COS 5-8
Functional trait analysis indicates deterministic assembly in experimental tallgrass prairies
Elucidating patterns and drivers of community assembly has long been a focus of ecologists. Since functional traits characterize, even if loosely, the ecological niches of species, utilizing them in assessing community assembly can prove insightful. The overall goal of this study was to determine how functional traits have changed over time in a long-term community assembly experiment in eastern Kansas to infer patterns and processes of community assembly. This experiment has the strengths of extensive replication, community states manipulated to a high degree of initial variation, and the unique ability to accurately track these communities changing over time. Abundance data from this experiment were combined with trait data from the TRY Database, and functional diversity was quantified by the functional dispersion (FDis) index, which measures the average distance between each species and the centroid of all species of a community in multidimensional trait space, indicating both the functional diversity and variation of the community. A significant decrease in mean FDis over time indicates functional convergence of the communities, a deterministic result likely due to environmental filtering; a significant increase in mean FDis over time suggests deterministic assembly caused by competitive interactions, favoring functionally diverse communities. No change indicates stochastic assembly.
Results/Conclusions
Results of this analysis show a significant increase in mean FDis over the six-year course of the experiment: the overall increase is a significant one, and from year to year Fdis never decreases, but only stays the same or increases. This pattern indicates that over time, the constituent species of a community are occupying a greater volume of trait space. That is, since FDis is weighted by species abundance, assembly is resulting in fewer individuals of species that possess similar function and more individuals of species with diverse function. This non-random pattern is likely produced by species-species interactions, in line with the principle of competitive exclusion. While this result does not exclude stochastic processes from influencing community assembly patterns (and in fact, stochastic and deterministic processes quite likely co-occur), it does provide strong evidence for the existence of deterministic processes in shaping communities over time.