Scaling Biodiversity: The distribution of species: occupancy, scale, and rarity

Introduction Species occupancy is typically measured as the number of cells occupied by the species in a study area. Because it is easy to document and interpret and it correlates with species abundance, occupancy is widely used for measuring species rarity and for assessing extinction risk on which conservation decisions are made (Gaston, 1994; Fagan et al., 2002; Hartley & Kunin, 2003; Wilson et al., 2004). Ecologists and conservation practitioners, however, have long realized that occupancy often fails to capture significant spatial features of distribution. It is possible that two species having the same occupancy can exhibit very different patterns (Fig. 3.1). Most species in nature are discretely distributed due to the patchiness of landscapes, or due to intrinsic reproductive or dispersal behavior of the species. An outstanding problem concerning species distribution in space is how to describe the patchiness of a species and to measure the effect of changing spatial scale (cell size) on the patchiness for the purpose of predicting distribution at fine scales from coarse scales. There are two primary approaches to addressing this question. The first one is to use existing measures and methods to describe patchiness and scale effect. Many fragmentation indices in landscape ecology can be used for this purpose (Turner, Gardner & O’Neill, 2001; Wu et al., 2003). These include edge length (perimeter), the number of patches, perimeter/area ratio and many other indices to capture the spatial features of species distribution. An interesting development on this front is the percolation models for edge length and the number of patches proposed by He and Hubbell (2003). These models unify edge length, the number of patches, spatial scale and abundance and show that the edge length and number of patches of a distribution can accurately be predicted by the abundance of the species and the degree of fragmentation. These models are useful for analyzing and comparing species distributions.

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