Search

Offspring quality and quantity are subject to a trade-off. Depending on species, ant colonies can produce either numerous winged queens that start new colonies alone (independent colony foundation [ICF]) or one propagule consisting of reproductives helped by workers (fission). Some species do both. In the Rhytidoponera impressa group, colonies with winged queens use ICF, whereas colonies with mated workers use fission. We assessed the effect of latitude and environment on colonial reproduction by collecting 79 colonies along the Australian east coast. A population dynamical model predicted that from tropical north to temperate south, seasonal fluctuations and harsher conditions should lead to a decrease in ICF success (-48%), making fission a suitable alternative. Yet, our empirical data showed that ICF persists at a higher rate than expected, presumably because it allows aerial dispersal, unlike fission. Investigation of colony demography and measurement of lean and fat weights in virgin winged queens and workers showed that, from north to south, colonies increase quality of queens relative to workers (+50%) at the cost of quantity (-86%). This modification limits the decrease in ICF success (-34%). A tremendous range of offspring phenotypes associated with quality-quantity trade-offs make ants competitive in diverse habitats.

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.

We analyze the evolutionary consequences of host resistance (the ability to decrease the probability of being infected by parasites) for the evolution of parasite virulence (the deleterious effect of a parasite on its host). When only single infections occur, host resistance does not affect the evolution of parasite virulence. However, when superinfections occur, resistance tends to decrease the evolutionarily stable (ES) level of parasite virulence. We first study a simple model in which the host does not coevolve with the parasite (i.e., the frequency of resistant hosts is independent of the parasite). We show that a higher proportion of resistant host decreases the ES level of parasite virulence. Higher levels of the efficiency of host resistance, however, do not always decrease the ES parasite virulence. The implications of these results for virulence management (evolutionary consequences of public health policies) are discussed. Second, we analyze the case where host resistance is allowed to coevolve with parasite virulence using the classical gene-for-gene (GFG) model of host-parasite interaction. It is shown that GFG coevolution leads to lower parasite virulence (in comparison with a fully susceptible host population). The model clarifies and relates the different components of the cost of parasitism: infectivity (ability to infect the host) and virulence (deleterious effect) in an evolutionary perspective.

Though predation, productivity (nutrient richness), spatial heterogeneity, and disturbance regimes are known to influence species diversity, interactions between these factors remain largely unknown. Predation has been shown to interact with productivity and with spatial heterogeneity, but few experimental studies have focused on how predation and disturbance interact to influence prey diversity. We used theory and experiments to investigate how these factors influence diversification of Pseudomonas fluorescens by manipulating both predation (presence or absence of Bdellovibrio bacteriovorus) and disturbance (frequency and intensity of disturbance). Our results show that in a homogeneous environment, predation is essential to promote prey species diversity. However, in most but not all treatments, elevated diversity was transitory, implying that the effect of predation on diversity was strongly influenced by disturbance. Both our experimental and theoretical results suggest that disturbance interacts with predation by modifying the interplay of resource and apparent competition among prey.