Native wild pigs (Sus scrofa) at Pasoh and their impacts on the plant community

Although many large-bodied terrestrial mammals are presently extinct or exceedingly rare at Pasoh Forest Reserve (Pasoh FR), the native, wild pigs (Sus scrofa) are thriving. Line transect surveys conducted in 1996 and 1998 yielded density estimates of 47.0 and 27.0 pigs/km2, respectively. These are among the highest density estimates ever recorded for this species. Important factors contributing to the maintenance of such high pig density at Pasoh FR are likely the absence of large carnivores and an abundant year-round food supply in the oil palm plantations that virtually surround the reserve. Several studies have recently addressed some of the effects that such a high density of pigs may have on the understory plant community at Pasoh FR. To quantify the effects of soil rooting and seed predation by pigs on woody saplings, pig exclosures were constructed in the primary forest at the center of the reserve. After two years the number of woody plant recruits, total stem density, species richness, and height growth were greater inside enclosed areas than paired control plots to which pigs had access. Another study examined the prevalence of nest building by pigs. When ready to deliver young, pregnant females snap off or uproot tree and liana saplings 40–350 cm in height. These stems are meticulously piled into dome-shaped structures under which the female gives birth. Annual surveys of pig nests were conducted in the western 25-ha of the permanent 50-ha tree plot. More than 600 nests were located over a four-year period, for an average density of 6.0 pig nests constructed/ha/ year. Based on examinations of 10 nests and damage to the surrounding areas, each nest contained on average 145 uprooted stems and an additional 122 stems that were snapped off, leaving behind stumps. Pigs gathered these stems from an average area of 244 m2 surrounding each nest, damaging or killing 53% of woody understory vegetation > 70 cm tall and < 2.0 cm diameter at breast height. We estimated that pigs caused 29% of the total mortality of trees in the 1–2 cm Diameter at Breast Height (DBH) size class. Pigs also preferentially used saplings of the Dipterocarpaceae as nest construction material. Nest construction created > 85,000 stumps/km2 at Pasoh FR, suggesting that understory regeneration and future species composition may be influenced considerably by the resprouting abilities of damaged plants. More than 1,800 stumps were examined for 36 months to investigate resprouting. There were large differences in resprouting success among species with different life history characteristics and taxonomic associations. Stumps of the Dipterocarpaceae had by a wide margin the lowest survivorship at 36 months of the 19 most common families in the study. Overall, the data suggest that if pigs continue to be hyper-abundant in the reserve there could be a shift away from the economically and ecologically paramount Dipterocarpaceae.

Kalan Ickes & Sean C. Thomas
Pasoh: Ecology of a Lowland Rain Forest in Southeast Asia
Toshinori Okuda, N. Manokaran, Yoosuke Matsumoto, Kaoru Niiyama, Sean C. Thomas, & Peter S. Ashton
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