Island biogeography, competition, and abiotic filtering together control species richness in habitat islands formed by nurse tree canopies in an arid environment.

The theory of island biogeography predicts that island size is a key predictor of community species richness. Islands can include any habitat surrounded environments that are inhospitable to the resident species. In arid environments, nurse trees act as islands in an environment uninhabitable to many plant species, and the size of the canopy controls the size of the understory plant community. We predicted that plant species richness will be affected by the area of the habitat and decrease with habitat isolation. We sampled the adult and seedling plant communities at canopy center, canopy edge, and outside canopy microhabitats. We found that species richness in both adult and seedling communities increases with increasing island area. However, richness in seedling communities was greater than in adult communities, and this effect was greatest at the canopy center microhabitat. Competition has been demonstrated to be more important in controlling species distributions near the canopy center, and stress is more important near the canopy edge. Thus, our results suggest that neutral forces, biotic interactions, and abiotic filtering act together to control species richness in these island communities.