Bats Boom on Organic Farms, Study Says

John Pickrell in England
for National Geographic News
February 2, 2004
Organic produce, grown on farms that steer clear of typical pesticides
and chemical fertilizers, is becoming increasingly popular. Now, new
studies of British bat and insect numbers bolster claims that organic
farms are popular with wildlife too.

Bat activity may be up to 60 percent greater on organic farms than conventional farms in the United Kingdom, according to a new report. The study also revealed that one endangered European bat was completely absent on non-organic farms tested. A second study, to be published later this year in the science journal Conservation Biology, reveals that insect families which are key to British bat's diets are significantly more abundant and diverse on organic farms.

"Theoretical links between bat declines and the intensification of agriculture have been suggested, but until now there has been little solid data to back up those claims," said Liat Wickramasinghe, a zoologist at the Mammal Research Unit of Bristol University, England, and lead scientist behind the studies.

Though the findings are specific to the U.K., they have implications for insect-eating bats in any intensively farmed part of the world, she said.

Evidence of Declines

Bats' small size and shy nocturnal habits make them a challenge to monitor. But growing evidence suggests that European bats are declining. Six of the U.K.'s 16 species have special action plans in place for their conservation.

The greater horseshoe bat (Rhinolophus ferrumequinum), for example, has disappeared from much of its one-time range in England. Some experts believe its population has declined 90 percent during the 20th century, said Wickramasinghe.

Agricultural intensification has been suggested as a major cause of these declines—more than three-quarters of the U.K. is now farmland. This increased agricultural output has gone hand in hand with the increased use of insecticides and chemical fertilizers, but at the expense of biodiversity.

One report argued that out of 28 farmland birds studied, such as the grey partridge (Perdix perdix), 24 have shown range contractions linked to agricultural intensification since 1970. Insect declines have also been evident, said Wickramasinghe, a particular concern as Britain's bats are insectivorous.

In contrast to conventional farming techniques, organic farms in the U.K. limit the use of chemicals, and are controlled by strict certifying bodies. This makes them an ideal comparison for testing the impact of agricultural intensification.

Acoustic Sampling

As they report in the December Journal of Applied Ecology, doctoral student Wickramasinghe and biologist colleagues at Bristol University measured farmland bat abundance on summer nights in 2000 and 2002. The team used novel computer methods for recording the echolocation calls of bats and identifying them to species on 24 pairs of organic and conventional farms in England and Wales. "Being tiny, fast-flying, nocturnal creatures, it is impossible to get accurate abundance measures of bats directly," said Wickramasinghe.

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 activity—determined by a special type of call, known as a feeding buzz, which bats emit when attempting to catch prey—was 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.

Insect Abundance

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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