How does a re-colonizing population of Asian elephants affect the forest habitat?


Per Wegge, Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, PO Box 5003, 1432 Ås, Norway.
Tel: +47 64965762; Fax: +47 64965801


The Asian elephant Elephas maximus is currently re-colonizing the Bardia National Park in lowland Nepal. We studied their impact on woody vegetation in the nutrient-rich floodplain and in the relatively nutrient-poor sal forest. The types and extent of tree impact were recorded along fixed-width transects (335 km). Species composition, density and size classes ≥8 cm diameter breast height (dbh) were recorded in 15-m radius random plots (n=95). Impact was higher in the floodplain complex than in the sal-dominated forest. Our hypothesis that elephants were more selective on species in the nutrient-poor sal forest was only partly supported; the niche breadth of impacted trees was slightly higher in the floodplain complex. Pushed-over trees accounted for the highest proportion of impact (55%), followed by killed trees (39%). Of the pushed trees, 10% were not used for food. Among food trees, elephants selectively impacted size class 12–16 cm dbh, whereas non-food trees were impacted independently of size. A large proportion of the freshly browsed trees had been felled previously, indicating that most felled trees survived, enabling elephants to feed on them again. This may reflect an evolutionary adaptation among long-lived species with high site fidelity. Owing to preferential use but low abundance, two species in sal forest, Grewia spp. and Desmodium oojeinense, were found to be particularly vulnerable to local extinction due to elephants. Although the elephants had impacted a large number of species (62, 73% of all), 56.4% of the impacted trees consisted of Mallotus phillippinensis. A recently observed increase in the density of M. phillippinensis and the concurrent reduction of the hardly utilized Shorea robusta indicates that the rapidly growing elephant population may modify the composition of the forest by increasing its preferred food species.