Superheroes of the Night Skies

Bats, reservoirs for such viruses as Ebola, are increasingly villainized and require special conservation.

Fruit bat Pteropus seychellensis in the Seychelles Islands (superimposed on a micrograph of the Ebola virus)

Marion Schneider & Christoph Aistleitner (bat), NIAID (Ebola)

Approximately sixty miles southwest of Bangkok, the much-photographed Damnoen Saduak floating market does brisk commercial business. Here, Thai women paddle canoes and ply their fresh flowers, fruits, vegetables, sweets and meats, cold drinks, quickly cooked snacks, hats, bags, and various commemorative merchandise to a daily horde of tourists, myself included, who arrive to take a brief boat ride through traffic-jammed canals. Along the sides of the canals, stalls sell more souvenirs, including popular framed display boxes containing preserved and mounted bats, socalled bat boxes that can also be found for sale online.Watching the tourists clutch their dead wildlife keepsakes under their arms as they pile back into buses and taxis, I’m perplexed and repulsed.

Bat boxes are popular souvenirs in southeast Asia and can easily be purchased online.

Dawn Starin

I question an Australian tourist who has a bat box in hand about his souvenir. “We’re doing the locals a favor by helping them get rid of the bats. Just think of all the diseases they spread—rabies, malaria, cholera, TB,” he says. Another tourist chimes in, “They should be thanking us for buying these bat boxes.”

Bats, the only mammals capable of powered flight, clearly have a bad reputation in many parts of the world. Weighed down with sinister cultural baggage and frequently thought of as simply disease vectors, bats are underappreciated for the crucial role they perform in ecosystems worldwide. Some bats do host several dangerous viruses: fruit bats of the Pteropodidae family, for instance, are considered to be the natural hosts of the Ebola and Marburg viruses. The source of the recent outbreak has not been identified, though some speculation has once again implicated fruit bats. Yet scientists working on the front line, trying to understand and prevent the spread of deadly diseases, are not calling for the eradication of bat populations— rather, for their protection and preservation in the face of extirpation or even extinction.

Daniel G. Bausch, a virologist and associate professor at the Tulane University School of Public Health and Tropical Medicine, works regularly with the World Health Organization. Currently tackling the Ebola outbreak in West Africa, Bausch considers fruit bats to be the likely reservoir for the virus strain that has so far killed more than 3,000 people. Contact most often comes when bats are caught, cooked, and eaten—a regular practice throughout Africa and Southeast Asia. Noting that bats are an important part of the ecosystem, Bausch recommends that “people avoid hunting and preparing bats as food because contact with bat blood or bodily fluids or eating inadequately cooked infected meat may result in human infection with Ebola virus.”

Bats for sale at an Indonesian food market

Suzume Chan

What kinds of ecological consequences could arise if bats were killed en masse? David T.S. Hayman, currently a senior lecturer in Veterinary Public Health at Massey University in New Zealand, has spent much of the last decade studying the transmission among bats of viral diseases, including the filoviruses—two genera of RNA viruses, Marburgvirus and Ebolavirus, that cause severe hemorraghic fevers in primates, including humans. Like other specialists working in the field, Hayman emphasizes the ways in which bats help to maintain healthy forests by keeping insect populations in check, pollinating plant species, and distributing fruit seeds far and wide. He and colleagues explained the case last year: “[P]athogen transfer should be an active area of research in order to develop evidence-based policies to minimize risks, while conserving bats and the irreplaceable ecosystem services they provide.” As a more immediate deterrent to panicky bat slaughter, disease ecologist Fabian Leendertz of the Robert Koch Institute in Berlin, who led the team that tracked the current Ebola outbreak to Guinea, points out that if people living in or near forests attempt to eradicate bat colonies, it would “only increase human contact with potentially infected animals.”

As Ebola spreads through West Africa and bats get blamed, it is also often forgotten—or maybe not known—that increased urbanization, mining, development, and deforestation lead to the displacement of bats and greatly increase the risk of emergence of zoonotic diseases—those diseases that can jump
between species. According to veterinarian and epidemiologist Jonathan H. Epstein, associate vice president of conservation medicine at EcoHealth Alliance, who also studies Ebola, “The majority of emerging zoonotic diseases can be linked to human activities, and that’s where we need to focus our efforts in order to prevent the pandemics. Humans are the major drivers of emerging diseases. Things like agricultural expansion and deforestation . . . and certainly travel and trade—these are things that manipulate our environment and allow pathogens to get from animal hosts to people and then travel around the world.” Epstein adds, “Disease spillover is not a malicious act of wild animals, rather an unfortunate consequence of people continuing to put pressure on ecosystems, creating increased contact with wildlife through urbanization, agricultural expansion, travel, and trade.”

