Quantifying the relative importance of competition, predation, and environmental variation for species coexistence

1Coexistence theory and food web theory are two cornerstones of the longstanding effort to understand how species coexist. Although competition and predation are known to act simultaneously in communities, theory and empirical study of the two processes continue to be developed independently. Here, we integrate modern coexistence theory and food web theory to simultaneously quantify the relative importance of predation, competition, and environmental fluctuations for species coexistence. We first examine coexistence in a classic multi-trophic model, adding complexity to the food web using a novel machine learning approach. We then apply our framework to a parameterized rocky intertidal food web model, partitioning empirical coexistence dynamics. We find that both environmental fluctuation and variation in predation contribute substantially to species coexistence. Unexpectedly, covariation in these two forces tends to destabilize coexistence, leading to new insights about the role of bottom-up versus top-down forces in both theory and the rocky intertidal ecosystem.