COS 189-2 - Evidence of positive plant-plant interactions and why they are important for understanding species coexistence

Abstract: Our understanding of species coexistence is heavily embedded in ecological history and the assumption that competition rules ecological systems. Facilitation and positive density (and frequency) dependence, though acknowledged as important ecological phenomena, are typically assumed to be of negligible importance in the study of species coexistence and diversity maintenance. Though this assumption is mathematically pragmatic and in line with competition-focused ecological theory, it does not match up with observations of the real world and may contribute to why we struggle to predict coexistence in diverse natural systems. In this talk I present results from a series of studies to answer the questions: how common are plant-plant facilitation and positive density/frequency in a natural plant community? and how consistently evident are these types of interactions across environmentally heterogeneous communities? Using data from an annual plant system in Western Australia, we show that positive density dependence and non-monotonic density dependence are just as common as negative density dependence among four common annual species. In the same system we use a novel interaction networking approach to show that facilitation represents approximately 25% of all interactions, including intra-specific interactions, under a range of environmental conditions. These results highlight the need to rethink how we study coexistence in diverse natural systems. I finish with a short discussion of alternative approaches to studying species coexistence and remaining gaps in our understanding of how to include positive interactions in the study of coexistence and diversity maintenance in natural plant communities.