Spatial patterns in the geographic range sizes of bird species in the New World

Abstract

The attempt to identify and explain pattern in the extent of species’ geographical distributions at regional scales has been central to macroecology. However, with the exception of abundance, consistent relations between other variables and species geographic extent have not been forthcoming. One reason may be that studies often encompass the entire geographic ranges of only a fraction of the species in the taxon under consideration, setting biologically artificial boundaries to the area of study, and only revealing part of the pattern in question. Here, we examine patterns in the geographic range sizes of birds in the New World. By testing for patterns in the entire avifauna of a geographically isolated region (95% of species are endemic), we avoid many of the problems of previous studies. Most New World bird species have small geographic ranges, although the frequency distribution of logarithmically transformed ranges is left-skewed. The geographic range size-body size relation is approximately triangular. Small-bodied species may have either large or small ranges, whereas large-bodied species have only large ranges. Species threatened with extinction more often fall nearer to (or below) the lower edge defined by the majority of species in this triangle than do non-threatened species, suggesting that this represents the minimum area needed to sustain viable populations of species of different sizes. The maximum range size attained by species is relatively constant across body sizes, but falls short of the maximum possible given the land area of the New World, and so cannot be limited by this constraint. What does limit maximum range size is thus unclear. There is a latitudinal gradient in the size of species geographic ranges. Species which have the latitudinal mid-point of their geographic ranges at high latitudes either side of the equator tend to have large range sizes, whereas those with mid-points at lower latitudes tend to have small range sizes (as expected from Rapoport’s rule). However, this pattern is not symmetrical about the equator, but rather, at about 17° N. It appears to be a consequence of the biogeography of the New World, and implies that mechanisms suggested to explain Rapoport’s rule based on climatic variability are incorrect. Migrant birds have larger geographic ranges, on average, than do residents. They are also larger-bodied, and tend to inhabit more northerly latitudes than residents, but their larger ranges are not the simple consequence of these other patterns. The patterns we demonstrate, in particular those relating to maximum range size across body sizes and to latitudinal variation in range size, have significant consequences for the understanding of what determines species geographic range sizes.