Regulation of diversity: maintenance of species richness in changing environments

sites undergoing environmental change, we examined three data sets on long- term trends in taxonomic richness and composition: (1) 22 years of rodent c ensuses from a site in the Chihuahuan Desert of Arizona; (2) 50 years of bi rd surveys from a three-county region of northern Michigan; and (3) approxi mately 10,000 years of pollen records from two sites in Europe. In all thre e cases, richness has remained remarkably constant despite large changes in composition. The results suggest that while species composition may be hig hly variable and change substantially in response to environmental change, species diversity is an emergent property of ecosystems that is often maint ained within narrow limits. Such regulation of diversity requires maintenan ce of relatively constant levels of productivity and resource availability and an open system with opportunity for compensatory colonizations and exti nctions. In addition to studying the effects of diversity on biogeochemical processes, it will often be useful to think of species richness as an emer gent consequence of ecosystem processes.