Distribution of forest birds in the Andaman islands: importance of key habitats

Aim The aim of this study was to assess the influence of island area, distance to source pool, latitude, habitat diversity and habitat type on species richness of forest birds in the Andaman islands. Location The Andaman islands (India) in the Bay of Bengal. Methods The distributions of 47 species of forest birds were surveyed on 45 islands in the Andaman islands across a latitudinal gradient. The size of the island and distance to the nearest large island were assessed on a satellite image of 1 : 250,000 scale. The number and types of habitats and the species richness of birds on each island were recorded during a field survey. The effects of the variables measured on the species richness of forest birds were assessed using regression analyses. The best fit models were selected for interpretation of the results. Separate analyses were conducted with selected islands to eliminate the effects of latitude and to control the effects of area and habitat diversity. Results The number of species of forest birds was strongly influenced by island area and habitat diversity. However the key determinant of species richness was habitat type, particularly the presence of wet forests. Wet forests, either semi-evergreen or evergreen tropical forests are more common towards the southern islands and are usually restricted to larger islands. Main conclusions Area, habitat diversity and the presence of wet forests on islands significantly influenced species richness of forest birds. The wet forests maintain the biodiversity of the Andamans and should be regarded as a 'keystone habitat'. This is probably because wet forests are species rich and also because the Andaman biota has affinities with that of the Malay peninsula where wet forests predominate. Therefore biogeographical history probably plays an important role in influencing biodiversity at a regional scale.