Why ecologists struggle to predict coexistence from functional traits

A promising use of functional traits is in the prediction of plant species coexistence. Two ideas underly this effort: species coexist when their use of the environment differs enough to overcome competitive disparities, and functional traits provide a proxy for species’ use of the environment.Despite successes in using plant functional traits to understand ecosystem function and responses to environmental change, ecologists have struggled to predict species coexistence with functional traits.Mechanistic models of resource competition, which explicitly connect coexistence outcomes to species traits, the environment, and resource dynamics, reveal that coexistence rarely depends on simple differentiation in functional traits, potentially explaining the challenge of connecting individual traits to coexistence. The rationale behind trait-based ecology is that shifting focus from species’ taxonomic names to their measurable characteristics (‘functional traits’) leads to greater generality and predictive power. This idea has been applied to one of ecology’s most intractable problems: the coexistence of competing species. But after 20 years, we lack clear evidence that functional traits effectively predict coexistence. Here, we present a theory-based argument for why this might be the case. Specifically, we argue that coexistence often depends on special quantities called ‘process-informed metrics’ (PIMs), which combine multiple traits and demographic characteristics in non-intuitive ways, obscuring any direct ties between individual traits and coexistence. We then lay a path forward for trait-based coexistence research that builds on mechanistic models of competition. The rationale behind trait-based ecology is that shifting focus from species’ taxonomic names to their measurable characteristics (‘functional traits’) leads to greater generality and predictive power. This idea has been applied to one of ecology’s most intractable problems: the coexistence of competing species. But after 20 years, we lack clear evidence that functional traits effectively predict coexistence. Here, we present a theory-based argument for why this might be the case. Specifically, we argue that coexistence often depends on special quantities called ‘process-informed metrics’ (PIMs), which combine multiple traits and demographic characteristics in non-intuitive ways, obscuring any direct ties between individual traits and coexistence. We then lay a path forward for trait-based coexiste