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.

We analysed a 50-year dataset of avian species observations to determine how richness and community composition varied over a period of landscape-scale environmental change. Our study area, northern lower Michigan, has experienced substantial land-use and land-cover change over time. Like much of the northern Midwest, it has shifted from a largely unpopulated, post-logging shrubland to a moderately populated closed-canopy forest. Such changes are generally expected to influence overall richness and community composition. We found that regional richness per year remained virtually unchanged over the study period. Yearto-year variation in species number was surprisingly low. Richness totals included vastly different species groups as the composition of the regional bird community changed substantially over time.Changes in the types of species present appear to reflect deterministic changes in habitat. The number of grassland and open-habitat species decreased, for example, while species associated with older forests and urban habitats increased. Our results suggest that habitat changes at the landscape scale do not necessarily lead to changes in the number of species a region can support. Such changes, however, do appear to influence the types of species that will occupy a region, and can lead to substantial changes in community composition.

Abstract. Local habitat characteristics are often used to describe a species’ niche despite the fact that habitat use can vary across the geographical range. We sought to quantify variation in habitat preferences by asking how nesting habit varies within and between populations of Bell’s vireo (Vireo bellii) across its geographical range. Analyses of detailed nest placement data from three localities (Kansas, eastern New Mexico and western New Mexico) showed a general trend towards use of dense vegetation. However, there was substantial variation in nest placement and vegetation at nest sites between localities. Furthermore, a review of nest placement data from the literature shows strong differences in nest heights and species of trees even between populations less than 100 km apart. We evaluate these results in light of conservation and suggest that to be most effective, habitat conservation plans should be based on data collected at the locality where the population of interest occurs.

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