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Bug-Eating Bats Help Plants, According To Studies

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While some bats — vampire bats — live up to their Halloween reputation of sucking blood out of animals with their fangs, many more bats subsist on a diet that is more appealing, at least in its effects on the environment.

They eat lots and lots of bugs.

In the tropics, bats eat so many bugs that they put a significant damper on the number of bugs crawling on plants, two studies report in Friday’s issue of the journal Science.

Bug-eating bats are thus a boon to farmers there, reducing the need for insecticides. The findings also highlight incomplete knowledge about the role bats play in many ecosystems.

Using their sonar-like abilities, many bats can snatch insects out of the air in pitch blackness.

There are a lot of bats around, and bats must eat lots of insects,” said Kimberly Williams-Guillen, a postdoctoral researcher at the University of Michigan and lead author of one of the Science papers.

But few scientists have tried to see whether bats have a discernible influence on insect populations. Further, because bats fly around at night, scientists sometimes forget about them entirely.

In earlier experiments, scientists covered some plants with netting and cages, protecting insects from being eaten, and then months and years later, they counted the number of insects on the caged plants versus those on uncaged plants.

Fewer insects crawled on the uncaged plants because, the scientists concluded, birds flying around during the day were eating a lot of them.

Dr. Williams-Guillen wondered if hungry nocturnal bats might perhaps account for part of the insect control effect.

Instead of snatching moths and other flying bugs in midair, some bats hang in a tree and, with their extra-sensitive ears, wait until they hear the chirp of a grasshopper — “or even the sound of a caterpillar chewing on a leaf,” Dr. Williams-Guillen said — and swoop down to snatch the morsel.

At an organic coffee plantation in Mexico, she and her University of Michigan colleagues conducted an experiment similar to the earlier ones, except they compared coffee plants caged only during the day, plants caged only at night, plants caged all day long and plants not caged at all.

What the researchers found was that bats accounted for a large part of the insect consumptions, especially during the wet season in summer, when bats reproduce and mother bats have to eat prodigiously to nurse their offspring.

The opposite is true during the winter months when an influx of migrating insect-eating songbirds arrive from the United States and Canada.

Bats are as important as birds in limiting the insects in these coffee plants,” Dr. Williams-Guillen said.

A second team of scientists led by Margareta Kalka of the Smithsonian Tropical Research Center performed a similar experiment, but in a natural forest in Panama.

No other study had ever measured the effects of bats’ insect consumption in a natural forest,” said Dr. Kalka, who had been among the first to realize the importance of bats in controlling insects on plants, after he observed them with night vision cameras.

The findings could have implications for farmers; the number of bats is believed to be declining in many parts of the world.

It could also influence the method of farming. The coffee plantation studied by Dr. Williams-Guillen grew coffee plants under a canopy of shade trees.

Higher yields of coffee can be grown by eliminating the shade trees — but fewer species of bats frequent such open plantations, and that could mean fewer bats eating the bugs and a greater need for pesticides.

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