reported, particularly in control involving parasitoids and predators, rather than pathogens. Although it is conceivable that this reflects biases in reporting or frequency of application, we suggest that there is a puzzle here worthy of scrutiny, and we outline several potential underlying causes. In order of discussion (not necessarily of importance), these are: (1) lack of genetic variation; (2) genetic constraints on selection; (3) weak selection; (4) temporally varying selection; and (5) coevolutionary dynamics. We, in particular, focus on the potential for weak selection on the host for increased resistance, despite effective control. The very spatial mechanisms (e.g., refuges, metapopulation dynamics) believed to facilitate the persistence of many natural enemy-victim systems with strong biological control may also incidentally provide an environment where selection is weak on target pests to evolve improved resistance to control agents, thereby biasing coevolution toward the enemy. The basic insight is that in a spatially heterogeneous environment, a strong limiting factor on a population can be a weak selective factor. The hypotheses presented here provide ingredients needed to predict which biological control systems might be evolutionarily stable, and which not. Our aim in this thought piece is to stimulate more attention to the evolutionary dimension of biological control systems.

Abstract: Ceratosolen silvestrianus and C. arabicus are two species of agaonine fig wasps that co-occur in the figs of Ficus sur in West Africa. Our work in the Ivory Coast shows that both are active pollinators, and that they may be found together in the same syconia. In our study area, C. silvestrianus is much more abundant than C. flabellatus. We found no competitive effect for oviposition sites on C. silvestrianus, and hypothesize that habitat preferences allow the coexistence of two sympatric active pollinators in the same Ficus host in the Ivory Coast.

Abstract: We investigate how age-structure and differences in certain demographic traits between residents and immigrants of a single species act to determine the evolutionarily stable dispersal strategy in a two-patch environment that is heterogeneous in space but constant in time. These two factors have been neglected in previous models of the evolution of dispersal, which generally consider organisms with very simple life-cycles and assume that, whatever their origin, individuals in a given habitat have the same bio-demographic characteristics. However, there is increasing empirical evidence that dispersing individuals have different demographic properties from phylopatric ones. We develop a matrix model in which recruitment depends on local population densities. We assume that dispersal entails a proportional cost to immigrant fecundity. which can be compensated by differences in survival rates between immigrants and residents. The evolutionarily stable strategies (ESS) for dispersal are identified using a combination of analytical expressions and numerical simulations. Our results show that philopatry is selected (1) when dispersal rates do not vary in space, (2) when the metapopulation is a source-sink system and (3) when dispersal rates vary in space (asymmetric dispersal) and immigrants do not compensate for their reduced fecundity. We observe that non-zero asymmetric dispersal rates may be evolutionarily stable when (1) immigrants and residents are demographically alike and (2) immigrants compensate totally for their reduced fecundity through an increase in adult survival. Under these conditions, we find that the ESS occurs when the fitnesses at equilibrium in the two habitats, measured in our model by the realized reproductive rates, are each equal to unity. A comparison with previous studies suggests a unifying rule for the evolution of dispersal: the dispersal rates which permit the spatial homogenization of fitnesses are ESSs. This condition provides new insight into the evolutionary stability of source-sink systems. It also supports the hypothesis that immigrants have adapted demographic strategies, rather than the hypothesis that dispersal is costly and immigrants are at a disavantage compared with residents.

Abstract: Metapopulation studies of single species have shown that the size and spatial arrangement of patches of assumed uniformly 'suitable' habitat can influence their population dynamics and persistence. We investigated whether variation in the spatial arrangement of 'suitable' habitat of varied quality within a single site can affect the abundance and persistence of interacting species. We accomplished this by extending a field-based spatial simulation model of four interacting species at two trophic levels (an endangered butterfly, its larval food-plant, and two ants). The habitat on sites with the same average and range of qualities was rearranged to give varying degrees of local spatial heterogeneity or 'site ruggedness'. We found that the ant species that acts as host to the butterfly caterpillars decreased with site ruggedness. The impact on the butterfly was more substantial: it often failed to persist on very rugged sites. Despite being free-ranging over the whole area, the butterfly's persistence depends on the arrangement of habitat quality at a finer spatial scale, due to its interactions with species possessing narrower habitat niches and more localized dispersal. Ruggedness also influenced the rate of recovery of the host ant, and hence community structure, for more than a century following the butterfly's extinction.

