Temporal variation and coexistence: Ecological but not evolutionary coexistence of generalists and specialists in a two-season world

Background/Question/Methods

            Environmental variation has long been one of the favorite explanations to describe why so many competing species manage to coexist. In 1980, Armstrong and McGehee showed that 2 species can persist on a single biotic resource, but it requires key differences between the species and essentially a tradeoff between performance at high and low resource densities. These investigations all concern endogenously generated variation, but often the environmental variation is exogenously forced, either by other species beyond the scope of the model or by climate or weather.

            We consider the conditions for coexistence between 2 or more species experiencing an externally forced, two-season environment. Starting with the simplest of systems, non-interacting, linear growth functions and building up to a full resource competition, type II functional response, we look at how species that differ along a single trait axis coexist both ecologically and evolutionarily using Successional State Dynamics and Adaptive Dynamics modeling techniques.

Results/Conclusions

            Unlike Armstrong and McGehee, we found that even in the linear case, species competing for a single resource could coexist through seasonal specialization. In this case, the two seasons are essentially separate resources or niches, and so their coexistence can be said to conform to the competitive exclusion principle. With type II functional responses, where the differentiation between species comes in makes all the difference; if placed in the linear portion of the growth equations, i.e. maximum growth rate, either a pair of specialists or a single generalist will dominate, across parameter space. When the difference in environmental responses is manifested in the carrying capacity, the non-linear part of the growth equations, 3 species can coexist, a generalist and 2 specialists, with the generalist taking advantage of the beginning of each season before the specialist dominates. These triplets, however, are not evolutionarily stable and evolve toward a slightly less specialized specialist pair. This disparity between the mechanism of differential environmental responses provides important insights into the role of variation in maintaining species diversity. The sensitivity of coexistence to evolutionary change highlights the need to consider the timescale on which we are interested in understanding coexistence.