Top-down control of rare species abundances by native ungulates in a grassland restoration

Ecological restorations are predicted to increase in species diversity over time until they reach reference levels. However, chronosequence studies in grasslands often show that diversity peaks after the first few years and then declines over time as grasses become more dominant. We addressed whether bison grazing and seed additions could prevent this decline in diversity. Exclosures that prevented bison grazing were compared with grazed plots over 4 years, and seed additions were conducted inside and outside exclosures to test for seed and microsite limitations. A previous study conducted 4-months post seeding found that local species richness was primarily seed limited, but that grazing could sometimes increase seedling emergence. Here, we tested whether increased seedling emergence led to longer-term increases in the species diversity of the plant community. We found that the seed addition effect grew smaller and the grazing effect grew stronger over time, and that seed additions affected the abundance of added species only when plots were grazed. Grazed plots had higher species diversity and lower biomass and litter buildup compared to non-grazed plots. Our results suggest that moderate grazing by bison or management that mimics grazing can maintain diversity in grass-dominated situations. Our results also emphasize the need to follow seed additions over several years to assess correctly whether seed limitation exists.