Abstract: We compiled life tables for 78 holometabolous herbivorous insect species to quantify levels of apparent enemy-induced mortality of immature insects. Enemies were classified by type (predator, parasitoid, or pathogen), and mortalities caused by each type in each herbivore immature stage were tested with Analysis of Deviance for differences associated with four ecological characteristics of preadult herbivores: feeding biology, invasion status, and the cultivation status and latitudinal zone of the habitat. Total enemy-induced mortality is higher in the late developmental stages, and overall, parasitoids kill more herbivores than do either predators or pathogens. Among the ecological variables, both feeding biology and latitude showed significant enemy effects in at least one late developmental stage, whereas neither cultivation status nor invasion status was associated with enemy-induced mortality in any stage. Bonferroni adjustment of probabilities for multiple comparisons resulted in few significant interactions between enemy type and the ecological variables. However, raw probabilities and comparisons across herbivore immature stages suggest several patterns that deserve attention in future studies: (1)endophytic herbivores suffer lower mortality by predators and pathogens than exophytics, and endophytic leaf miners suffer the greatest parasitoid-induced mortality, while endophytic gallers/borers/root feeders suffer the least; (2) overall enemy-induced mortality is similar in natural and cultivated habitats; (3) exotic insects do not suffer lower enemy-induced mortality rates than natives; and (4) predation and disease may be greater in tropical/subtropical habitats, whereas parasitism is greater in the temperate zone. These results identify several general patterns in insect demographics that should be useful for hypothesis testing.

Abstract: I employ a mathematical model integrating the population and co-adaptive dynamics of an insect host and its specialist parasitoid wasp to investigate the competitive evolution of hva forms of host resistance: concealment from adult parasitoid location and encapsulation of parasitoid eggs. When only one form of resistance is permitted to evolve, concealment always evolves to frequencies equal to or higher than encapsulation. When both forms of resistance evolve, the outcome depends on the capacity of the parasitoid to evolve counter-measures. Evolution of the host in the presence of the most virulent parasitoid clone results in no differences between the equilibrium frequencies of the two forms of resistance, unless host clone densities are somehow unequally perturbed. When the parasitoid is allowed to co-evolve, the frequency of hosts concealing themselves exceeds the frequency of those capable of encapsulating parasitoid eggs. At equilibrium, there is generally a negative correlation between resistance measures in the host population. and a positive one for countermeasures in the parasitoid population. The mechanism driving the asymmetries between the two forms of host defence is the loss of more reproductive effort by parasitoids when the host encapsulates as compared to when an encounter is missed due to concealment. I predict that hosts should pay greater costs to maintain concealment as compared to encapsulation when they are relatively common and;or parasitism rates relatively low; that is, when regulation of the host population tends to be influenced bq other density dependent forces.

