Using local–regional richness plots to test for species saturation: pitfalls and potentials

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Abstract
1. Plots of local versus regional species richness are an exciting new tool for testing for species saturation in ecological communities. In this method, the local richness of a community is plotted as a function of its regional richness for different biogeographical regions. A proportional relationship between local and regional richness is interpreted as evidence for an unsaturated community, that is, a community with strong evolutionary limits to local richness. There will be no correlation between local and regional richness in a saturated community, that is, a community whose local species richness is limited largely by ecological processes. 2. Although at least 36 data sets have now been analysed using local–regional richness plots, there has not been much critical evaluation of the method. This paper provides such a critique, focusing on the selection of communities for comparison, the prevalence of pseudoreplication and multiple null models, and the effects of differing region size. 3. Local–regional richness plots are best suited for comparing similar habitats between different regions, not different habitats in a single region. In the latter, taxa effects and species pool effects are confounded. 4. Four very different types of local–regional richness plots have been published. Each type of plot has important underlying assumptions which are often not addressed by ecologists. 5. Of the 36 data sets reviewed in this paper, 13 were spatially pseudoreplicated, and 2 were temporally pseudoreplicated. Furthermore, ecologists differ in their choice of null model, with the result that the same local–regional plot could be interpreted as evidence for saturation by one ecologist, and for lack of saturation by another. 6. Differences in region size can result in pseudosaturation, the appearance of saturation by an unsaturated community. A simple model demonstrates this phenomenon. Other sources of error in estimating the regional species pool are also of concern. 7. In conclusion, local–regional richness plots are a potentially useful tool for distinguishing saturated from unsaturated communities, but should be used cautiously, and in conjunction with other supporting evidence (such as the presence or absence of competitive exclusion, resource limitation, density compensation and the effects of species invasions).