Wild Pigs (Sus scrofa) Mediate Large-Scale Edge Effects in a Lowland Tropical Rainforest in Peninsular Malaysia

Harrison RD (2012) Wild Pigs (Sus scrofa) Mediate Large-Scale Edge Effects in a Lowland Tropical Rainforest in Peninsular Malaysia. PLoS ONE 7(5): e37321. doi:10.1371/journal.pone.0037321 Wild Pigs (Sus scrofa ) Mediate Large-Scale Edge Effects in a Lowland Tropical Rainforest in Peninsular Malaysia Junichi Fujinuma 0 Rhett D. Harrison 0 Justin Wright, Duke University, United States of America 0 1 Graduate School of Environmental Science, Hokkaido University , Sapporo, Hokkaido , Japan , 2 Key Laboratory for Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Science , Mengla, Yunnan , China Edge-effects greatly extend the area of tropical forests degraded through human activities. At Pasoh, Peninsular Malaysia, it has been suggested that soil disturbance by highly abundant wild pigs (Sus scrofa), which feed in adjacent Oil Palm plantations, may have mediated the invasion of Clidemia hirta (Melastomataceae) into the diverse tropical lowland rain forest. To investigate this hypothesis, we established three 1 km transects from the forest/Oil Palm plantation boundary into the forest interior. We recorded the distribution of soil disturbance by wild pigs, C. hirta abundance, and environmental variables. These data were analyzed using a hierarchical Bayesian model that incorporated spatial auto-correlation in the environmental variables. As predicted, soil disturbance by wild pigs declined with distance from forest edge and C. hirta abundance was correlated with the level of soil disturbance. Importantly there was no effect of distance on C. hirta abundance, after controlling for the effect of soil disturbance. Clidemia hirta abundance was also correlated with the presence of canopy openings, but there was no significant association between the occurrence of canopy openings and distance from the edge. Increased levels of soil disturbance and C. hirta abundance were still detectable approximately 1 km from the edge, demonstrating the potential for exceptionally large-scale animal mediated edge effects. - Tropical forests are threatened by deforestation and forest degradation [1]. Forest degradation arises when factors such as hunting, logging, fire, invasive species, and pollution, often acting in concert, alter the ecology of the forest and reduce its capacity to maintain biodiversity [2,3]. Whereas the area of deforestation is relatively easily defined, forest degradation varies hugely in spatial extent and the severity of its consequences depending on the agents responsible [4,5]. Edge effects occur where a sharp boundary is created between the forest and an open habitat, such as a road, cattle pasture or Oil Palm plantation [6]. The microclimate of the forest edge is changed, particularly with respect to temperature, humidity, wind shear and turbulence [7,8], and these microclimatic changes alter ecological processes at the edge [5,8,9,10]. Irregularly shaped forest fragments and linear cuttings, such as roads, power-line corridors, and pipelines, create a large proportion of edge and can, therefore, lead to forest degradation over substantial areas. Nevertheless, direct edge effects are usually confined to within 400 m of the edge [5,11]. However, animals may also respond to edges and as a consequence of their mobility can potentially alter plant communities over much larger scales [4,11,12]. Changes in animal abundances at edges have been reported and these are predicted to affect animal-mediate ecological functions, such as pollination [13,14], decomposition [15], predation [16,17], herbivory [16,18], and seed dispersal [19]. However, to date, animal mediated edge effects have rarely been studied [11,16]. Colonization of habitats by exotic species is a major problem for biodiversity conservation [20,21]. Through release from natural enemies [22] or functional traits that confer a competitive advantage over native species [23,24], exotic species may become invasive. Such species command a large proportion of the energy flow through an ecosystem, altering its ecology and displacing native species [25]. Invasive plants are most common in open habitats and in particular those that have been highly disturbed [2]. In general, diverse natural communities, such as undisturbed continental tropical rain forests, rarely suffer from such crises [26,27]. However, contrary to expectations, Pasoh Forest Reserve, Peninsular Malaysia, a highly diverse lowland dipterocarp forest, has been invaded by the South American pioneer shrub Clidemia hirta (L.) D. Don. (Melastomataceae). It has been suggested that this invasion may have been mediated by the activities of wild pigs (Sus scrofa), which are very abundant at Pasoh [18]. It is thought that soil disturbance caused by the wild pigs, when they root and grub for food, may create colonization micro-sites for C. hirta and thereby have abetted the invasion of the rain forest by this alien plant. Moreover, as the pigs are known to feed in the Oil Palm surrounding the reserve, Peters [18] predicted the existence of a gradient of soil disturbance frequency with distance from forest/ Oil Palm plantation boundary, and suggested that the abundance of C. hirta would decline in a similar fashion. If this conjecture is correct, the invasion of the rain forest at Pasoh by C. hirta may be considered a consequence of a large-scale edge effect mediated by wild pigs. Wild pigs are native throughout Europe and Asia, including tropical SE Asia. Their activities, including feeding, wallowing, and nest construction, have important effects on the ecology of forests [2830]. Where abundant, their feeding activities can disturb the soil surface over large areas [31]. Pigs grub for insect larvae and fruit in the leaf litter and surface soil using their noses to turnover the soil. Trampling with their sharp hooves further churns the soil, particularly where it is water-logged. In addition, a single pig may cut over 50 seedlings in the construction of a sleeping nest [28]. The activities of wild pigs have shaped invasive plant community composition on oceanic islands [32] and where wild pigs have been introduced they are often considered invasive. Two possible factors have been put forward to explain the high abundance of wild pigs in the forest at Pasoh: the local extinction of tigers who would previously have been the main predator of pigs [33] and a super-abundance of food in the Oil Palm plantations around reserve [34]. However, tigers have been extirpated over much of their former range in SE Asia and are rare where they still occur, but high abundances of wild pigs are localized phenomena. Moreover, during the early 2000s the Oil Palm plantations around Pasoh were cut down and replanted and during this period the populations of wild pigs declined drastically, before recovering again after the young Oil Palms began fruiting [34]. Thus, a super-abundance of food in the surrounding Oil Palm plantations is strongly suspe (...truncated)