Linking patterns, causes and functional consequences of changing biodiversity: Dung beetles and forest fragmentation

Habitat loss and fragmentation are rapidly modifying tropical forest ecosystems which contain the majority of the Earth's biodiversity. While there is growing knowledge of how species respond to disturbance, little is known about the processes underlying this response or its consequences for the functioning of ecosystems. Understanding causes of species declines provides generalizable information for predicting and preventing future responses. It is also necessary to understand how these changes affect the ecological processes that maintain the long-term integrity of entire ecosystems. I used a holistic approach to examine dung beetle communities on an archipelago of forest islands recently created by flooding in Venezuela. Beetle species richness, density and biomass all declined sharply with decreasing island area and increasing island isolation. Some species appeared to require at least 85 ha, which is more than that required by many large mammal species. Species richness was highly nested, indicating that extinction order was non-random. Extinction prone species were large-bodied, specialized to forest habitat, and occurred at low population density. Most species were forest specialists which appeared to be sensitive due to physiological and behavioral constraints. These species were often unable to reflect from forest boundaries, leading to rapid accidental emigration from smaller fragments. Upon leaving suitable habitat, beetles could not navigate towards forest and died quickly in the matrix, although a few savannah-adapted species may have been an exception. Experimental manipulation of howler monkeys suggested that local beetle extinctions are driven more strongly by area-dependent emigration than by food availability, at least in small fragments. Large species may have been especially area-sensitive due to high wing-loading and fast, wide-ranging flight. Because large species were the most extinction prone and the most functionally important for burying dung, fragmentation led to unexpectedly rapid disruption of dung burial. Due extremely weak density compensation, declining beetle abundance also contributed to loss of dung burial. By burying dung, dung beetles play several important ecological roles, including secondary seed dispersal, nutrient cycling, and regulation of vertebrate parasites. Consequently, the loss of dung beetle diversity in forest fragments is likely to have cascading effects throughout the ecosystem.