In order to assess how diversity changes over time at sites undergoing environmental
change, we examined three data sets on long-term trends in taxonomic richness and
composition: (1) 22 years of rodent censuses from a site in the Chihuahuan Desert
of Arizona; (2) 50 years of bird surveys from a three-county region of northern Michigan;
and (3) approximately 10,000 years of pollen records from two sites in Europe. In
all three cases, richness has remained remarkably constant despite large changes in
composition. The results suggest that while species composition may be highly variable
and change substantially in response to environmental change, species diversity is
an emergent property of ecosystems that is often maintained within narrow limits.
Such regulation of diversity requires maintenance of relatively constant levels of
productivity and resource availability and an open system with opportunity for compensatory
colonizations and extinctions. In addition to studying the effects of diversity on
biogeochemical processes, it will often be useful to think of species richness as
an emergent consequence of ecosystem processes.