The Habitat Volumes of Scarce and Ubiquitous Plants: A Test of the Model of Environmental Control

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Abstract
This study tests Brown''s environmental-control model by examining the expected relationship between the ubiquity of species in samples and the realized edaphic habitat volume of these species. The habitat space of a suite of plants from southern Western Australia was defined by principal-components analysis of soil data. The habitat volumes of 36 scarce and 41 ubiquitous species were not significantly different. An alternative hypothesis is suggested, the scarce plants do not have restricted environmental tolerances but, rather, fill their habitat space more sparsely. Strong correlations between species frequencies in samples and habitat volume were observed in the total data set and 8 of 10 plants guilds, corroborating the predictions of the model. However, only half the correlations within guilds were significant at the 5% levels whenthe statistical bias from sampling frequency was taken into account. Sampling bias has a clear and important effect, and if it is ignored, measurements of relative environmental tolerance and habitat volume may be misleading.