UC Davis

Against the backdrop of resource competition, ecological coexistence appears mysterious: if species must specialize in order to exist, how can dozen of similar species inhabit the same environment? Many explanations have been put forward, but efforts have remained primarily theoretical, with simple models demonstrating coexistence via simple mechanisms. Therein lies the problem --- explanations are simple by design, but nature is complex. Here, we improve upon Modern Coexistence Theory, a mathematical framework that can be used to measure the relative importance of different explanations for coexistence. Innovations range from the methodological to the conceptual.

Additionally, we attempt to understand the causes of unexpected population crashes, also known as catastrophes or ecological "black swan" events. Many such crashes do not fit the common explanation of tipping points resulting from changing environmental conditions. We provide an alternative and general mechanism for population crashes in a static environment, namely stochasticity (of any variety) combined with multiple episodes of density dependence. This explanation is confirmed in experimental microcosms of the red flour beetle, and connections to real-world population crashes are discussed.