Spatial patterns in the species richness of birds in the New World

Spatial patterns in species richness of the complete New World avifauna were analysed, using data previously employed to examine spatial trends in geographic range size. This allowed variation in the patterns to be compared. Species richness was highest around the equator, and decreased towards higher latitudes in both hemispheres. This decrease was asymmetrical; at equivalent latitudes, richness was higher in the southern than in the northern hemisphere, although the reverse was true for a measure of endemism. Controlling for latitude, species richness was higher in the west than in the east. The net primary productivity of, and solar radiation received by an area were both correlated with species richness. However, neither explained more variation in richness than did latitude. No single mechanism developed to explain spatial patterns in species richness satisfactorily explains the patterns observed in the New World avifauna. We discuss reasons why this might be the case. Finally, we point out that species richness at low latitudes is not simply a multiplication of richness at high latitudes; species found at high and low latitudes are unlikely to be ecologically equivalent. Any mechanism that is proposed to explain richness patterns in New World birds will need also to account for this observation.