When can few-species models describe dynamics within a complex community?

TL;DR

This paper investigates when simple models with few species can accurately describe the dynamics within complex ecological communities, providing methods to approximate focal species' behavior despite community complexity.

Contribution

It introduces two novel methods for approximating focal species dynamics within diverse communities, even when community effects are significant.

Findings

Effective models often closely match true dynamics

Focal species dynamics can be approximated despite complex interactions

Methods work well under various community interaction scenarios

Abstract

Dynamics of species' abundances in ecological communities are often described using models that only account for a few species. It is not clear when and why this would be possible, as most species form part of diverse ecological communities, with many species that are not included in these few-variable descriptions. We study theoretically the circumstances under which the use of such models is justified, by considering the dynamics of a small set of focal species embedded within a diverse, sparsely-interacting community. We find that in some cases the focal species' dynamics are high-dimensional, making a few-variable description impossible. In other cases we show that such a description exists, even though the effect of the surrounding community on the focal species' dynamics is not small or simple. We give two different methods for approximating the dynamics, by using effective parameters that depend on the surrounding community, which are relevant under different assumptions on the relation between the explicitly modeled focal species and the rest of the species. Both methods work surprisingly well in many of the cases that we check, with effective dynamics that are often very similar and sometimes indistinguishable from the true dynamics, even when the effect of the community on the focal species is significant.