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Bats protect crops from insects

Bats significantly reduce insect damage to plants, report researchers from the Smithsonian Tropical Research Institute and the University of Michigan.

Using nets to control the presence of bats and birds at certain times of the day in a Panamanian rainforest and a Mexican coffee plantation, the scientists found that bats can be more effective pest control agents than birds.

In Panama, Smithsonian Tropical Research Institute (STRI) biologists found that “bats can consume roughly twice as many plant-eating insects as do birds.”

“Uncovered control plants accessed by both bats and birds lost merely 4.3 percent of their leaf area to insect herbivores. When only birds were excluded, plants lost 7.2 percent of their leaf area,” reported STRI. “When only bats were excluded, plants lost a striking 13.3 percent of their leaf area”

The University of Michigan study showed similar results at a 740-acre organic coffee plantation in Chiapas, Mexico.

The researchers say the results demonstrate the ecological significance of bats.

“Bats are impacting ecological systems in all kinds of ways” said Kimberly Williams-Guillén, a tropical ecologist at the University of Michigan who was the lead author of the coffee plantation paper.

Smithsonian Researchers Show Major Role of Bats in Plant Protection
Smithsonian Tropical Research Center – April 3, 2008

Researchers at the Smithsonian Tropical Research Institute report that bats significantly reduce insect abundance and damage on plants. In a lowland tropical rainforest in Panama, bats can consume roughly twice as many plant-eating insects as do birds. This landmark study in the journal Science is the first to compare the ability of bats and birds to protect plants via insect predation in a natural forest ecosystem.

A big-eared bat of the genus Micronycteris. Bats of this genus are found in Mexican coffee plantations, where they glean insects from foliage and help limit pest populations. Photo by Merlin D. Tuttle, Bat Conservation International

A previous study by the authors suggested that bats were underestimated predators of plant eating insects, based on video recordings of feeding events.

In the current study, Smithsonian short-term fellow Margareta Kalka, and co-authors Elisabeth Kalko, institute staff scientist and professor at the Institute of Experimental Ecology at the University of Ulm, and Smithsonian postdoctoral fellow Adam Smith, separated the insect-control effects of bats and birds by placing netting enclosures over five common tropical plant species only at night or only by day. Uncovered control plants accessed by both bats and birds lost merely 4.3 percent of their leaf area to insect herbivores. When only birds were excluded, plants lost 7.2 percent of their leaf area. When only bats were excluded, plants lost a striking 13.3 percent of their leaf area, demonstrating that in the tropical forest understory bats can be more effective pest control agents than birds.

Caterpillars, katydids, beetles and other insects devour tropical plant leaves. Plants directly defend themselves by producing tough leaves and toxic chemicals. Phyllis Coley, STRI research associate and University of Utah professor, who has documented tropical plant defenses for many years, considers this study to be a major contribution: “The role of insect predators, such as birds and bats, is key to plant survival. However, the magnitude of this “top-down” pest control is still not well understood.”

Previously, researchers estimating the top-down effects of birds on herbivory excluded large insect-eaters by placing netting enclosures over entire plants, leaving the nets in place around the clock. By doing so, they quantified the combined effect of birds and bats but attributed it merely to birds.

“Most researchers are outside in the daylight, when they can see birds actively hunting insects. Bats, however, hunt insects at night, which is inherently more secretive and harder to observe,” said Sunshine Van Bael, a Smithsonian researcher involved in earlier exclosure projects.

Kalka speculates that the documented greater effect of bats as insect predators in Panama could be attributed to the absence of migratory birds in the area during the study period. This explanation is supported by a similar study presented by researchers from the University of Michigan in the same issue of Science. There, the authors report a seasonal shift in top-down effects of bats and birds on herbivory of shade-grown coffee plants in Mexico. Birds are more important insect predators in the dry season, when migratory birds are present, but are less important than bats in the rainy season, when migrants are absent.

It is clear from both studies that bats play an extremely important role in the food chain in the tropics and probably in temperate areas as well. Bats should be considered in both conservation planning and in management strategies for agricultural areas.

