Establishment limitation reduces species recruitment and species richness as soil resources rise
M. Henry H. Stevens (tel. 513 529 4206; fax 513 529 4243; e-mail HStevens@muohio.edu).
- 1At local spatial scales, species richness tends to fall as productivity rises. Most explanations have focused on increased extinction, but, instead, we test experimentally whether increased soil fertility reduces recruitment. Specifically, we test whether variation in recruitment is due to source limitation, germination limitation or establishment limitation, and how litter accumulation and seed predation contribute to these processes.
- 2We established four crossed experimental treatments in a perennial-dominated early successional plant community over 3 years. We added seed of 30 species, manipulated access by selected seed predators, removed litter and added slow release fertilizer at four levels (0, 8, 16 and 32 g N m−2).
- 3Species recruitment and richness both decreased with increasing fertility, but, counter to our expectations, we found that neither seed additions nor litter removal could counteract the negative effects of fertilizer.
- 4Seed additions increased seedling density at all fertilizer levels, and seed predation appeared to have no influence on seedling densities. In spite of high seedling densities at all fertilizer levels, final stem density declined by 70% as fertilizer increased. A strong stem density–species richness relationship suggests that declines in final stem density caused more than half of the decline in species richness along this fertility gradient.
- 5These results suggest that establishment limitation, i.e. the reduction of growth and survival from seedling to adult, controls species recruitment in highly fertile sites.
- 6The high degree of recruitment limitation commonly observed in productive habitats suggests that high productivity causes establishment limitation, thereby isolating these communities from the regional species pool. We suggest that such isolation provides a mechanism to explain why the species composition of productive communities exhibits higher variability than the composition of less productive communities within the same regional source pool.