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 (r2 = 0.53, p < 0.001) and was significantly related to the species richness of 10 of the 11 life-form classes ofplants 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 (r2 = 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 (r2 = 0.23 -0.45, p < 0.05) than soil drainage heterogeneity (r2 = 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 r2 = 0.39; refuge size, partial r2 = 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. Influencia de la Heterogeneidad Geomorfol6gica en la Biodiversidad II. Una Perspectiva de Paisaje Resumen: La influencia de la heterogeneidad geomorfol6gica en la riqueza de especies de plantas vasculares se estudi6 en 26 refugios de Rhode Island Audubon que variaron en tamanio de 1.4 a 58.6 ha. Se estimaron indices de heterogeneidad abi6tica reflejantes de una variaci6n espacial en pendiente, aspecto y drenaje del suelo a partir de bases de datos de Sistemas de Informaci6n Geografica. Una vez removida la influencia del tamafno del refugio tanto para la diversidad bi6tica, como para la heterogeneidad geomorfol6gica, la riqueza de plantas vasculares se encontr6 altamente relacionada con la heterogeneidad geomorfol6gica. Tan solo la diversidad en las clases de drenaje del suelo explic6 50% de la variabilidad en la riqueza total de especies de plantas (r2 = 0.53, p < 0.001) y estuvo significativamente relacionada con la riqueza de plantas en 10 de las 11 clases de plantas estudiadas. Cuando dos refugios considerados matematica y biol6gicamente casos extremosfueron removidos del analisis de regresi6n, la heterogeneidad del drenaje del suelo explic6 mas del 65% de la varianza de la riqueza total deplantas (r2 = 0.67, p < 0.001). La heterogeneidad del aspecto, la heterogeneidad de la pendiente y el indice de composici6n de las tres mediciones de heterogeneidad geomorfol6gica estuvieron menos relacionadas con la riqueza total de plantas que la heterogeneidad del drenaje del suelo sola, perofueron en conjunto mas efectivas para predecir la riqueza de las especies nativas de maderas (r2 = 0.23, p < 0.05) que la heterogeneidad del drenaje del suelo (r2 = 0.08, p > 0.05). En un andlisis de regresi6n multiple por pasos, la heterogeneidad del drenaje del suelo y el tamanlo del refugio explicaron 66% tAddress correspondence to K T Killingbeck; email keith@uriacc.uri. edu Paper submittedJuly 8, 1996; revised manuscript acceptedJune 4, 1997.