The influence of geomorphological heterogeneity on biodiversity II. A landscape perspective

The influence of geomorphological heterogeneity on vascular plant species richness was studied in 26 Rhode Island Audubon refuges ranging in size from 1.4 to 58.6 ha. Indices of abiotic heterogeneity reflecting spatial variation in slope, aspect, and soil drainage were calculated from extant databases in a geographic information system. After removing the influence of refuge size on both biotic diversity and geomorphological heterogeneity, vascular plant species richness was found to be highly related to geomorphological heterogeneity. Diversity in soil drainage class alone accounted for more than 50% of the variance in total plant species richness (r(2) = 0.53, p < 0.001) and was significantly related to the species richness of 10 of the 11 life-form classes of plants studied. When two refuges considered to be mathematical and biological outliers were removed from the regression analysis, soil drainage heterogeneity accounted for more than 65% of the variance in total plant species richness (r(2) = 0.67, P < 0.001). Aspect heterogeneity, slope heterogeneity, and the composite index of all three measures of geomorphological heterogeneity were each less related to total plant richness than soil drainage heterogeneity alone, but they were all more effective at predicting richness of native woody species (r(2) = 0.23-0.45, p < 0.05) than soil drainage heterogeneity (r(2) = 0.08, p > 0.05). In a stepwise multiple regression analysis, soil drainage heterogeneity and refuge size explained 66% (p < 0.001) of the variation in species richness (soil drainage heterogeneity, partial r(2) = 0.39 refuge size, partial r(2) = 0.27). These results explicitly support the tenet that geomorphological heterogeneity plays a major role in determining species richness. Because biotic and abiotic diversity were intricately linked at the scale of the landscape, conservation of geomorphological heterogeneity may be an effective strategy for conserving biodiversity.