Abstract: Life history, the schedule of when and how fast organisms grow, die and reproduce, is a critical axis along which species differ from each other. In parallel, competition for resources determines the potential coexistence of multiple species. While there are a number of studies integrating deterministic competition and coexistence theory with life history, there are relatively few that treat competition as a stochastic process, and even fewer general, analytical results combining life history and stochastic competition. Thus, a missing knowledge gap in ecological theory is a fully-developed program to understand the interaction of life history and stochastic competition. This project addresses that conceptual gap by introducing a new theoretical model, based on stochastic differential equations, to predict what happens when stochastic competition meets life history variation. Our first result is in the limit that equalizing mechanisms govern coexistence. In the absence of life history differences, neutral models have shown that many species can persist for long timescales on a single resource when their fitnesses are equal. We’ll show that that outcome changes when species with different life history strategies compete in a stochastic model of community assembly. Even when those species have equal fitness, different life history strategies can dramatically reduce the timescales over which these species persist. On the other hand, we prove that certain combinations of life history strategy are complementary, meaning that pairs of species with such complementary life history strategies are more likely to coexist than those without. We test this quantitative prediction in empirical data for herbaceous perennials, showing that pairs of co-occurring species are more likely (compared to a null expectation) to have complementary life history strategies, as predicted by the theory. We will subsequently generalize these results for the case that pairs of species can differ in all three of niche, fitness, and life history, and show that up to specific thresholds in niche and fitness differences, life history differences qualitatively matter in determining when species will coexist.
