The Influence of Leaf-Monkeys on their Feeding Trees in a Cyclone-Disturbed Environment

Abstract

A cyclone in November 1978 caused extensive damage to a site of natural dry evergreen forest in Sri Lanka. The cyclone destroyed more than 50 percent of the woody vegetation that had produced most of the food for two species of leaf-eating monkeys or langurs. Apparently, this caused an imbalance between these langur populations and their natural food supply and resulted in overbrowsing on those feeding trees which were not destroyed by the cyclone. Preferentially browsed tree species that were relatively rare and/or small in size died at significantly greater rates due to overbrowsing than those which were buffered against overbrowsing by virtue of being large in tree size and/or relatively abundant in the forest. The virtual disappearance of three overbrowsed tree species from the forest suggests that langurs may contribute to the change in floristic diversity in cyclone-disturbed areas. However, such an effect of langur folivory is thought to be short-lived and specific to this kind of rare disastrous environmental situation. ON NOVEMBER 23, 1978, A CYCLONE SWEPT across the island of Sri Lanka in the Indian Ocean causing extensive damage. Studies concerning the forest and its primate inhabitants were conducted before and after the cyclone at a site, Polonnaruwa, which lay directly in its path. This provided a unique opportunity to examine the effects of such a natural disaster on the relationship between the primates and their natural forest habitat. Earlier (Dittus 1985) I described the cyclone damage to the forest at Polonnaruwa and considered the influence of recurrent cyclones on the dry evergreen forest of Sri Lanka in general. At Polonnaruwa one immediate effect of the cyclone was the destruction of much of the food supply for two folivorous monkeys, the grey langur Presbytis entellus and the purple faced langur Presbytis senex (Figs. 1 and 2). My aim in this paper is to examine the effect that browsing by these langurs had on the survivorship of several of their feeding trees within 42 months after the cyclone. The focus is on the trees rather than on the langurs during what probably represents a transient phase of imbalance between the langur populations and their feeding trees. The examination is of interest on two counts. First, it clarifies the nature of the relationship between the langur populations and their feeding trees at a time of sudden drastic environmental change. Second, as overbrowsing by langurs contributed to the cyclone damage and consequent death of several of their feeding trees, it defines langur browsing as an additional agent influencing the floristic diversity of dry evergreen forest of Sri Lanka in areas that have recently undergone cyclone damage. The meteorological details of the cyclone, its path, and the kind and degree of damage it inflicted on the forest at Polonnaruwa were described earlier (Dittus 1985). Cyclone damage involved the uprooting of trees (tree falls), bre kage of trunks and large branches (crown damage), and extreme defoliation and loss of twigs particularly in t e upper layers of the forest. In addition, many trees that survived the immediate effects of the cyclone died within 42 months thereafter. Some of this post-cyclone tree mortality could be attributed to extensive crown damage; thus trees which lost 40 percent or more of their branches and trunks died at significantly greater rates after the cyclone than trees with lesser crown damage. But, not all post-cyclone tree mortality could be thus explained.