Thank the Bats for Organic Coffee
Good Java Courtesy of Mexican Long-eared Bats
Coffee is a staple food for most people, as well as being a gourmet delight, especially shadegrown organic coffee. Who knew we had bats to thank for the great taste?
Organic coffee plantations in Mexico must not use pesticides, but the crops attract a range of leaf and flower eating insects. More than 200 species of insects feed on, or can otherwise damage, coffee plants. To control the pests, growers depend on natural predators, and until recently gave all the credit to local birds.
Now, 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.
The researcher, Kimberly Williams-Guillén, a tropical ecologist and a postdoctoral fellow at the U-M School of Natural Resources and Environment, said these bats use a different hunting technique from the swooping we usually associate with bats.
"Bats are impacting ecological systems in all kinds of ways, and I just want them to get the credit they deserve," she said.
At a time when bat populations are declining worldwide, this newly recognised benefit to organic coffee farmers is another example of how these much-maligned mammals provide ecological services that go largely unnoticed. As well protecting agriculture, bats pollinate wild plants, disperse fruit seeds, and gorge on disease-spreading mosquitoes.
Netting Stopped Bats’ Nocturnal Hunting
The bat's role in controlling coffee-eating insects has been overlooked for two reasons, Dr Williams-Guillén said. A flaw in the design of ‘exclosure’ experiments used to study the impacts of various animals on coffee plants prevented bats from hunting round the plants.
The exclosures - net-covered wood-and-plastic frameworks - were placed over coffee bushes day and night. 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 were generally given the credit for the reduction on the unprotected plants.
But because the netting remained in place day and night, bats also had been excluded, Dr Williams-Guillén said.
To determine the relative contributions of birds and bats at the Finca Irlanda plantation, Dr Williams-Guillén and her 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, showing the bats’ protective effect exceeded the impact of birds.
Perch and Wait Approach Catches Bugs
The second reason bats had not been recognised as good insect eaters was because their style of hunting is different from the aerial swooping of many bats, who zero in on flying prey using echolocation techniques. About 45 different species of bats have been identified at the Chiapas plantation.
"People believed that all the bats were flying around in mid-air and taking mosquitoes and moths," Dr 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.
"But it turns out that foraging modes in bats are much more diverse than people had thought," she said.
Instead of flying around, these big-eared bats of the Micronycteris and Lonchorhina genuses rely on an approach called foliage gleaning. They patiently ‘perch and wait’ in the tree canopy above the coffee bushes, hanging inverted, 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.
And as the exclosure experiments showed, they are far more efficient than birds at getting a good feed and protecting the crop at the same time.
Dr Williams-Guillén’s research was published in Science magazine in April.
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