Small-scale species richness in forest canopy gaps: the role of niche limitation versus the size of the species pool

The form of the relationship between local species richness and the number of species in the surrounding region can be used as a test between competing theories of community structure. For 32 canopy gaps in New Zealand Nothofagus forest, we examined the relationship between the number of vascular plant species in 0.2-m(2) quadrats within the gap and the species richness of the whole gap. We found no evidence that competition for a limited number of niches placed an upper limit on the number of locally co-occurring species. Rather, the mean number of species in quadrats within canopy gaps increased in direct proportion to gap species richness. This relationship held after we controlled for potentially confounding factors, including variation in forest, floor substrate, and gap size, age, shape and orientation. Our results suggest that even over relatively small spatial scales, local species richness can be constrained by the size of the species pool in the immediately surrounding region.