Abstract: In recent spatial models describing interactions among a myrmecophilous butterfly Maculinea rebeli, a gentian Gentiana cruciata and two competing species of Myrmica ant, we predicted that apparent competition should exist between gentians (the food of young M. rebeli caterpillars) and Myrmica schencki, which supports M. rebeli in its final instar. Here we extend and quantify model predictions about the nature of this phenomenon, and relate them to ecological theory. We predict that: (i) Within sites supporting the butterfly, fewer M. schencki colonies occur in sub-areas containing gentians than in identical habitat lacking this plant. (ii) Where G. cruciata and M. schencki do co-exist, the ant colonies will be less than half the size of those living >1.5 m from gentians; (iii) The turnover of M. schencki colonies will be much greater than that of other Myrmica species in nest sites situated within 1.5 m of a gentian. All three predictions were supported in the field on 3-6 sites in two mountain ranges, although the exact strength of the apparent competition differed from some model predictions. Field data were also consistent with predictions about apparent mutualisms between gentians and other ants. We suggest that apparent competition is likely to arise in any system in which a specialist enemy feeds sequentially on two or more species during its life-cycle, as occurs in many true parasite-host interactions. We also predict that mor: complex patterns involving other Myrmica species and G. cruciata occur in our system, with apparent competition existing between them in some sub-areas of a site being balanced by apparent mutualism between them in other sub-areas.

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Abstract: We test the ability of a complex spatial model to predict the size of M. rebeli populations in two mountain ranges. The published model was based on nineteen parameters that described the population dynamic interactions between the butterfly, its initial food plant Gentiana cruciata, its subsequent host Myrmica schencki, and other competing non-host species of Myrmica. All original model parameters were measured on one site in the Spanish Pyrenees. In this study we measured seven of these parameters, concerning gentians and their habitat, at 12 new sites in the French Alps and at another in Spain. We show that model predictions using these values explained 86% of the variation in M. rebeli egg numbers, independently measured on the other sites. Three other (non-intuitive) model predictions were also partly confirmed: (i) A priori, our model (which had been parameterised from a single, atypical site with very high gentian densities) predicted a close, indirect, non-linear relationship between gentian and butterfly densities, with egg-numbers being negatively correlated at high gentian densities and positively correlated over low densities. The prediction would be unusual for phytophagous butterflies and is counter-intuitive for a species which experiences only moderate competition on its food plant before spending most of its life inside ant nests, where it is regulated by strong, density-dependent competition. Although this was not fully tested due to a lack of new sites supporting high gentian densities, the empirical results showed a very close positive relationship between egg numbers and gentian densities on sites supporting < 1200 gentians per hectare. Furthermore, our original mechanistic model also accurately predicted both the slope and the range of the positive relationship found at low densities. (ii) Annual fluctuations in populations of M. rebeli are exceptionally small. The fit between observed and predicted egg densities was significantly closer than expected if M. rebeli populations are assumed to fluctuate with the amplitude of the least variable phytophagous British butterfly. (iii) Myrmica schencki populations on the new sites occurred at the low densities predicted by the model as a consequence of parasitism by the butterfly. The accuracy of these predictions over a wide range of sites encourages us to apply the model to simulate the effects of different conservation options for M. rebeli.

Abstract: We investigate the roles of several factors in determining patterns of parasitism on the oak galler species Neuroterus quercusbaccarum (L.). We employ exclosures around growing galls to identify the windows of vulnerability of the galler host to two primary parasitoids and one inquiline parasite. There are phenological and temporal patterns in the incidence of parasitism among the three parasite species, with the inquiline Synergus sp. tending to attack small galls early, followed by Mesopolobus tibialis which attacks medium-sized galls, and by Torymus auratus, which attacks the largest galls. Despite the importance of gall size in structuring the parasitoid complex, gall size does not act as an absolute refuge from parasitism. Finally, both M. tibialis and T. auratus have significant effects on gall growth in reducing the final gall size. This result is in agreement with the idiobiontic life style of these species, since gall growth stops with the death of the gall maker.

