Complex interactions can create persistent fluctuations in high-diversity ecosystems

Abstract

When can ecological interactions drive an entire ecosystem into a persistent non-equilibrium state, where many species populations fluctuate without going to extinction? We show that high-diversity spatially heterogeneous systems can exhibit chaotic dynamics which persist for extremely long times. We develop a theoretical framework, based on dynamical meanfield theory, to quantify the conditions under which these fluctuating states exist, and predict their properties. We uncover parallels with the persistence of externally-perturbed ecosystems, such as the role of perturbation strength, synchrony and correlation time. But uniquely to endogenous fluctuations, these properties arise from the species dynamics themselves, creating feedback loops between perturbation and response. A key result is that fluctuation amplitude and species diversity are tightly linked: in particular, fluctuations enable dramatically more species to coexist than at equilibrium in the very same system. Our findings highlight crucial differences between well-mixed and spatially-extended systems, with implications for experiments and their ability to reproduce natural dynamics. They shed light on the maintenance of biodiversity, and the strength and synchrony of fluctuations observed in natural systems.

Emergent phases of ecological diversity and dynamics mapped in microcosms

“…S1). Our results suggest that persistent fluctuations and high diversity require and allow each other, as theoretically shown in previous work (32,33).…”

“…May and others have suggested that large or strongly interacting communities will generically be unstable (20)(21)(22)(23)(24)(25)(26)), yet we still do not understand how such large communities may arise, nor the dynamical behavior of communities that are not stable. In particular, it has been shown that species can go extinct before the community loses stability (27)(28)(29)(30), and also that unstable communities can display a range of dynamics including periodic (limit cycle) oscillations or chaotic fluctuations, that in some cases play a role in sustaining diversity (31)(32)(33)(34)(35)(36)(37). This body of theory has however been difficult to validate because driving parameters such as ecological interactions are often challenging to estimate (38,39).…”

Biodiversity as insurance: from concept to measurement and application

“…On the other hand, non-equilibrium coexistence theory suggests that fluctuations can also enhance the coexistence of interacting species (Barab as, D'Andrea & Stump, 2018). Thus, while May (1972) envisioned the instability of complex communities as a limitation, Roy et al (2020) recently showed that the selfsustaining fluctuations of such communities can also enable more species to coexist. With arguments paralleling biological insurance theory, they identified conditions under which population responses become asynchronous and differentiated, leading to more species persisting in a chaotic state than at equilibrium.…”

Community composition exceeds area as a predictor of long-term conservation value

“…The negative association between temporal turnover and long-term conservation value likely arises due to the fact that turnover in the model is purely autonomous, driven by ecological interactions and dispersal (O'Sullivan et al, 2021) as opposed to external processes such as abiotic fluctuations or regional-scale invasions. Autonomous turnover requires ecological redundancy as well as spatial connectivity to function; without a pool of species in adjacent sites capable of passing through local abiotic filters, communities converge on fixed points in the absence of external driving forces (Roy et al, 2020;Pearce et al, 2020;O'Sullivan et al, 2021). Those sites undergoing rapid autonomous turnover have least impact on regional diversity since the ecological redundancy required for steady state dynamics means the metacommunity can readily respond to their loss.…”