Seed size and provenance mediate the joint effects of disturbance and seed predation on community assembly

Local plant community assembly is influenced by a series of filters that affect the recruitment and establishment of species. These filters include regional factors that limit seeds of any given species from reaching a local site as well as local interactions such as post-dispersal seed predation and disturbance, which dictate what species actually establish. How these filters interact to influence recruitment into local assemblages, and whether they act differentially on individual species based on traits such as seed size or their provenance (i.e. native vs. exotic), has not been well examined. Such studies, however, are crucial for understanding community assembly and for making predictions about what species might be favoured under specific ecological circumstances. We added 20 native and 19 exotic species that varied in seed size to undisturbed or experimentally disturbed subplots in and out of larger rodent exclusion plots at ten grassland sites across the Blackfoot River drainage in western Montana, USA. Individually, exclusion of rodent seed predators and disturbance substantially increased cumulative (summed across all species) seedling recruitment. Exclusion of rodent seed predators enhanced recruitment to a greater extent in disturbed rather than undisturbed plots and for native species compared with exotics, while disturbance enhanced recruitment to a greater extent for exotics compared with natives. Examination of individual species responses indicated that results were generalizable across species within each group and not driven by the response of a few species. Seed size mediated these patterns. Notably, the positive effect of rodent exclusion on recruitment was greater for large- versus small-seeded species, while the impact of disturbance on recruitment was more pronounced for small-seeded exotics relative to other groups. Synthesis. These results reveal that local filters such as post-dispersal seed predation and disturbance can individually and collectively impose strong limitation on seedling recruitment into local assemblages. Seed size importantly predicts how strongly individual species are influenced by these local filters. Interestingly, in situ community filters have differential effects on native versus exotic species, suggesting that processes that limit native recruitment may not have the same inhibitory influence on exotics.