Limits to species richness in terrestrial communities

Are communities limited by biotic interactions, or are they random draws from regional species pools? One way to tell is to compare total species counts in geographic regions to average counts in ecological samples falling within those regions. If species richness is limited regionally, then the relationship should be curvilinear even in a log-log space. Global sets of samples including trees and 10 groups of animals are analysed to test this hypothesis. Most relationships are indeed curvilinear. To explain these patterns, a simple model is proposed that invokes biotic interaction-limited speciation or immigration rates combined with extinction or extirpation rates that fall as the number of occupied patches increases. Local and regional richness come into balance as the rates trade off, causing global richness to also be limited. Surprisingly, however, the data for trees break the pattern, suggesting that the great adaptive radiation of seed plants may still be unfolding.