Spatial turnover in the global avifauna - PubMed (original) (raw)

Comparative Study

Spatial turnover in the global avifauna

Kevin J Gaston et al. Proc Biol Sci. 2007.

Abstract

Despite its wide implications for many ecological issues, the global pattern of spatial turnover in the occurrence of species has been little studied, unlike the global pattern of species richness. Here, using a database on the breeding distributions of birds, we present the first global maps of variation in spatial turnover for an entire taxonomic class, a pattern that has to date remained largely a matter of conjecture, based on theoretical expectations and extrapolation of inconsistent patterns from different biogeographic realms. We use these maps to test four predictions from niche theory as to the form that this variation should take, namely that turnover should increase with species richness, towards lower latitudes, and with the steepness of environmental gradients and that variation in turnover is determined principally by rare (restricted) species. Contrary to prediction, we show that turnover is high both in areas of extremely low and high species richness, does not increase strongly towards the tropics, and is related both to average environmental conditions and spatial variation in those conditions. These results are closely associated with a further important and novel finding, namely that global patterns of spatial turnover are driven principally by widespread species rather than the restricted ones. This complements recent demonstrations that spatial patterns of species richness are also driven principally by widespread species, and thus provides an important contribution towards a unified model of how terrestrial biodiversity varies both within and between the Earth's major land masses.

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Figures

Figure 1

Figure 1

Global distributions of species gain and loss between neighbouring grid cells. The mean number of species (a,d) shared (matching component a), (b,e) gained (matching component b) and (c,f) lost (matching component c) in comparisons between each focal grid cell and its adjacent neighbours, expressed as raw numbers (a_–_c) and as a proportion of the total number of species in the focal cell. The colour scales are histogram equalized and the quartile values are indicated.

Figure 2

Figure 2

Global distributions of spatial turnover indices and species richness. (a) _β_w, (b) _β_sim, (c) _β_j and (d) species richness. The colour scales are histogram equalized and the quartile values are indicated.

Figure 3

Figure 3

Relationships of three indices of spatial turnover with species richness and latitude. Associations are shown for species richness (a_–_c), and latitude (d_–_f), for (a,d) square-root transformed _β_w, (b,e) _β_sim and (c,f) _β_j. The range of values at each latitude and species richness, respectively, is shown in grey and the latitudinal or species richness medians are plotted as open circles. In order to show these relationships more clearly, the graph for _β_w is truncated at 3 and that of _β_sim at 0.3.The omitted values constitute 1.8% (_β_w) and 0.7% (_β_sim) of the dataset: all have low species richness (max=186, median=4) but cover a wide range of latitudes. Southern latitudes are indicated as negative, northern ones as positive.

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