It has long been recognized that consumer species often evolve in response to competition for some or all of their resources by another consumer species. Divergence in the relative use of different resources is the best known response. However, convergence and parallel change in relative use are also known to occur, as are changes in the absolute rate of resource intake. We know less about how these adaptive evolutionary shifts change the population densities of the competitors and resources. Simple mathematical models of competition between two consumer species for two resource types are used to explore the likely changes in population densities that result from adding a second consumer species to a system with one resident consumer species. The models assume that there exists a trade-off in the resident's attack rates on its two resources; the trait that determines these two rates changes in a manner that increases individual fitness. The models also assume that the invading competitor only uses one of those resources.
Results/Conclusions
One of the most surprising aspects of adaptive evolutionary change in relative resource use by the resident generalist is that it often produces a decline in the resident's density. This decline is frequently much greater than the initial decline caused by the invader's consumption of resources. For resources that are nutritionally substitutable, the resident always declines in population size as it diverges from its competitor, provided that the trait does not reach its maximum or minimum value. Another consequence of competition is that the resource used by the invading competitor can ultimately increase in population density as a consequence of the invasion. When resources are nutritionally essential, evolution may change the impact of the invader on the resident consumer from negative to positive; i.e., the resident increases about its pre-invasion density as a consequence of the 'competition'. Consumer population regulation and trade-off shape are shown to have several other important effects on the population-level consequences of evolutionary shifts in response to competition.