Computer-linked bat detectors were used to record the number of high-frequency echolocation calls (inaudible to human ears) at various habitat types on each site. Each organic farm was paired with a similar conventional farm not more than five kilometers (three miles) away, and bat surveys were completed on consecutive nights to control for environmental variation.
The team found that not only was the total bat activity 61 percent higher on organic farms tested, but also that foraging activitydetermined by a special type of call, known as a feeding buzz, which bats emit when attempting to catch preywas 84 percent more common.
The most significant differences in activity were found above aquatic habitats, such as ponds or streams. One threatened species, the lesser horseshoe bat (Rhinolophus hipposideros), was only detected on organic farms.
Bat numbers may be linked to factors such as water quality and hedgerow height, but are most likely linked to differences in insect abundance, said Wickramasinghe.
A further insect-trapping study, carried out concurrently by researchers, disclosed that insects were 64 percent more abundant on organic farms versus conventional ones. The researchers also found that out of 18 insect families which bats are known to rely on for food, including moths, beetles and flies, five were significantly more common in organic habitats.
Different bats specialize on insects of varying sizes and use distinct call frequencies to seek out their prey, commented British bat expert Allyson Walsh, currently director of the non-profit Lubee Bat Conservancy in Gainseville, Florida. "Some bats eat itsy-bitsy insects. Others eat huge beetles. Thus a greater range of flying insects of varying sizes attracts a greater diversity of bats," she said, noting that the situation is likely to be the same in the U.S.
"One alternative to pesticides is to turn the tables and attract bats to your farm," she said. "In the U.S., farmers have been putting up bat boxes and successfully reducing the need to spray to control insect pests." Walsh pointed to one project where electronic "fake bats" were set up in Texas cornfields to emit mock echolocation calls. These devices successfully reduced the number of moth eggs laid, either by attracting more bats, or spooking moths into laying elsewhere, she said.
The studies add evidence to the argument that minimizing pesticide use, however accomplished, offers clear advantages for biodiversity.
Organic enterprises could have unintended consequences for bat conservation, said Wickramasinghe. "Less intensive farming benefits bats, and as the number of organic enterprises increase it may help to reverse bat population declines," she said.
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