Asian elephants are essential as seed dispersers in a disturbed tropical forest

Abstract

Large animals are disproportionately prone to extinction, and the resulting effects on ecosystem processes are unclear. Megaherbivores--animals weighing over 1000 kg--are thought to be functionally unique in their contribution to many ecological processes. One such ecosystem process is seed dispersal, but few studies have measured the relative importance of a megaherbivore species and sympatric seed dispersers. This dissertation explores how the loss of Asian elephants (Elephas maximus) would affect the dispersal of three large-fruited tree species. Research was conducted in Buxa Tiger Reserve, India, a disturbed tropical forest system resembling much of the region. I found that elephants were a top frugivore of all three tree study species: Dillenia indica (chalta), Careya arborea (kumbhi), and Artocarpus chaplasha (lator). The main alternative frugivores were domestic bovids (the cow, Bos primigenius, and the buffalo, Bubalus bubalis), and Rhesus macaques (Macaca mulatta). Elephants consumed the most D. indica fruit; once eaten, seeds from D. indica were over 16 times more likely to pass undigested into elephant dung than into the dung of domestic bovids. Elephants ate 18% and 10% of frugivore-consumed kumbhi and lator fruits, respectively, defecating 2-3 times as many seeds per fruit as domestic bovids. I show that seed predation of D. indica seeds from elephant dung is unlikely to negate elephants' role in their dispersal. Seeds taken from elephant dung germinated as well or better than seeds from bovid dung or directly from fruit. Elephants were calculated to move seeds up to 10 times as far as domestic bovids. An empirical probability model estimated that the loss of elephants would result in reductions of about 66%, 42%, and 26% in the number of successfully dispersed seeds of each species without compensation. In compensation scenarios, other frugivores could ameliorate reductions in dispersal, making them as low as 6% if species such as gaur (Bos gaurus) persist. Thus the importance of elephants as seed dispersers is amplified by the population reductions of other large disperser species throughout tropical Asia. My findings suggest that losing the largest and most physiologically unique species is likely to have measurable effects on ecological processes.