Complex ecosystems lose stability when resource consumption is out of niche

Abstract

Natural communities exhibit diverse dynamics, encompassing global stability, multi-stability, periodic oscillations, and chaotic fluctuations in species abundances. Resource-consumer interactions provide a lens for mechanistic understanding of community behaviors, yet only a globally stable equilibrium can exist in the original MacArthur resource-consumer model. Here we find that diverse dynamics emerges when species consume resources that contribute little to their own growth. Key to understanding these results is comparison of niche range (difference between growth-promoting resources of similar species) and consumption range (difference between growth-promoting resources and the resources that are actually consumed). If the consumption range is small, we observe global stability as in the MacArthur model. But when consumption range increases to about the niche range, stability is lost, giving rise to emergent alternative stable states, globally stable states but with species extinction, and slightly later, persistent fluctuations. Given its importance in dictating stability, we define the ratio between consumption and niche ranges as encroachment, and find that it predicts key community properties like diversity and attractor basins even after the instability transition. In particular, after the loss of stability (encroachment greater than one), species extinction increases approximately linearly with encroachment. Since encroachment relies only on intrinsic species properties, community stability is resilient to environmental changes such as resource supply and mortality rate. Encroachment provides a framework for capturing resource competition due to growth-consumption inconsistency in complex communities as well as a robust quantitative characterization of the resulting emergent dynamics.