One Large Organic Shade-grown Coffee, Please—with Extra Bats
University of Michigan – April 3, 2008

If you get a chance to sip some shade-grown Mexican organic coffee, please pause a moment to thank the bats that helped make it possible.

At Mexican organic coffee plantations, where pesticides are banned, bats and birds work night and day to control insect pests that might otherwise munch the crop.

Until now, the birds got nearly all the credit. But a new study from University of Michigan researchers shows that during the summer wet season, bats devour more bugs than the birds at Finca Irlanda, a 740-acre organic coffee plantation in Chiapas, Mexico.

And they often do it using a “perch and wait” hunting technique that is proving to be far more common than bat researchers had believed. A report on the study appears in Friday’s edition of the journal Science.

At a time when bat populations are declining worldwide, this new-found benefit to organic coffee farmers is another example of how these much-maligned mammals provide ecological services that go largely unnoticed. In addition to aiding agriculture, bats pollinate wild plants, disperse fruit seeds, and gorge on pesky mosquitoes by the ton.

“Bats are impacting ecological systems in all kinds of ways, and I just want them to get the credit they deserve,” said Kimberly Williams-Guillén, a tropical ecologist and a postdoctoral fellow at the U-M School of Natural Resources and Environment.

The bat’s role in controlling coffee-eating insects has been overlooked for two reasons, Williams-Guillén said. The first involves a flaw in the design of “exclosure” experiments used to study the impacts of various animals on coffee plants.

In previous experiments, the exclosures—simply net-covered wood-and-plastic frameworks—were placed over coffee bushes around-the-clock. After several days, scientists counted the insects on the protected plants and compared the tally to totals from nearby unprotected plants. The protected plants usually had higher pest counts, and birds generally received the credit.

But because the netting remained in place day and night, bats also had been excluded, Williams-Guillén said. And their impact went unnoticed.

To determine the relative contributions of birds and bats at the Finca Irlanda plantation, Williams-Guillén and her U-M colleagues established four types of exclosures: birds-only excluded during the day, bats-only excluded at night, both excluded day and night, and control plants with no netting.

They found that during the summer wet season, the bat-only exclosures resulted in an 84 percent increase in the density of insects, spiders, harvestmen and mites—exceeding the impact of birds.

Williams-Guillén’s co-authors on the Science paper are Ivette Perfecto of the U-M School of Natural Resources and Environment and John Vandermeer of the U-M Department of Ecology and Evolutionary Biology.

The second reason the bat’s contribution to coffee-plantation pest control had been overlooked has to do with hunting techniques.

Bats are well known for a foraging strategy called aerial hawking, which involves fluttering through the night sky, zeroing in on prey using echolocation, and gulping countless flying bugs. A bat can eat half its body weight in a single night using this technique.

But many of the bats at the Chiapas plantation—about 45 species have been recorded there so far—rely largely on an approach called foliage gleaning. They patiently “perch and wait” in the tree canopy above the coffee bushes, inverted and clutching a branch with their feet, sometimes for hours at a stretch. Their large, pointy ears listen intently for the sounds of insects chewing, crawling across leaves, or chirping.

Then they swoop down and snatch the bug off the leaf or stem.

“People had believed that all the bats were flying around in mid-air and taking mosquitoes and moths,” Williams-Guillén said. “And if that’s all they were going for, then you wouldn’t expect them to have an effect on insects that were just hanging around on the plants,” such as katydids and leaf-eating beetles.

“But it turns out that foraging modes in bats are much more diverse than people had thought,” she said. More than 200 species of insects feed on, or can otherwise damage, coffee plants.

Funding for the U-M Mexico study was provided by the National Science Foundation and Bat Conservation International.

For more on Williams-Guillen

M.B. Kalka et al (2008). “Bats Limit Arthropods and Herbivory in a Tropical Forest” and K. Williams-Guillen et al (2008) “Bats Limit Insects in a Neotropical Agroforestry System.” Science 4 April 2008.

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