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Abstract

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.

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