Spatial heterogeneity indirectly influences the coevolution of relevant traits of interacting species by affecting geographic patterns in species interactions. Species survival and reproduction differ predictably over space, thus environmental gradients in landscapes are likely to be important in coevolutionary dynamics. Spatially varying antagonistic interactions can lead to a spatial mosaic of selection pressures for the evolution of victims’ defenses and enemies’ counter measures. In addition, migration of individuals can introduce trait variation and can either facilitate or hinder local adaptation. Although space is an obviously important factor in antagonistic interactions, its effects on coevolution has not yet been well studied. A host-parasitoid system as an example, we theoretically studied the effects of environmental gradients on the coevolution of host resistance and parasitoid virulence. We constructed a discrete-time coupled host-parasitoid metapopulation model with separate environmental gradients for the host and parasitoid populations represented by Gaussian curves. We assessed how the shape of trade-offs, the steepness of gradients, and migration rates altered coevolutionary outcomes.
We found that, conforming to previous other studies, resistance and virulence evolve to be the strongest at the optimal patches along the gradients. Polymorphism is higher when the trade-off between reproduction/recruitment and resistance/virulence is weak and when environmental gradients are steep. Higher migration rates tend to reduce the degree of polymorphism by spreading individuals with high resistance and virulence from the optimal locations to marginal habitats. Results suggest that the degree of polymorphism can substantially vary between optimal and marginal habitats. We conclude that spatial patterns of coevolutionary outcomes in host-parasitoid interactions can exhibit higher or lower heterogeneity than underlying environmental gradients.