Kalan Ickes

NATURAL‐ENEMY RELEASE FACILITATES HABITAT EXPANSION OF THE INVASIVE TROPICAL SHRUB CLIDEMIA HIRTA

Nonnative, invasive plant species often increase in growth, abundance, or habitat distribution in their introduced ranges. The enemy-release hypothesis, proposed to account for these changes, posits that herbivores and pathogens (natural enemies) limit growth or survival of plants in native areas, that natural enemies have less impact in the introduced than in the native range, and that the release from natural-enemy regulation in areas of introduction accounts in part for observed changes in plant abundance. We tested experimentally the enemy-release hypothesis with the invasive neotropical shrub Clidemia hirta (L.) D. Don (Melastomataceae). Clidemia hirta does not occur in forest in its native range but is a vigorous invader of tropical forest in its introduced range. Therefore, we tested the specific prediction that release from natural enemies has contributed to its ex- panded habitat distribution. We planted C. hirta into understory and open habitats where it is native (Costa Rica) and where it has been introduced (Hawaii) and applied pesticides to examine the effects of fungal pathogen and insect herbivore exclusion. In understory sites in Costa Rica, C. hirta survival increased by 12% if sprayed with insecticide, 19% with fungicide, and 41% with both insecticide and fungicide compared to control plants sprayed only with water. Exclusion of natural enemies had no effect on survival in open sites in Costa Rica or in either habitat in Hawaii. Fungicide application promoted relative growth rates of plants that survived to the end of the experiment in both habitats of Costa Rica but not in Hawaii, suggesting that fungal pathogens only limit growth of C. hirta where it is native. Galls, stem borers, weevils, and leaf rollers were prevalent in Costa Rica but absent in Hawaii. In addition, the standing percentage of leaf area missing on plants in the control (water only) treatment was five times greater on plants in Costa Rica than in Hawaii and did not differ between habitats. The results from this study suggest that significant effects of herbivores and fungal pathogens may be limited to particular habitats. For Clidemia hirta, its absence from forest understory in its native range likely results in part from the strong pressures of natural enemies. Its invasion into Hawaiian forests is apparently aided by a release from these herbivores and pathogens.

Effects of native pigs (Sus scrofa) on woody understorey vegetation in a Malaysian lowland rain forest

Large mammals often play important roles in determining the struc- ture and composition of plant communities. This study focused on the extent to which wild pigs (Sus scrofa) influence the dynamics of tree seedlings and saplings in a lowland rain forest at Pasoh Forest Reserve in West Malaysia. Native wild pigs are common in the study area and may significantly influence growth and survivorship of woody plants in the understorey through several activities - namely, nest building, soil rooting and seed predation. To test experimentally the impact of pigs on the plant community, eight 49-m 2 exclosures were constructed. After 2 y, the number of recruits inside exclosures was three times greater than in unfenced control plots. Stem density was highly correlated with species richness, which also increased significantly inside exclosures. Height growth of plants was greater in the exclosures by 52.5% for trees between 1 and 7 m tall. Trees less than 1 m tall, however, exhibited no differences in growth. Mortality of plants also did not differ between treatments. The observed differences between exclosure and control plots can be attributed to soil-rooting and seed predation, suggesting that these two behaviours of wild pigs are important to plant dynamics in the understorey.

Impacts of nest construction by native pigs (Sus scrofa) on lowland Malaysian rain forest saplings.

Isolation of remnant forest patches, coupled with anthropogenic changes in the surrounding landscape, often leads to changes in the population density of forest-dwelling mammals. At Pasoh Forest Reserve in Peninsular Malaysia, densities of native wild pigs (Sus scrofa) are 10–100 times greater than historical levels due to the local extinction of feline predators and the presence of abundant food sources in areas adjacent to the forest. Female pigs build reproductive nests out of understory vegetation; at Pasoh these nests are constructed primarily of woody saplings several meters tall that pigs snap or uproot, causing substantial local damage. We documented the prevalence of nest building by pigs at Pasoh within a 25-ha area from 1995 to 1998 and investigated the impacts of this mammalian behavior on the understory plant community. In total, 643 pig nests were enumerated, providing an estimate that 6.0 nests·ha−1·yr−1 were constructed in the survey area. Pigs avoided constructing nests adjacent to trails but otherwise built nests throughout the 25-ha survey area. A single pig nest contained, on average, 267 ± 86 (mean ± 1 sd) woody saplings, of which 45% had been uprooted and 55% had the main stem snapped. The understory area affected by the construction of a single pig nest averaged 244 ± 112 m2, with 53% of all free-standing woody plants ≥70 cm tall and <2.0 cm dbh in this area being uprooted or snapped. As a result of nest building, pigs caused an estimated 29% of the observed tree mortality of saplings 1–2 cm dbh, and 43% of sapling mortality and damage combined. Pigs affected plant families differently, with individuals from the economically and ecologically paramount Dipterocarpaceae being about two times more likely to be used in nest construction than other taxa. Our results indicate that pig nest-building activities are a major source of sapling mortality, and that their effects probably will result in substantial shifts in tree community composition in this forest.

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 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.

Woodpecker Participation in Mixed Species Flocks in Peninsular Malaysia

Abstract Woodpeckers (family Picidae) are widely distributed and have been documented as participants in mixed species flocks in both temperate and tropical regions. However, they usually are not considered frequent flock participants, and the presence of multiple species of woodpeckers participating simultaneously in flocks is virtually undocumented. We determined the frequency of this association for the six most common woodpecker species at Pasoh Forest Reserve, West Malaysia. During the two-month study period, woodpeckers participated regularly in flocks with other woodpecker species, including congeners. Observations of multiple species of woodpeckers participating in foraging flocks in other parts of Malaysia, as well as Thailand and Indonesia, suggest that this might be a regular occurrence in Southeast Asia.

Woodpeckers (Picidae) at Pasoh: Foraging Ecology, Flocking and the Impacts of Logging on Abundance and Diversity

Woodpeckers are extremely diverse in the lowland rain forests of Southeast Asia. In these forests, up to 16 species can be found, representing the highest syntopic diversity for woodpeckers in the world. Understanding how foraging resources are partitioned in this species-rich guild is of both ecological and management concern. We investigated the foraging ecology of 13 species of woodpecker at Pasoh Forest Reserve (Pasoh FR) from May-July 1998. We obtained interpretable sample sizes for four species (Meiglyptes tristis, M. tukki, Picus mentalis and Reinwardtipicus validus). These four species exhibited distinct foraging behaviors and partitioned resources by using different substrates or attack maneuvers, or by foraging at different heights. Although the overall abundance of woodpeckers was similar between logged and unlogged forest, there were significant differences in the relative abundance of individual species. The most likely reason for these differences is the lack of heterogeneity in the mature logged forest compared to the unlogged forest. The relatively low abundance of snags, treefalls, and treefall gaps in the logged forest probably limit the number of woodpeckers that use these resources. While conducting this study, we also documented flocking interactions among several woodpecker species, and between woodpeckers and the Greater Racket-tailed Drongo (Dicrurus paradiseus). Understanding the complex foraging associations of this diverse guild warrants further study.