Nonlinear thinking in ecology and evolution: applying the threshold elemental ratio across levels of ecological organization.

Nonlinear dynamics govern ecological processes; thus, understanding thresholds is important for measuring and forecasting the effects of climate change and management of natural resources. However, identifying whether and how such thresholds transfer across ecological levels of organization remains challenging. We argue for a broadening of a foundational organismal concept from ecological stoichiometry theory, the threshold elemental ratio (TER), to study how nonlinear dynamics driven by shifts in limitation operate in evolutionary and ecological processes from organisms to ecosystems. Traditionally, TERs are used to describe the elemental ratio at which the limitation of organismal growth shifts from one element to another. Building on this definition, we make a case for broadening the TER beyond organisms to include populations, clades, communities, and ecosystems. We discuss how TERs may be detected and translated across different ecological levels and evolutionary processes through simulation modeling, literature review, and synthesis of empirical examples from diverse systems and scales including: cyanotoxin production in lakes, alder-salmon dynamics, and the Cambrian explosion. Collectively, we argue that TERs are likely widespread and consequential across levels of ecological organization and that such thresholds manifest from a diversity of mechanisms. Thus, applying the TER concept across ecological levels of organization holds promise for advancing our understanding of nonlinear dynamics from the micro-evolutionary to the macro-ecological.