Gardner volumes and self-organization in a minimal model of complex ecosystems

TL;DR

This paper investigates how large, complex ecosystems self-organize over time using a minimal nonlinear model, revealing that the system's dynamics are constrained by Gardner volumes and linked to stability thresholds akin to May's criteria.

Contribution

It introduces a novel minimal model connecting ecosystem self-organization with Gardner volumes and random matrix theory, providing insights into stability and extinction dynamics.

Findings

Solutions are confined to time-varying Gardner volumes.

Diversity decreases exponentially over time.

Spectrum contractions determine extinction or unbounded growth.

Abstract

We study self-organization in a minimally nonlinear model of large random ecosystems. Populations evolve over time according to a piecewise linear system of ordinary differential equations subject to a non-negativity constraint resulting in discrete time extinction and revival events. The dynamics are generated by a random elliptic community matrix with tunable correlation strength. We show that, independent of the correlation strength, solutions of the system are confined to subsets of the phase space that can be cast as time-varying Gardner volumes from the theory of learning in neural networks. These volumes decrease with the diversity (i.e. the fraction of extant species) and become exponentially small in the long-time limit. Using standard results from random matrix theory, the changing diversity is then linked to a sequence of contractions and expansions in the spectrum of the community matrix over time, resulting in a sequence of May-type stability problems determining whether the total population evolves toward complete extinction or unbounded growth. In the case of unbounded growth, we show the model allows for a particularly simple nonlinear extension in which the solutions instead evolve towards a new attractor.