Plant traits, community assembly and coexistence

Background/Question/Methods A conceptual gap has arisen in community ecology between trait-based studies of community assembly and demographic theories of coexistence. Studies of coexistence initially focused on niche differentiation, primarily along axes of abiotic conditions, activity time (diurnal or seasonal), or resource availability (e.g. food size). Over the past several decades, a more diverse array of coexistence mechanisms have been proposed, including Janzen-Connell dynamics, colonization-competition tradeoffs, storage effects, and non-transitive competitive hierarchies. At the same time, comparative studies-particularly in plant ecology-highlight the distribution of species along dominant ecological strategy axes, defined by suites of correlated traits (e.g. seed size, specific leaf area, etc.). Many studies of community assembly have reported non-random distributions of ecological traits, including even spacing, high variance and platykurtosis. The current challenge, which we address in this talk, is to link species traits, and their distributions in communities, to potential mechanisms that will promote coexistence.

Results/Conclusions We present two approaches to this problem. The first treats ecological traits as proxies for species performance along traditional niche axes. The primary evidence for such relationships is correlations between species traits and distributions along environmental gradients. While such relationships exist, they are often weak; correlations between community level trait means and environmental conditions are invariably stronger than the corresponding relationships for individual species. The second approach views differentiation in species traits per se as a common property shared by most, if not all, theories of coexistence. We will review a range of coexistence mechanisms and how particular species traits may be associated with the required demographic properties sufficient to maintain coexistence. Further research is needed, in carefully chosen model systems, to directly link non-random trait distributions to mechanisms of coexistence, closing this gap in community ecology.