WHAT DETERMINES THE GRADIENT IN BIRD SPECIES DIVERSITY ACROSS THE HIMALAYAS?
Why are more species found in tropical than temperate regions? Explanations may be broadly divided into those rooted in history or in present day conditions. I describe our research on this issue, which uses birds of the Himalayas as a model system, and new methods to apportion these effects. Twice as many species are found in the east Himalayas as in the west Himalayas. About ¼ of all eastern species extend to the west, and we compare the attributes of the species with large ranges to those with small ranges. I summarize contributions from both the history of the region and contemporary conditions to the decline in species numbers from east to west.