Functional relationships between birds and fruits on an elevational gradient in Papua New Guinea

Birds constitute a vital component of tropical rainforests, filling a wide range of functional roles spanning from predation to seed dispersal to pollination. Tropical mountains are typified by high bird diversity, and provide a unique opportunity to examine changing intertrophic functional relationships within relatively small distances. However, the relationships between birds and their food resources along tropical elevational gradients are poorly understood. This thesis investigates various components of bird alpha- and beta-diversity along an elevational gradient in Papua New Guinea. It then focuses on an important tropical feeding guild (frugivores) and relates observed bird diversity patterns to those of fruits found along the gradient, concentrating on the functional relationships between them. Within a single (lowland) elevational band, bird beta-diversity was found to be very low. With increasing elevation on a tropical mountain, high bird beta-diversity and declining alpha-diversity did not seem to be driven by direct climatic effects. Functional and phylogenetic declines with increasing elevation may be driven in large part by a loss of large frugivores towards upland forest, corresponding to a decrease in large fruits at high elevations. Indeed, frugivorous birds at high elevations preferentially selected smaller fruits than those at lower elevations when given a choice, suggesting a close functional connection between frugivorous birds and the fruits they disperse. This research highlights the importance of functional diversity in maintaining intertrophic dynamics, and demonstrates the need to think beyond the species or even habitat level when considering measures to best protect biodiversity in a way that maintains these dynamics. By focusing on the relatively undisturbed forests of New Guinea, this research has demonstrated the importance of intertrophic functional connections which may have been lost in more degraded habitats.