The Influence of Man-made Trails on Foraging by Tropical Frugivorous Bats

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The diversity and abundance of fruit-eating bats in the neotropics reflect bats' importance as frugivores in this region. Frugivorous bats may fly several kilometers per night in search of patchy food resources, and many bats forage as they commute (Fleming et al. 1977). Such nightly movements are typically made along well-traveled flyways, which in dense tropical forest often include man-made trails (e.g., Tuttle 1976). Understory fruits along trails are exposed to the many bats using trails as flyways, whereas understory fruits away from trails should be exposed to fewer bats. We hypothesized that due to greater exposure, ripe fruits along trails would be taken more often by bats than ripe fruits away from trails. To test this hypothesis an experiment was conducted for four nights in dry forest at Parque Nacional de Santa Rosa, Provincia de Guanacaste, Costa Rica (in February 1982), and for nine nights in rain forest at Finca La Selva, Provincia de Heredia, Costa Rica (in March 1982). Mist-netting showed that Carollia spp., Glosophaga spp., and Artibeus spp. were the most common fruit-eating bats in both areas. At both sites four pairs of lines were baited with ripe fruits each night. One line of each pair was strung between trees adjacent to a trail and the other line of the pair was strung at the same height approximately ten meters away in an area off the trail. Off trail lines were strung in areas with approximately the same density of vegetation as trails (i.e., very small gaps) in order to minimize differences in detectability based on vegetation density. Each line was baited with five fruits, thus each test night twenty fruits were presented on trails and twenty fruits were presented approximately ten meters away from the trails. Fruits were hung on 10 cm segments of very fine wire, regularly spaced along 2 m lengths of nylon string. These strings were placed 1.8 m above the ground, and fruits were placed away from branches in order to exclude arboreal rodents. Lines were not placed near other fruiting vegetation. Although different bat-dispersed fruits were used during the experiments (Muntingia calabura, Solanum rugorum, and Ficus spp.), on any given night the fruits on all lines were from the same species of plant. The lines were baited just after sunset and the number of fruits taken was rallied just before sunrise. At La Selva the location of the lines was changed three times to avoid the effect of learning by bats. At La Selva, Wilcoxon's signed rank tests were used to compare the number of lines visited and the number of fruits taken by bats each night (Table 1). Significantly more lines were found by bats on the trail (P < .01), and more fruits were taken there than off the trail (P < .01). The data obtained in the dry forest at Santa Rosa showed the same trend, but these data were not analyzed statistically since the Santa Rosa experiment ran only four nights. The fact that bats more readily find fruits along trails may have several implications for tropical biology, since researchers working in dense forest tend to concentrate their work along trails. In particular, studies of fruit removal by bats may lead to overestimation if conducted along trails. It is not dear how the presence of trails used as flyways influences the dispersal of such fruits. Bats usually drop seeds and uneaten fruit parts near source plants (Fleming 1981), but they may also do so while commuting; Janzen et al. (1976) found that bats dropped fruits along a fence row used as a commuting landmark. Flight patterns of birds have been shown to influence seed dispersal; in a study at La Selva, Howe and Primack (1975) found a higher density of seeds and seedlings along the flight directions more frequently used by birds when leaving the source tree. However, our data are not suitable to test whether a batðgenerated seed shadow is present along trails used as flyways; further studies need to be done to determine the effects of bats on trail vegetation. Meanwhile, in order to prevent bias, caution should be taken in biological studies done along or near trails in tropical forests.

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Original version is available from the publisher at: http://www.ots.ac.cr/bnbt/2192.html