The distribution of bird species in the New World: Patterns in species turnover

Spatial patterns of species richness are most often analysed at large scales by covering the area in question with a grid and comparing the richness of different squares. This method ignores species turnover across squares, or beta diversity, which has the potential either to reduce or to exaggerate richness patterns demonstrated using grid methodologies, and may itself show interesting spatial patterns. A consideration of spatial patterns of turnover should accompany any analysis of spatial patterns in species richness. Here, we analyse latitudinal patterns in turnover for the avifauna of the New World, using a variety of different turnover measures. Depending on the measure used, turnover either decreases with latitude, mirroring the pattern in species richness, or shows no significant latitudinal variation. Thus, the latitudinal pattern in richness previously shown for New World birds is not caused by turnover. Turnover is negatively related to the mean geographic range size of species at a given latitude, although this is again method-dependent. We discuss the relationship between the different measures of turnover, and assess how appropriate each is as a measure of this quantity. We conclude that more studies are required before we can make any generalisations about patterns in turnover, and that dose attention to methodology will be required in all future considerations.