Abstract: We examine a spatially explicit 'case model' for the interaction between the lycaenid butterfly, Maculinea rebeli, and its specialist parasitoid, Ichneumon eumerus. This butterfly lives in small, closed populations, rarely numbering over a few thousand individuals, and the parasitoid is found at only a small subset of butterfly-harbouring sites. We explore how parasitoid searching intensity and behaviour, and host refuges from parasitism affect the dynamics of the host-parasitoid couple. In the absence of explicit host refuges, the parasitoid persists only for a very restricted range of search rates and searching behaviours. Absolute refuges to parasitism, modelled as a cue-threshold phenomenon in the elicitation of intensive search for the host, expand the persistence conditions. We link these results to the more general problem of what inferences can be drawn concerning the association between population-level variation in the distribution of parasitism and the population dynamics of the system. The parasitoid's persistence depends importantly on heterogeneity in the vulnerability of the host caterpillars to encounter with parasitoids. Although the host's persistence is also enhanced by such heterogeneity, it is actually intraspecific competition within ant nests that dominates host dynamics. Deductions of the stabilizing power of parasitoids from measures of spatial heterogeneity in parasitism will be spurious without information about the respective density-dependent influences of the parasitoid and other limitation factors affecting the host.

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Abstract: In the laboratory, the two species of copepods Lepeophtheirus thompsoni and Lepeophtheirus europaensis, ectoparasites of flatfishes, can meet and mate on at least one host species. In the wild however, these two species are found isolated on their sympatric hosts. Habitat selection theoretically represents a powerful enough mechanism to explain the maintenance of genetic heterogeneity in the wide sense. In this paper, the host colonization process is studied for both parasite species. It is shown that each parasite can develop and reach adult age on each host species. However, L. thompsoni is highly selective; it almost totally refuses to colonize hosts other than its natural one. Lepeophtheirus europaensis, on the contrary, readily infests turbot and brill in single-host experiments, but strongly prefers the brill when it has a choice. It appears that these two genetic entities are sympatrically maintained due to strong habitat selection. Such a pattern could theoretically only occur in a soft-selection context (density dependence). This point is discussed with respect to the different patterns in host use found in the geographical distribution of these parasites.

Abstract: We have investigated the theoretical consequences of character evolution for the population dynamics of a host-parasitoid interaction, assuming a monophagous parasitoid. In the purely ecological model it is assumed that hosts can escape parasitism by being in absolute refuges. A striking property of this model is a threshold effect in control of the host by the parasitoid, when host density dependence is weak. The approximate criteria for the parasitoid to regulate the host to low densities are (1) that the parasitoid's maximum population growth rate should exceed the host's and (2) that the maximum growth rate of the host in the refuge should be less than unity. We then use this ecological framework as a basis for a model which considers evolutionary changes in quantitative characters influencing the size of the absolute refuge. For each species, an increase in its refuge-determining character comes at a cost to maximum population growth rate. We show that refuge evolution can substantially alter the population dynamics of the purely ecological model, resulting in a number of emergent and sometimes counter-intuitive properties. In general, when the host has a high carrying capacity, systems are polarized either with low or minor refuge and 'top-down' control of the host by the parasitoid or with a refuge and 'bottom-up' control of the host by a combination of its own density dependence and the parasitoid. A particularly tantalizing result is that co-evolutionary dynamics can modify ecologically unstable systems into ones which are either stable or quasi-stable (with bouts of unstable dynamics, punctuating long-term periods of quasi-stable behaviour). We present five quantitative criteria which must all be met for the parasitoid to be the agent responsible for control of the host at a co-evolutionary equilibrium. The apparent stringency of this full set of requirements supports the empirically-based suggestion that monophagous parasitoid-driven systems should be less common in nature than those driven by multiple forms of density dependence. Further, we apply our theory to the question of whether exploiters may 'harvest' their victims at maximum sustainable yields and to the evolutionary stability of biological control. Finally, we present a series of testable predictions of our theory and methods useful for testing them.

