Zhijie Feng: Emergent competition shapes the ecological properties of multi-trophic ecosystems

Ecosystems are commonly organized into trophic levels -- organisms that occupy the same level in a food chain (e.g., plants, herbivores, carnivores). A fundamental question in theoretical ecology is how the interplay between trophic structure, diversity, and competition shapes the properties of ecosystems. To address this problem, we analyze a generalized Consumer Resource Model with three trophic levels using the zero-temperature cavity method and numerical simulations. We find that intra-trophic diversity gives rise to ``emergent competition'' between species within a trophic level due to feedbacks mediated by other trophic levels. This emergent competition gives rise to a crossover from a regime of top-down control (populations are limited by predators) to a regime of bottom-up control (populations are limited by primary producers) and is captured by a simple order parameter related to the ratio of surviving species in different trophic levels. We show that our theoretical results agree with empirical observations, suggesting that the theoretical approach outlined here can be used to understand complex ecosystems with multiple trophic levels. In the end of the talk, we will discuss our future direction of studying eco-evolutionary dynamics of invasions, where new species are repetitively introduced to ecosystems described by generalized Consumer Resource Models. With new insights from order statistics and extreme value theory, and our knowledge of the emergent behavior of ecosystems, we plan to study the asymptotic behaviors of these dynamics.