Dung beetle assemblages on tropical landbridge islands

Abstract

Understanding and predicting the effects of tropical forest fragmentation on biodiversity and ecosystem functioning is extremely important for conservation planning. This thesis provides one of the first comprehensive studies on a group of ecologically important invertebrates ¿ dung beetles, on forested landbridge islands in Lake Kenyir, Peninsular Malaysia. I show patterns of changes in dung beetle assemblages on islands of varying sizes and in their key ecological function. I also examined the dynamics and underlying drivers of dung beetle distribution on the small islands, and discuss the conservation value of small forest fragments based on these results.

Chapter 1 gives a general overview of our current understanding of forest fragmentation in the tropics and dung beetle ecology. In Chapter 2, I examined the overall effects of forest fragmentation on dung beetle assemblages. I found that below island area of 35.8 ha, species richness and community composition were driven by a small island effect (SIE), rather than by a direct relationship with area. Likely as a result of SIE, no significant nested pattern was found among the dung beetle assemblages on islands. Dung beetle species with low baseline density and inability to forage on forest edge were found to be rarer among sites hence likely more prone to local extinction. These results highlight the stochastic nature of dung beetle communities on small islands, and the need for better understanding of minimum fragment size, capable of retaining functional ecological communities, for effective conservation management.

In Chapter 3, I show that the foraging height of the little studied arboreal dung beetles (represented by Onthophagus sp. 7), were present in higher numbers at 5 m from the forest floor relative to at 15 m in the foliage on smaller islands, indicating a possible downward shift in their vertical stratification in response to forest fragmentation. This expands our current understanding of effects of forest fragmentation to a three-dimensional paradigm.

In Chapter 4, I specifically determined the population dynamics on the small islands. Results from the replicated dung supplementation experiment did not support the food limitation hypothesis. On the other hand, results from the Paragymnopleurus maurus translocation experiment indicate that dispersal limitation is likely an important driver of species presence on small islands. The results of the dispersal limitation experiment were confounded by the presence of invasive ant species, which highlights the need for extensive planning before utilizing assisted colonization as a conservation tool. These results highlight the potential of landbridge islands as experimental ground to test ecological theories, and highlight possible hope for the ¿functional¿ rehabilitation of small forest fragments.

Chapter 5 moves from species and community ecology to ecosystem function, and reports that dung burial rate generally decreased in small islands, with dung beetle abundance being the strongest correlate. I compare results with previous studies and suggest that, whether dung beetle diversity or abundance plays a more important role in dung burial may depend on the site and community composition. In this chapter, I also compare different sampling methods and show that human dung is the most effective bait for dung beetle diversity surveys. In Chapter 6, I provide an overview of the results and give overarching discussion and recommendations for future studies.