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Abstract: 1. Caterpillars of the butterfly, Maculinea rebeli, feed sequentially in flower-buds of Gentiana cruciata and in nests of Myrmica schencki ants, with which they have a sort of 'cuckoo bird' relationship. Ants feed them in preference to their own larvae, reducing the production of new workers and hence the colony size the following year. 2. Other Myrmica compete with M. schencki for nest-sites. They adopt caterpillars with equal facility, but fail to rear them to maturity and are consequently less damaged. 3. Using a mathematical model we explore how species interactions can influence the populations of the butterfly, M. schencki, and other Myrmica species. The model assumes a 1 ha square of 900 cells with a fixed population of gentians. A cell can contain at most a single Myrmica colony which can adopt a proportion of the caterpillars leaving the gentians. Both density-dependent and density-independent butterfly mortalities occur on the plant and within the nest. 4. M. schencki is more successful in hotter, drier habitat patches than other Myrmica. This is simulated by a one-dimensional gradient in the maximum yearly reproductive rate (R) of M. schencki, the gradient for other Myrmica species being approximately reciprocal to that of M. schencki. M. schencki and other Myrmica compete pre-emptively for vacant nest sites. 5. The model is seeded with nests and run for 50 generations, to stabilize the ant distributions, before being 'colonized' by a single female of Maculinea rebeli. Addition of the butterfly to the system reduces dramatically both the number and average size of M. schencki nests. As a result, the number of nests of other Myrmica increases as they encroach onto the drier areas. In intermediate areas, where most pre-emptive competition occurs, the nest-size of other Myrmica increases, but overall, their average size also falls because of the direct effect of the caterpillars. 6. The system stabilizes at about 268 nests of M. schencki, 73% being in cells containing G. cruciata and so able to adopt caterpillars and 27% being in plant-free cells, avoiding parasitization. Only 16% of cells are plant-free, demonstrating the negative effect of the butterfly on the spatial distribution of M. schencki and its ability to mediate apparent competition between plants and ants. 7. The model predicts that: (a) the largest populations of butterflies should occur in systems of intermediate plant density (c. 1000-1500 plants ha-1); (b) the greatest turnover in ant nests should occur in dryer areas, where nests of M. schencki are relatively most damaged by the butterfly, despite a pronounced impact on the ant populations in the first years following colonization; (c) only c. 0.2% of nests should go extinct each subsequent year as a consequence of butterfly parasitization; and (d) only about half of the potential rearing capacity of the M. schencki population should be exploited by the butterfly in any one year. 8. We suggest that a subtle combination of changes in the environment can either benefit or endanger the butterfly population depending on how each of the ant species are affected. 9. From a conservation stand-point, population size is not necessarily an accurate predictor of the fragility of the butterfly population; as long as the butterfly population is safe from stochastic mechanisms of local extinction, the intrinsic and resource-based components of the butterflies' basic reproductive rate are better indicators of persistence.

Abstract: 1 Using a spatially explicit cellular automaton model we explore the effects of tree demography, fire-induced mortality, and seed dispersal on the spatial spread of a single tree species in a humid savanna at Lamto in West Africa. 2 The model system is described by six parameters and consists of a grass-surrounded square grid of connecting cells, each being either inhabited by grass alone or by grass and an individual tree. In the baseline numerical simulations the tree can only recruit seedlings in immediately adjacent cells. These seedlings may perish from annual grass fires in their first year of life if they are not protected from the advancement of the fire by neighbouring reproductively mature trees. 3 Based on preliminary parameter estimates from data collected at field sites at Lamto, we predict that fire slows, but does not stop, the spread of the tree. In the absence of fire the doubling rate of the tree population is about 6 years, whereas we predict that yearly fires prolong this to at least 30 years. 4 The temporal dynamics of the tree population are fairly smooth and predictable as long as there are more than c. 100 cells in the system. As the number of cells is decreased below c. 100 the trajectories become increasingly variable from year to year. 5 Mortalities from fire act in an inverse spatially density-dependent fashion, enhancing tree aggregation. The role of fire in enhancing tree aggregation is supported by additional simulations in which dispersal of seeds to non-adjacent cells can occur. When a small amount of dispersal is possible the rate of tree population growth is greatly accelerated as compared to when no such dispersal occurs. 6 We present several hypotheses to explain why the savanna at Lamto is not tree-dominated as would be predicted by the model, discuss how seed dispersal and fire influence tree dynamics, and make predictions for future testing.

