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 ac ross squares, or beta diversity, which has the potential either to red uce or to exaggerate richness patterns demonstrated using grid methodo logies, and may itself show interesting spatial patterns. A considerat ion of spatial patterns of turnover should accompany any analysis of s patial patterns in species richness. Here, we analyse latitudinal patt erns 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 richn ess, or shows no significant latitudinal variation. Thus, the latitudi nal pattern in richness previously shown for New World birds is not ca used by turnover. Turnover is negatively related to the mean geographi c range size of species at a given latitude, although this is again me thod-dependent. We discuss the relationship between the different meas ures of turnover, and assess how appropriate each is as a measure of t his quantity. We conclude that more studies are required before we can make any generalisations about patterns in turnover, and that dose at tention to methodology will be required in all future considerations.