ANTAGONISTIC COEVOLUTION OVER PRODUCTIVITY GRADIENTS

This Study addresses the question of how spatial heterogeneity in prey productivity and migration act to determine geographic patterns in an tagonistic coevolution with a predator. We develop and analyze a quant itative coevolutionary model for a predator-prey interaction. If the m odel is modified appropriately, the results could broadly apply to mul tispecies communities and to herbivore-plant, parasite-host, and paras itoid-host associations. Model populations are distributed over a grad ient in prey birth rate (as a measure of productivity). Each populatio n, 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 pa tches. The most pervasive patterns are the following: investments in p redator offense and prey defense are both maximal in the patches of hi ghest prey productivity; when there are no constraints on maximal inve stment, mean predation evolves to highest levels in the most productiv e patches; similarly, the predator has a greater impact (measured as t he percentage reduction in prey density) on the prey population in hig h productivity patches as compared with low productivity ones-in spite (even after evolution) of prey abundance being highest in the most pr oductive patches; and migration has the net effect of shunting relativ ely offensive and defensive strains from productive patches to nonprod uctive ones, potentially resulting in the elimination of otherwise rar e, low-investment clones. A modification of the model to gene-for-gene type interactions predicts that generalist strains (in terms of the r ange of strains the predator can exploit or the prey can fend off) dom inate in productive areas of the prey, whereas specialists prevail in marginal habitats. Assuming a wide range of productivities over the pr ey's geographical distribution, the greatest strain diversity should b e found in habitats of intermediate productivity. We discuss the impli cations of our study for adaptation and conservation. Empirical studie s are in broad accord with our findings.