Abstract: We review empirical studies bearing on the effects of parasites on the age of maturity of their hosts. The few cases already published support theoretical predictions, namely a decrease of host prereproductive life-span unless parasites are benign. Host responses may be due either to phenotypic plasticity or to genetic differences, and even though very few studies on this topic have already been published both mechanisms occur. Promising areas of research include the distribution of age-specific potential costs of resistance to parasitism, as well as the evolution of age-specific parasite preferences under the concomitant evolution of host life-history traits.

Abstract: An important question in ecology is the extent to which populations and communities are governed by general rules. Recent developments in population dynamics theory have shown that hosts' refuges from their insect parasitoids predict parasitoid community richness patterns. Here, the refuge theory is extended to biological control, in which parasitoids are imported for the control of insect pests. Theory predicts, and data confirm, that the success of biological control is inversely related to the proportion of insects protected from parasitoid attack. Refuges therefore provide a general mechanism for interpreting ecological patterns at both the community level (their species diversity) and population level (their dynamics).

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Abstract: We develop a formal theory, based on simple population dynamics constructs, to identify how proportional refuges to parasitism may act to determine the number of parasitoid species sampled per host species. A series of mathematical population models based on biologically realistic phenomena predict one of two basic species richness patterns. If the host population is sufficiently exploited, then a domed relationship results between refuge and richness; otherwise, depending on the types of parasitoids present and the initial population levels, the basic pattern is either domed or simply monotonically decreasing. We suggest that simple rules, centering on proportional refuges to parasitism and the finite rate of increase of the host population, explain the richness patterns present in real parasitoid assemblages.

Abstract: A central problem in ecology is predicting the diversity of communities. Insect parasitoids may encompass 20 percent of all insect species; hence, establishing the mechanisms that drive parasitoid species richness represents a major step in understanding the diversity of terrestrial communities. An assemblage model, based on population dynamic constructs, shows how the presence of refuges from parasitoid attack can generate diversity patterns that are in good accord with global data on structural protection from parasitism resulting from host feeding biology. This theory offers a simple ecological explanation for the range of diversifies observed in real parasitoid assemblages. Predicting parasitoid diversity may be a realistic goal, at least for those systems in which the basic demography is well understood.

Abstract: 1. The five European species of Maculinea butterflies are among the most endangered invertebrates in the world. To attain a better understanding of the mechanisms which are most influential to their population dynamics, we develop a mathematical population model for one of these species, Maculinea rebeli, based on its dependence on two sequential resources, the cross-leaved gentian, Gentiana cruciata, and a red ant, Myrmica schencki. One of the main objects of this modelling study is to raise questions about the ecology and conservation of this rare and fascinating butterfly. 2. The model has two main biological components: (i) recruitment and competition on the plant for the first few weeks of butterfly life, and (ii) recruitment by and competition within nests of the red ant for the remainder of the butterfly life-cycle. 3. We estimate all of the parameters for the model based in part on data collected at a 4-ha study site in the Spanish Pyrenees, one of the largest known populations of Maculinea in the world. 4. The model is used to evaluate how different ecological processes and densities of the plant and ant resources may affect the butterfly population, including its density, resilience from perturbation. and persistence. 5. The model clearly indicates that, for the Spanish study site, the survival of the butterfly in ant nests is disproportionately more important to its population than is competition on the plant. Changes in the observed population of the plant have virtually no impact on the butterfly population. 6. Future studies should take account for instance of changes in the density of ant nests and the population of a specialist ichneumonid parasitoid.

