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Individual study: Effects of exclusion of wild pigs Sus scrofa on woody understorey regeneration at Pasoh Forest Reserve, Negeri Sembilan, Malaysia

Published source details

Ickes K., Dewalt S.J. & Appanah S. (2001) Effects of native pigs (Sus scrofa) on woody understorey vegetation in a Malaysian lowland rain forest. Journal of Tropical Ecology, 17, 191-206

Summary

Although wild pigs Sus scrofa are known to have detrimental effects on forest communities where introduced, little is known about their impact on communities to which they are native.  A large proportion of their food is subterranean in the animals regularly rooting through soil to locate food. Rooting may adversely affect vegetation by killing seedlings, by altering soil properties or facilitating the spread of exotics. Conversely, rooting may be beneficial to native plants, for example by creating open patches where seedlings may germinate and establish.

In a lowland rain forest  in West Malaysia, the extent to which wild pigs influence the dynamics of tree seedlings and saplings was investigated by creating exclosures.

Study area: Pasoh Forest Reserve (110 km southeast of Kuala Lumpur) in the state of Negeri Sembilan, comprises a core area of 650 ha of primary lowland dipterocarp forest and a 650 ha buffer zone (forest regenerating from selective logging that occurred 40 years earlier). Another 650-1000 ha of primary hill dipterocarp forest comprises the eastern portion of the reserve. The reserve is fenced which excludes most larger herbivores such as Asian elephants Elephas maximus and most deer. Smaller species however can move freely through the fencing. Native wild pigs are common and it was thought that thay might significantly influence growth and survivorship of woody plants in the understorey through several activities: nest building, soil rooting and seed predation. This in turn might have implications for long-term forest management.

Exclosure design: Eight 49-m² exclosures were constructed between 30 June and 11 July 1996. Two control plots were paired with each exclosure. These were placed 1 m outside of the exclosures on two of the four sides ( on the two sides that subjectively most closely resembled the vegetation within the experimental plot).

Exclusion of vertebrates: Due to the wide mesh size of the fence and open top design, it was assumed that birds, rodents, civets and primates could readily enter exclosures. The only herbivores excluded were therefore wild pigs, Malayan porcupine Hystrix brachyura, brush-tailed porcupine Atherurus macrourus and lesser mouse-deer Tragulus javanicus. Both porcupines are present at apparently low densities. Lesser mouse-deer neither build nests nor disturb soil by digging, as pigs do. They are granivorous, and therefore excluding this species may have confound the interpretation of results with regard to seed predation, mostly presumed to be by pigs.

Vegetation sampling: In all plots between 12 August and 21 September 1996, woody, free-standing plants over 30 cm height were identified, mapped and measured for basal diameter, dbh and height up to 740 cm.

A second survey took place in August 1998, the same measurements being taken and any mortality noted.  All new plants that had recruited into the 30 cm height category were added (hereafter called recruits). Additionally, in 1998, all seedlings of woody plants < 30 cm tall were counted (hereafter called seedlings).

Plant survival: After two years, no differences were found in woody plant mortality between experimental and control plots. However, taller plants had significantly higher odds of survival than shorter plants.

Plant recruitment: Overall, the number of recruits in exclosures was three times greater (and also slightly taller ) than in the controls.  In 1998, exclosures had 56% more seedlings  than controls;  woody stem density  was much higher in exclosures; species diversity was lower in exclosures but number of species, however, was greater. The lower recruitment in the controls was attributed to activities of wild pigs.

Growth: Overall height growth of stems initially between 7 m (± 1m) tall was 50% greater inside exclosures. No differences were found for tree stems initially between 100 cm (± 30cm) tall. No treatment differences were detected in height growth for free-standing lianas initially.

Conclusions: This study indicates that  wild pigs play an important role in understorey plant dynamics at Pasoh.  Although the current density of pigs in the reserve is higher than historic levels, a number of large herbivorous mammals are absent from the fenced reserve, thus their abscence and influence on forest vegetation, may be inpart balanced by these higher pig densities.

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