Ecological Gradients in Diversity and Abundance: A Search for Patterns and Processes in Small Mammal Communities

Abstract

One of the most fundamental questions in ecology is: what are the patterns of diversity and the mechanisms that produce them? Many biological theories have been proposed to explain latitudinal and elevational diversity gradients, but no accepted, general explanation for the distribution of biodiversity has surfaced. Two necessities for establishing a general diversity theory are utilizing more rigorous statistical methods to test hypotheses, and including more comparative analyses. With these aims in mind, I examine the predictability of null models and biological diversity hypotheses for latitudinal and elevational gradients in diversity and abundance of small mammals. The unimodal distribution of diversity in North American desert rodents was highly consistent with the mid-domain effect—a spatial constraint null model incorporating the overlap of variably sized ranges within a bounded region. Deviations from the null model demonstrated a localized pulse in richness caused by a local hard boundary, the Baja peninsula. The small mammal diversity along an elevational transect in Costa Rica was unimodal with species richness highest between 1000-1300 m. The spatial constraints of montane topography appear to influence the diversity pattern, although climatic conditions including an intermediate rainfall and temperature regime, and distance from the persistent cloud cap also are correlated with the pattern. The global analysis of elevational diversity trends for non-volant small mammals revealed a ubiquitous pattern of mid-elevational peaks in species richness. The mid-domain null model was not generally predictive across all datasets. Diversity peaks occurred at higher elevations on taller mountains (Massenerhebung effect), which is consistent with climatic factors working in concert to produce elevationally correlated habitat bands. Gamma diversity patterns demonstrated higher altitudinal peaks in species diversity as latitude increased. An examination of replicates in alpha diversity studies along elevational transects found high variability both temporally and spatially, emphasizing the necessity of replication in well-designed studies of diversity gradients. In an examination of range size-abundance trends no strong relationship was found between abundance or body size with elevational range size. Local and regional abundances across elevational ranges generally revealed a trend toward higher abundances at mid-range, although usually not centered at the range midpoint.