Pollinating and seed-dispersing bats get research attention and well-deserved appreciation for their role in maintaining tropical forests. But Margareta Kalka says insect-eating bats of the tropics have long been overlooked and under-appreciated. So she conducted what is apparently the first experimental research to quantify the importance of bats in protecting plants from insects in forests and cacao plantations in Panama.
Kalka, affiliated with the Smithsonian Tropical Research Institute in Panama while pursuing her Ph.D., was supported in part by a BCI Student Research Scholarship funded by the U.S. Forest Service International Programs.
Part of the problem in understanding bats’ values in the tropics, she found, involved a consistent flaw in previous field experiments that examined the role of birds in reducing insect abundance and damage. The studies compared damage to plants that were covered with nets to keep birds away with that of similar plants that were not netted. The difference between netted and open plants constitutes the damage prevented by predators. Kalka noted, however, that the nets were “left in the field day and night and therefore excluded bats [at night, as well as birds during the day], but attributed all reductions in [insect] abundance and damage only to birds.”
So she set up three plant groups for study: one covered with mesh netting during the day but not at night, one netted only at night and a control that was always open to both day- or night-flying predators. Over a period of about six weeks, she gauged damage by taking pictures of leaves and examining them with image-analyzing software. She counted insects on the plants both day and night at 10-day intervals.
Her results strongly indicate that bats reduce insect abundance and damage to tree saplings in her study area. Plants “without bat protection” had 149 percent more insects per square meter than control plants. The night-netted plants suffered more than three times the damage of control plants, while those that birds could not reach received only about twice the damage. Her research did not distinguish among several species of insectivorous bats.
At an organic cacao farm (cacao seeds used to produce chocolate), Kalka placed naturally occurring caterpillars on “platforms” of cacao leaves wired to branches. Two platforms were erected in each of 45 trees, one exposed to all predators and the other surrounded by mesh that kept birds and bats away. Half the platforms were loaded with caterpillars at sunrise and checked each evening to determine how many had been eaten by birds; the others were loaded at sundown and checked at dawn for bat activity. Bats consumed more caterpillars than birds, although the results, while strongly suggestive, fell just short of statistical significance. They nonetheless document that bats consume cacao-damaging caterpillars.
Kalka clearly demonstrated the long-overlooked importance of insect-eating bats in maintaining tropical forests and found indications of a similar role at cacao agroforests. Understanding this ecological and economic value should, Kalka hopes, reduce the persecution of bats in the region and lead to their inclusion in future conservation plans.