Effect of Ants on Elevational Diversity Pattern of Birds in the Eastern Himalaya

Based on the latitudinal diversity gradient, one may expect species diversity to decline while moving up from tropical low-elevations to temperate mid-elevations. Many elevational species diversity gradients follow this expectation and show a decline in diversity with increase in elevation. However, several other studies report a hump-shaped pattern of species diversity along elevational gradients. In this dissertation, I examine how these patterns of species diversity along elevational gradients may be shaped by the distribution of resources and competitors. In chapter 1, I conducted a formal meta-analysis of patterns of summer arthropod abundance along elevational gradients. I found that arthropod abundance tends to peak at higher elevations at mid-latitudes. Shorter growing seasons at these sites leading to synchronous emergence and reproduction of arthropods might be responsible for this observed pattern. In chapter 2, I asked how arthropod abundance pattern might be shaped by extensive ant predation at low elevations and how might affect bird species diversity pattern. Bird species richness peaks at mid-elevations in the eastern Himalaya, where ants are essentially absent. Low elevations in the eastern Himalaya are home to a highly aggressive insectivorous, arboreal ant species, the Asian weaver ant (Oecophylla smaragdina). I found that removal of these ants from a tree increases the abundance of large arthropods (>4mm) and their diet overlaps significantly with diets of birds from low and mid-elevations. Thus, competition for arthropod prey from these ants contributes to lower diversity of birds at low elevations. In chapter 3, I assessed whether availability of nesting cavities contributes to the mid-elevational peak of bird species richness in the eastern Himalaya and if ants play a role in reducing the availability of cavities at low elevations. I recorded extensive use of nest boxes by birds at 200m elevation and none at 1200m, suggesting nest-site limitation at low elevations. Moreover, ants occupied about 70% of the nest boxes at 200m elevation compared to only 13% at 1200m. Thus, ants and birds might also compete for nesting cavities at low-elevations. Overall, these results suggest that resource abundance patterns play an important role in shaping the diversity and distribution of species. Supplementary tables including data from each chapter are available online.