Beta diversity among prairie restorations increases with species pool size, but not through enhanced species sorting

1. Understanding variation in community composition across space, or beta diversity, is of long-standing interest in ecology, yet the determinants of beta diversity remain poorly known. In part, this results from a lack of manipulative tests of hypothesized drivers. The size of species pools is one putative driver, but few studies have provided a direct test of this mechanism through manipulation of clearly defined species pools independent of local communities. Furthermore, we know little about underlying mechanisms, such as enhanced species sorting, or whether a species pool size-beta diversity relationship is scale-dependent or modified by environmental conditions. 2. Here, we evaluate 29 prairie plant communities restored from bare soil with known species pools (seed mixes) to address those questions. To address the generality of beta diversity drivers across scales, we investigated how the size of species pools during restoration influenced beta diversity in the plant community at two scales: among prairies and within prairies (among plots). 3. Among a group of prairies sown with larger species pools, among-site beta diversity was greater than among a group of prairies assembled from smaller pools, but not because of enhanced species sorting. We found an interaction between species pool size and an environmental filter, whereby beta diversity was higher among prairies restored with species-rich seed mixes, but only when soil moisture was also high. We detected neither greater beta diversity nor stronger species sorting among plots within prairies sown with species-rich mixes. 4. Synthesis. This work provides what is to our knowledge the first large-scale manipulative test of how species pool size influences beta diversity. We found higher beta diversity among restored prairies sown with species-rich seed mixes, but little evidence for species sorting as a causal mechanism. Our results, based on manipulated real-world communities, provide an important link between previous theoretical and observational studies and small-scale experimental approaches. Of applied importance, our findings show that by creating communities of high beta diversity, ecological restoration can counteract widespread anthropogenic biotic homogenization.