A key challenge at the interface of ecology and conservation is to predict how and why biodiversity responds to environmental change. In many ecosystems, biodiversity often shows divergent responses to similar types of disturbance. However, the extent to which these divergent responses reflect stochastic outcomes of community assembly due to ecological drift, deterministic outcomes of community assembly due to niche selection, or interactive effects of dispersal and disturbance history remains unclear. In this study, we experimentally tested how dispersal interacts with fire and predation to influence the assembly of woody plant communities in a temperate oak-hickory forest in the Missouri Ozarks. In a factorial, landscape-scale field experiment, we manipulated fire disturbance in three large (2.5-ha) sites, dispersal by adding seeds of 15 tree species to plots within paired burned and unburned sites, and predation (seed predation and herbivory) using mammal exclosures. In each of 300 plots, we censused the abundance and composition of woody plant species over two years following prescribed burns to test the hypotheses that 1) dispersal increases local species diversity and decreases variation in community composition (beta-diversity), and 2) fire and predation mediate the effect of dispersal on local diversity and beta-diversity.
Results/Conclusions
Increasing dispersal through seed addition increased local plant species richness and diversity (P<0.0001). Moreover, the effects of dispersal on local diversity were mediated by both fire disturbance and predation. Dispersal increased local diversity (P<0.0001) and species richness (P<0.0001) more inside than outside of mammal exclosures (dispersal x exclosure interaction for diversity: P=0.0026; dispersal x exclosure interaction for richness: P=0.0061). Furthermore, fire increased compositional dissimilarity among communities (beta-diversity) (P<0.0001). Collectively, our results highlight the importance of considering interactive effects of dispersal limitation, fire disturbance, and predation on community assembly. In unburned landscapes, dispersal limitation limits community diversity. When species are released from dispersal limitation, seed predators and herbivores limit local diversity. However, fire may increase environmental harshness or heterogeneity and thereby override the effects of both dispersal and predation. Identifying the nature and strength of interactions among these assembly processes can advance understanding of restoration outcomes and conservation strategies in ecosystems altered by environmental change.