Coexistence of specialist and generalist plants in variable environments

Tradeoffs between the ability to grow well under optimal vs. suboptimal conditions are the basis of much of our understanding of species turnover along environmental gradients. Such tradeoffs may also be important for coexistence in heterogeneous environments. Species that tradeoff maximum population growth rate in an optimal environment for increased growth in suboptimal environments (i.e. habitat generalists) experience less environmental variation than those that do not (i.e. habitat specialists). Coexistence of these strategies is possible because each grows best relative to the other in a certain area of the environment. In terms of mutual invasion, generalists are able to invade specialist residents by accumulating in suboptimal sites and specialists are able to invade generalist residents by being better competitors in optimal sites. Using a model of perennial plants in a variable environment, I find coexistence of these strategies depends on the scale of spatial and temporal variation in the environment traits of the species. Coexistence is strongest where the variation in the environment is correlated over large spatial and temporal scales and the habitat specialist experiences long-range within-species competition. This combination of traits and environment allow the specialist to best concentrate its population in the optimal sites and is effective at promoting coexistence whether the environmental variation affects germination, fecundity or mortality.