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Abstract: There are a number of ways in which a host can respond in evolutionary time to reductions in survival and reproduction due to a virulent parasite. These include evolving physiological morphological, or behavioural mechanisms of resistance to infection (or to proliferation, once infection has occurred). But a more unexpected tactic is also possible. This is for hosts to reproduce (slightly) sooner when in the presence of a virulent parasite as compared to when the parasite is less virulent or absent. As such, hosts which reproduce younger may be at a selective advantage, since they can both evade parasitism in time and, even when parasitised, can reduce the likely impact of the parasite on survival and reproductive success. We employ a simple mathematical model to propose that parasites and pathogens can act as important agents in the evolution of the timing of reproduction and associated life-history characters (e.g. body size). Once established in a semelparous host population, evolutionary increases in parasite virulence should result in the evolution of shorter lived hosts; whereas the evolution of less virulent forms of the parasite should be accompanied by the evolution of longer lived hosts. We argue that in the presence of a sufficiently virulent parasite the evolution of longer pre-reproductive life-spans should require the previous or concomitant evolution of morphological, behavioural or physiological resistance to parasitic infection and proliferation.

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Abstract: This study considers how non-linearities in the transmission of microparasitic infections affect the population dynamics of host-parasite systems in which the disease is potentially lethal to the host. Non-linearities can either lead to a locally stable or unstable host-parasite equilibrium point, depending on the respective contributions of healthy and infected hosts to the functional form of the transmission rate. Analysis of the non-linear transmission model results in a revealing pair of local stability criteria. Specifically, stability requires sufficient total levels of intrinsic growth of the host population and total levels of density-dependent transmission. The most stable systems occur when increases in the density of healthy hosts result in increases in transmission efficiency, and increases in the number of infected hosts result in small decreases in transmission efficiency. These appear to be very reasonable relationships for directly transmitted microparasites.

Abstract: (1) This study explores the extra-host interactions between the braconid endoparasitoid, Apanteles glomeratus, and the granulosis virus (GV) of Pieris brassicae, focusing on factors influencing the transmission of the virus within and between host broods. (2) Transmission of the virus by A. glomeratus adult females was shown to correlate positively with the time since death of the virus-killed donor larvae. Transmission was most likely to occur from donors which were dead for 24-48 h. (3) In the absence of the parasitoid, the risk of infection to a given P. brassicae larva was dependent on (i) the host age at the time of the introduction of the virus, and (ii) the initial number of the larvae infected. The initial number of uninfected hosts, however, had no effect on the per capita risk of infection. (4) The parasitoid accelerated the spread of the GV in the host brood, but the eventual number of hosts succumbing to the infection was unaffected. (5) Whereas a field experiment demonstrated the parasitoid as an important disseminator of the GV, percentage parasitism and the number of virus-infected cadavers were significantly negatively correlated. This latter result suggests that the parasitoid is somehow repelled by virus-killed hosts.

Abstract: (1) We develop a mathematical population model for Oryctes rhinoceros, where it is limited both by density-dependent larval mortality and a baculovirus. (2) The model divides the phenology of the female beetle into three distinct life-stages: juveniles (egg through young adult), feeders (young to middle-aged adults which are either mating or initiating attacks on palms), and breeders (older adults which lay their eggs at breeding sites). (3) The rates of six different transmission pathways of the baculovirus are estimated using a least squares minimization criterion based on field prevalences of disease. This technique reveals that transmission from infected to susceptible feeding adults is the dominant route. This is in agreement with the literature on the subject (Zelazny 1976). (4) The numerical simulation of the system, although differing from field observations in the first 6 months of the simulation, is in accord with longer-term field observations on the population densities, prevalence of disease, and depression of the pre-baculovirus release equilibrium of Oryctes. The reason for the short-term differences is thought to be the omission from the model of spatial aspects of the spread of the disease. (5) Inundation by the fungus Metarhizium anisopliae var. majus is modelled as a long-residual, density-independent, 'biopesticide' which kills both juveniles and breeding adults. (6) Numerical simulations show that increasing fungus inundation rates result in substantial decreases in the juvenile beetles. However, the fungus has no perceptible effect on the economically important populations of healthy (i.e. uninfected) feeding adults, and at high rates of application leads to an amplification in population cycles (i.e. instability) and the eventual elimination of the baculovirus from the system.

