Department of Biology, Washington University in St. Louis Eureka, MO, United States
Background/Question/Methods
Foundation species are abundant species that have strong effects on communities and ecosystems. While important for maintaining species diversity at regional scales, it is unclear to what extent foundation species affect the major processes of community assembly: selection, dispersal, and ecological drift. We explored how a candidate foundation species in forest understories, Asimina triloba (Pawpaw), influences the relative importance of selection and drift in plant community assembly. We tested two hypotheses. First, the selective-sweep hypothesis posits that foundation species are superior competitors, resulting in lower species richness (alpha diversity), lower variation in species composition (beta diversity), and non-random patterns of species composition in sites dominated by foundation species.
Second, the ecological-drift hypothesis posits that foundation species increase the importance of demographic stochasticity, resulting in lower alpha diversity, higher beta-diversity, and more random patterns of species composition. We tested these hypotheses by measuring the composition and diversity of herbaceous and woody plant species in 50 quadrats (1 x 1 m) stratified among 5 blocks in a temperate forest-dynamics plot in the Missouri Ozarks. Each block contained 5 quadrats located inside a large patch of A. triloba and 5 plots in a paired control plot without A. triloba.
Results/Conclusions
Local species richness and species composition differed significantly between A. triloba patches and control plots. The control plots contained 41 more morpho-species than the A. triloba patches, and the inverse Simpsons scores were significantly different in a mixed effects model P < 0.0001). Beta-diversity among A. triloba patches and control plots was not statistically different. However, overall species composition differed significantly between A. triloba patches and control plots (ANOSIM: P < 0.0001). These preliminary findings suggest that A. triloba influences community assembly. At local scales, reduced species richness within patches dominated by A. triloba may reflect selection for species that can tolerate competition for aboveground and/or belowground resources, random local extinctions due to small population sizes under resource-limited conditions, or a combination of these deterministic and stochastic processes. Our study illustrates ways in which the presence of foundation species may determine spatial variation in biodiversity by affecting community assembly processes.
