Can we predict the evolution of differences among species that allow for their coexistence?

Background/Question/Methods

There are many theories that posit how differences among species may explain their coexistence. For example, fast growers coexist with good survivors through succession, and species that have specialist pathogens may coexist through Janzen Connell mechanisms. However, there is little theoretical work that explores how the differences among species necessary for their coexistence arise and are maintained in the face of evolution, immigration, and even individual plasticity. For a given tradeoff that allows ecologically stable coexistence of two species at its extremes, it is often the case that a single intermediate strategy can out-compete both extreme strategies. This then raises the question as to how species coexistence is maintained when the arrival of intermediate strategies is always possible.We study the possibilities of evolutionarily and ecologically stable coexistence arising only from simple tradeoffs, and, where perfect stable coexistence is impossible, study the implications of the imperfections of ecological and evolutionary optimization on potential coexistence. We use simple representations of shade-tolerance tradeoffs and pathogen transmission to establish ecologically stable coexistence. We use adaptive dynamics analyses to investigate the potential for evolutionarily stable coexistence and simulations to test the importance of ecological realism on predicted coexistence.

Results/Conclusions

We have found that, although ecologically stable coexistence can arise from some simple and physiologically realistic tradeoffs for plant species, this coexistence is not resistant to invasions by all intermediate strategies. We show, however, that the finite temporal, spatial, and evolutionary nature of ecology likely leads to the stable coexistence among strategies. Specifically, the shifting of environmental selective forces, the discrete nature of phenotypic change from evolutionary mechanisms, and finite population sizes of all strategies increase predicted coexistence through increasing the time necessary for the single competitively-dominant intermediate strategy to drive all others extinct. This work brings together and confirms many long-standing ecological theories. The study highlights the challenges in developing predictive models of species coexistence.