Abstract
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.
Highlights
Tropical forests are threatened by deforestation and forest degradation [1]
Approximated 18% of the total transect length was categorized as disturbed by wild pigs (Figure 3)
There was a significant decline in soil disturbance with increasing distance from the forest/Oil Palm plantation boundary
Summary
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 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]. Shaped forest fragments and linear cuttings, such as roads, power-line corridors, and pipelines, create a large proportion of edge and can, lead to forest degradation over substantial areas
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