Collaborative Research: Higher order interactions, functional trait changes, and species coexistence in an annual plant community

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Natural communities are often composed of dozens or hundreds of interacting species. However, for simplicity, the study of species interactions in ecology most often focuses on interactions between pairs of species. Ecologists often then assume that our understanding of interactions between species pairs can be used to make sense of the dynamics of entire communities. This core assumption is challenged by the potential for Higher Order Interactions (HOIs), which occur when the interaction between a pair of species is changed by the presence of a third species. HOIs can either make it easier or more difficult for species to coexist in a diverse community, depending on the strength and the nature of the effects. The possible existence of these interactions therefore fundamentally challenges our understanding of how biodiversity is maintained in natural communities. However, we know very little about how strong or widespread HOIs might be in nature. The research will quantify the strength and prevalence of HOIs among plant species in a grassland community and test if certain physical or physiological characteristics of plant species make HOIs more likely to occur. The project will address a fundamental challenge in community ecology, train a graduate student and postdoctoral researcher, and be used to improve courses offered at the high school, undergraduate and graduate level. The research will combine large-scale field experiments, detailed measurements of plant functional traits, and the application of mathematical theory to ask three questions: (1) How strong are Higher Order Interactions (HOIs) among competitors in nature? (2) How do HOIs affect opportunities for species coexistence? (3) Can ecologists predict the magnitude of these interactions in nature with knowledge of plant functional traits? The project will quantify the magnitude of nearly 150 three-way HOIs through a large field experiment in a California annual plant community subjecting focal species to simultaneously varying densities of two different competitor species. Models with HOIs will then be fit to the demographic data collected. To address the coexistence consequences of the fitted HOIs, the invasion condition for coexistence and species persistence times will be evaluated in parameterized models that either include or exclude these HOI terms. To assess the predictability of HOIs, the research will measure how plant functional traits plastically respond to the densities of other competitors, and this information will be used to predict the value of the fitted HOIs. Taken together, the research will allow ecologists to both quantify the importance of HOIs for species coexistence and identify their functional basis, laying the groundwork for further mechanistic studies of these interactions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.