Testing for local species saturation with nonindependent regional species pools

Identifying the factors controlling local community structure is a central problem in ecology. Ecologists frequently use regression to test fur a nonl inear saturating relationship between local community richness and regional species pool richness, suggesting that species interactions limit the numb er of locally coexisting species. However, communities in different regions are not independent if regions share species. We present a Monte Carlo tes t for whether an observed local-regional richness relationship is significa ntly different from that expected when regions are nonindependent and speci es interactions do not limit community membership. We illustrate this test with data from experimental microcosm communities. A conventional F-test su ggests a significant saturating relationship between realized community ric hness and species pool richness. However, the Monte Carlo test fails to rej ect the null hypothesis that species interactions do not affect community r ichness. Strong species interactions do not necessarily set an absolute upp er limit to the number of locally coexisting species.