Insect-eating bats get no respect in the tropics. The research focus and (well-deserved) credit for helping to maintain healthy tropical forests goes mostly to pollinating and seed-dispersing bats, while the insect eaters have long been overlooked and under-appreciated. To rectify that situation, I conducted the first experimental research to quantify the importance of bats in protecting plants from insects in forests and chocolate-producing cacao plantations in Panama.
My results demonstrate that not only do bats significantly reduce insect numbers and damage, but their beneficial impact can be significantly greater than that of birds, which have long been credited with helping to control damaging insects. This research was supported in part by a BCI Student Research Scholarship funded by the U.S. Forest Service International Programs.
Bat roles in reducing insect damage in the tropics have been overlooked primarily for two reasons: shortcomings in traditional methods of analyzing the diets of bats and a research focus on insectivorous birds without accounting for the contributions of bats.
Bat diets are typically determined by identifying insect remains extracted from feces or found beneath roosts. That, however, systematically underestimates the proportion of soft-bodied prey, such as caterpillars and immature katydids, that leave few identifiable remains.
A recent study I conducted with Professor Elisabeth K.V. Kalko of the University of Ulm, Germany, recorded infrared video of bat-foraging behavior, documenting that plant-eating insects account for more than 70 percent of the diet of the common big-eared bat (Micronycteris microtis) in a tropical lowland forest in Panama. (This is a gleaning bat that takes its prey off surfaces, such as leaves.)
More importantly, we showed that almost 50 percent of prey items, mostly plant-eating insects, would not have been detected by traditional methods of bat-diet analysis. Since that is the source of most data on bats diet, other insectivorous bats probably also eat many more insects than previously estimated.
Appreciation for insectivorous bats has also been limited because most research into the beneficial role of insect predators has focused on birds. This was highlighted by several recent studies in Panama, which concluded that birds significantly reduce insect damage to plants in natural forests and cacao plantations. Tree branches were covered with mesh nets to exclude insect-eating birds while allowing insects to pass through. Insect densities and damage were compared to that of uncovered control trees that were fully accessible to birds. The difference between netted branches and control plants represents the protective impact of predators.
But because the exclosures were left in place both day and night, the studies actually excluded both birds and bats, although they attributed all reductions in insect abundance and damage only to birds.
To correct that shortcoming, I designed an exclosure experiment that distinguishes between the impacts of bats and birds. I selected 35 sets of three young plants each in a lowland tropical forest on Barro Colorado Island, Panama. I subjected each set to three different treatments: nocturnal bat exclosures were covered from sunset to sunrise; daytime bird exclosures were covered from sunrise to sunset; and controls were left uncovered day and night. I also established seven additional pairs of plants, each with only nocturnal exclosures and controls. The exclosures were made of PVC frames covered by agricultural netting with holes big enough to allow access to insects and other arthropods, but not bats and birds.
During the 10-week study, I measured insect abundance six times during the day and night at 10-day intervals. At the end of the study period, I assessed insect damage by taking digital pictures of leaves and examining them with image-analysis software.
My results in natural forests strongly indicate that bats reduce insect abundance and damage to tree saplings. Night-netted plants without bat protection had 153 percent more insects per square meter (10.7 square feet) than control plants that were exposed to bats. Plants that birds could not reach had 65 percent more insects than uncovered controls. And plants without any access to bats suffered more than three times as much damage as control plants and almost twice that of plants where only birds were excluded.
To determine whether bats provide similar protective services to a commercially valuable plant, I undertook a six-week experimental study at an organic cacao farm on the mainland in Panamas Bocas del Toro Province. More than 3.5 million tons of cacao seeds, which are used to produce chocolate, are harvested each year, primarily in Africa, South America, Indonesia and Mexico.
I placed caterpillars on platforms of cacao leaves attached by wires to branches. The wires were treated with a sticky substance (called Tanglefoot) to exclude ants and keep the caterpillars from escaping.
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.
Predation rates were more than three times higher at night than during the day, indicating that bats are more active predators of caterpillars in cacao farms than birds. My preliminary results document that bats consume cacao-damaging caterpillars. I hope these preliminary results encourage further research to quantify the value of insectivorous bats throughout the tropics.
I discussed my work almost daily with people living in the study area. Most depend on cacao or plantain farming and considered bats detrimental to their crops and dangerous to themselves. Showing them the damage that bats are preventing may have begun the process of changing those negative views. My findings also made their way into the curriculum of an environmental education program being developed by local schools and the Smithsonian Tropical Research Institute in Panama.
With the importance of insect-eating bats in maintaining forests and cacao plantations now established, it is time for bats finally to be included in conservation and management plans for natural and commercial forests of the tropics.