Global patterns of geographic range size in birds

Abstract

Large-scale patterns of spatial variation in species geographic range size are central to many fundamental questions in macroecology and conservation biology. However, the global nature of these patterns has remained contentious, since previous studies have been geographically restricted and/or based on small taxonomic groups. Here, using a database on the breeding distributions of birds, we report the first (to our knowledge) global maps of variation in species range sizes for an entire taxonomic class. We show that range area does not follow a simple latitudinal pattern. Instead, the smallest range areas are attained on islands, in mountainous areas, and largely in the southern hemisphere. In contrast, bird species richness peaks around the equator, and towards higher latitudes. Despite these profoundly different latitudinal patterns, spatially explicit models reveal a weak tendency for areas with high species richness to house species with significantly smaller median range area. Taken together, these results show that for birds many spatial patterns in range size described in geographically restricted analyses do not reflect global rules. It remains to be discovered whether global patterns in geographic range size are best interpreted in terms of geographical variation in species assemblage packing, or in the rates of speciation, extinction, and dispersal that ultimately underlie biodiversity.

Figure 1. Species-Range Area Distribution for the Global Avifauna

(A) Untransformed range areas. (B) Log ₁₀-transformed range areas. (C) Normal probability plot for log ₁₀-transformed range areas, showing the expectation under a normal distribution (dashed line) and the observed distribution (open circles).

Figure 2. Geographic Distribution of Geographic Range Areas, Latitudinal Range Extent, and Species Richness for the Global Avifauna

(A) Median geographic range area (km ²). (B) Minimum geographic range area (km ²). (C) Variance in geographic range area (km ²). (D) Median latitudinal range extent (degrees). (E) Minimum latitudinal range extent (degrees). (F) Total species richness. The map scales are based on quartiles of the underlying distributions; the scale bars show the quartile values for each map. Parallels are shown at 45° S, the Equator, and 45° N.

Figure 3. Global Relationships between Geographic Range Area, Latitudinal Range Extent, Species Richness, Land Area, Island Area, and Latitude

(A) Median geographic range area and latitude. (B) Species richness and latitude. (C) Median range area and latitude for species with midpoints falling in each respective latitudinal band. (D) Total land area (km ²) within latitudinal bands. (E) Median latitudinal range extent and latitude. (F) Median geographic range area and species richness. (G) Median latitudinal range extent and species richness. (H) Proportion island area and latitude. For (A–C) and (E and F), open circles represent latitudinal means, and grey points show the spread of individual grid cell values. Southern latitudes are indicated as negative, northern ones as positive.