Impacts of seed limitation, rodent seed predation, and disturbance on native and exotic seedling recruitment: Are there general patterns?

Background/Question/Methods

    Local plant community assembly and community structure is influenced by a series of filters that constrain recruitment and establishment of species.  These filters can include the ability of seeds of any given species to reach a local site (which influences the degree of recruitment limitation) and local interactions such as competition and post-dispersal seed predation.  Although these processes have been studied in isolation, their joint effects have seldom been examined.  Furthermore, although intrinsic properties of species, such as seed size and evolutionary origin (native vs. exotic), can influence the magnitude of recruitment, how these properties interact with disturbance and seed predation to influence overall recruitment into local assemblages has rarely been explored.  In a field seed addition experiment, we examined the individual and joint effects of dispersal limitation, disturbance (which removed existing competitors) and post-dispersal seed predation on the recruitment of 20 native and 19 exotic species across ten grassland sites in western Montana.  Added species varied widely in their respective seed sizes. 

Results/Conclusions

    We found that the diversity of local assemblages was strongly recruitment limited; seed addition enhanced local diversity.  Once recruitment limitation was overcome by seed addition, both disturbance and exclusion of seed predators had a substantial positive influence on the number of seedlings (across all species) that recruited into plots.  Moreover, exclusion of seed predators enhanced recruitment to a greater extent: 1) in disturbed than undisturbed plots and 2) for native species compared to exotics.  Seed size mediated these patterns.  For example, the positive effect of disturbance on recruitment was generally greater for species with small versus large seeds.  In contrast, the positive effect of rodent exclusion on recruitment was greater for large versus small seeded species, particularly for natives compared to exotics.  These results show that regional and local “filters” jointly influence the magnitude of seedling recruitment, the relative abundance of recruiting species and ultimately the diversity of local assemblages.  They further highlight how intrinsic properties of species, such as their seed size and evolutionary origin, can help predict which groups of species are most influenced by local filters.