Variation in grassland restoration outcomes: Species pools and dispersal mediate local stochasticity

Christopher P. Catano, Department of Plant Biology, Michigan State University, East Lansing, MI and Lars Brudvig, Program in Ecology, Evolutionary Biology and Behavior, Michigan State University, East Lansing, MI; Plant Biology, Michigan State University, East Lansing, MI

Background/Question/Methods
Understanding why community assembly outcomes vary from place to place is central to biodiversity theories and ecosystem restoration. Variation in community assembly may depend on how regional factors — such as species pool size and dispersal rates — modify local stochastic and deterministic processes. For example, low dispersal rates can increase demographic stochasticity when fewer individuals establish in local communities, thereby increasing variation in species composition across communities (β-diversity). Larger species pools (those with more species) can also increase variation, albeit through alternative processes like enhancing species sorting along environmental gradients or through sampling effects caused by more possible community permutations. However, it remains unclear how species pool size and dispersal rates interact to cause variation in community assembly because experiments manipulating both factors simultaneously are rare. We established 12 grassland sites spanning a soil resource gradient, where we factorially manipulated species pool size and dispersal rates (seed arrival) within each site. We then asked how these treatments alter plant species β-diversity, local (α) diversity, and the strength of species sorting across a soil resource gradient.
Results/Conclusions
We found that dispersal rate and species pool size interacted to alter variation in plant species composition. Lower dispersal rates and larger species pools caused higher β-diversity and lower α-diversity. Moreover, the effects of both species pool size and dispersal rate appear mostly due to fewer individuals arriving per species. Fewer individuals arriving can increase chance recruitment and/or demographic stochasticity due to small population sizes. Consistent with this interpretation, we found that lower dispersal rates weakened the strength of species sorting along the soil resource gradient regardless of species pool size. Our results suggest regional factors that limit the number of individuals colonizing communities increases the importance of stochastic assembly processes, thereby increasing variability in assembly outcomes. Our findings have the potential to align predictive restoration ecology with contemporary community ecology theory by considering how local, regional, and stochastic forces co-structure assembling communities undergoing restoration.