How the equilibrium assumption is addressed in empirical tests of major ecological theories

The idea that natural systems tend to be at equilibrium dates back to the origin of the field of ecology and continues to underlie most ecological theory. However, empirical evidence for equilibrium dynamics in nature and in experiments is surprisingly elusive. Here, we address this conundrum by first exploring the history of equilibrium in ecological theory and the evidence for equilibrium dynamics in natural systems. We then search the literature to quantify how empiricists deal with equilibrium in their research, and we address barriers to integrating equilibrium into empirical work by providing step-by-step instructions for determining whether a population is at equilibrium. Next, we lay out three ways that equilibrium is embedded in theory, and for each, outline when meeting the equilibrium assumption in empirical tests is critical for scientific inference, and when it is possible to relax this assumption. And finally, we present concrete steps that empiricists and theoreticians can each take in order to meet in the middle when it comes to equilibrium. We hope that this paper will stimulate new discussions from researchers from across the theory-empirical divide about this longstanding issue.

Methods
This dataset is the outcome of a Web of Science search on July 24th, 2024 of the top cited articles that test five major equilibrium-based ecological theories: Modern Coexistence Theory, Tilman’s resource ratio (R* theory), the Metabolic Theory of Ecology, Metapopulation Theory, and Neutral theory. We developed search strings iteratively, adding in additional terms highly specific to those theories (e.g., "stabilizing niche differences" for Modern Coexistence Theory) in order to capture a substantial portion of the literature. Abstracts were screened to verify that the papers involved empirical tests of the theory’s predictions, and we retained 10 papers for each theory, for a total of 50 papers.