Abstract: (1) This study explores within-host mechanisms of intra- and interspecific competition involving the gregarious braconid endoparasitoid, Apanteles glomeratus, and the granulosis virus (GV) of Pieris brassicae. (2) Increasing doses of the GV resulted in increases in mortality and decreases in lifespan of the host. Susceptibility to the virus decreased markedly with host age. (3) High doses of the GV resulted in substantial reductions in production of pathogen progeny, consistent with intraspecific competition for a limited (but growing) resource. (4) Survival of the parasitoid brood within GV-infected hosts depended on the relative timing of parasitoid emergence (at the spinning of the host pupal mat) and host death due to the virus. (5) Interspecific competition between the natural enemies did not involve changes induced by the parasitoid in the pathogen's ability to infect the host. However, the expected life-span of hosts harbouring both competitors was significantly shorter than hosts infected with the virus alone. (6) Interspecific competition within the host was, on average, highly symmetrical. The presence of virus reduced the average weight of the parasitoid brood by 29%, whereas the parasitoid brood reduced the potential reproductive output of the virus by 28%. However, when the size of the parasitoid brood was taken into account, the parasitoid individual was best off when in small broods, whereas the virus did best when competing with larger parasitoid broods.

Abstract: I present evidence that gregariously feeding species of lepidoptera have evolved resistance to infection by their viral parasites, in terms of the larger amounts of virus needed to cause an infection as larvae age. First, I show a significant positive association between age-related resistance to infection by viruses and gregarious feeding behaviour in 13 cases studies involving temperate lepidoptera. Since pronounced levels of resistance to viral infection are not found in solitary species it can be inferred that this resistance has evolved in gregarious species at a cost. Further, that gregariousness itself often breaks down with larval age, suggests that the host responds to the selection pressure of the virus through not only physiological adaptations, but also behavioural ones. Second, I present evidence from a laboratory experimental system (Pieris brassicae and a granulosis virus) showing that observed age-related resistance is sufficient to explain the insensitivity of the per capita risk of infection to the number of larvae in the brood. The findings presented here are relevant to the use of pathogens for the biological control of insect pests.

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Abstract: Parasites have the potential to influence intraspecific competition within the population of a single host species in a manner analogous to a predator or a parasite influencing the outcome of interspecific competition between two competing species. I show how the competitive interactions of healthy and diseased hosts may influence the population dynamics of the host and, more particularly, the ability of susceptible and infected sub-populations of the host to persist. The dynamic character of the system can resemble interspecific competition between two species, predator-prey interactions, or, in some cases, a combination of the two. I suggest that in real systems it may be difficult to discriminate the relative contributions of parasitism and intraspecific competition to the limitation of host populations.

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Abstract: It is now well established that pathogens such as viruses, fungi bacteria and protozoans can have profound effects on the dynamics of their invertebrate host populations. Theoretical models of invertebrate host-pathogen interactions which assume uniform structure of the pathogen population may reasonably explain the oscillatory behaviour observed in some systems, but do not adequately describe the existence of more constant populations found in other host-pathogen interactions. An examination of the literature relating to these relatively stable systems suggests that the common thread is the eventual transmission of some of the more protected, longer-lived stages of the pathogen occurring in reservoirs, such as the soil, host cadavers on trees, or the live host itself. In this letter, I propose a new theoretical model which incorporates this population structure and accounts for the range of dynamics observed in natural systems. In particular, I show that host populations may be regulated to low and relatively constant densities if sufficient numbers of pathogens are trans-located from pathogen reservoirs to habitats where transmission can occur. An understanding of pathogen reservoirs may be of value in the design of biological control programmes and may greatly increase the effectiveness of pathogens as biological control agents.

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