Bats tend to carry and transmit a large number of diseases in part because they live and travel in such large communities. Compared to rodents, for instance, bats host more zoonotic viruses per species. In an article published last year in the Proceedings of the Royal Society, a group of biologists and zoologists (including Hayman) found sixty-one zoonotic viruses in bats and sixty-eight in rodents. Given that there are roughly twice as many rodent species as bat species, the high disease diversity in bats is striking. Still, infection rates remain incredibly low given the amount of bat-human contact.

Diseases aside for a moment, bats provide numerous invaluable services for our ecosystems, our economies, and us. According to Bat Conservation International, more than 300 plant species in the Old World tropics alone rely on the pollinating and seed-dispersal services of bats, and additional bat–plant relationships are constantly being discovered. Upwards of 500 species of plants worldwide likely rely upon bats for these services. Studies done in the neotropics have shown that bats dropped more seeds in the center of large openings in forests than birds did. Fruit bats defecate in flight; thus they often scatter seeds over degraded forests and scrublands, which in turn promotes tree growth and forest regeneration. Quite simply, huge bat colonies of a million individuals can disperse many millions of seeds every night.

Durian fruit, a popular and lucrative crop in Asia, depends upon pollination by bats.

Kalaiarasywikimedia commons

Without bats’ pollination and seed-dispersing services, local ecosystems in a variety of niches could gradually collapse as plants fail to provide food and cover for wildlife species near the base of the food chain. Rainforests, the lungs of the earth, rely on bats to help propagate trees and shrubs. In fact, because of their effectiveness at dispersing seeds into devegetated areas, bats have been described as the “farmers of the tropics.” Desert ecosystems also rely on bats: nectar-feeding bats for pollination and fruit-feeding bats for seed dispersal. Bat-reliant desert plants include organ pipe, saguaro cacti, cardón, agave, and, conservatively, at least another hundred species. On islands with few wildlife species, bats may be the sole pollinators and seed dispersers of local plants; bat biologist William E. Rainey says that in those cases fruit bats play a “keystone” role in forest maintenance and community structure. If they are eradicated, the whole system could disintegrate.

Many important agricultural plants also depend on bats to pollinate their flowers or distribute their seeds to new locations. These include mango, wild banana, avocado, dates, figs, cashews, cloves, peaches, guava, agave, and baobab. The stinky, spiky durian and the lesser known petai bean, two of Asia’s favorite foods, require pollination by bats. These two crops alone are worth an estimated two to three billion dollars. If these crops were to fail, many local agricultural communities could well be out of pocket.

As if that weren’t evidence enough of our reliance on this group of animals, bats also help keep crop pests and mosquitoes in check, and reduce the need for harmful pesticides. Research published in the journal Science concludes that bats save U.S. farmers more than $3.7 billion a year (possibly as much as $53 billion a year) in avoided crop damage and reduced pesticide needs. A single brown bat can catch 1,200 mosquito-size insects in just one hour. In Missouri alone, gray bats eat 540 tons of insects per year, while the 20 million Mexican freetails from Bracken Cave in Texas eat approximately 200 tons of insects nightly. The bats that live in Khao Chong Pran cave in Thailand consume close to 20 tons of insects each night. Far from being spreaders of malaria, as the Australian tourist in Thailand told me, bats eat insects that could otherwise infect humans with various diseases.

Even bat byproducts have proved themselves to be of importance. In caves where bats nest, their droppings, consumed by microorganisms and tiny invertebrates, provide vital nutrients for fish, salamanders, frogs, and other animals higher up the food chain. This bat guano is also one of the world’s best fertilizers, and as such, a big moneymaker. It is a critical resource, both commercially and for subsistence farming in much of the developing world. In Ratchaburi, Thailand, from Khao Chong Pran Cave alone—housing more than two million free-tailed bats—bat guano, collected by local villagers paid by the Buddhist monks of the adjoining temple, sells for an estimated $135,000 annually and supports the temple school. Unfortunately, as in all harvesting of resources, overexploitation can occur, and in some areas irresponsible guano harvesting has damaged many local colonies.

We depend on bats, but do bats need us for protection? Because bats are the slowest-reproducing mammals on earth for their size—females of most species give birth to only one pup per year—they are exceptionally vulnerable to extinction, and not just in the tropics. In eastern North America, white nose syndrome, a fungal disease, has killed at least 6 million bats since 2006 and is now moving westward, threatening the existence of all North American hibernating bat species. So far it has spread to twenty-five states and five Canadian provinces east of the Rocky Mountains, and has caused some populations of bats to decline by more than 80 percent, according to a U.S. Geological Survey study. According to Rob Mies, the executive director of the Organization for Bat Conservation, “Most bat species in the world are in decline. We estimate that about half of the 1,300 known bats are at risk of extinction in the next twenty-five years.”

And sometimes the dangers to bats come from unexpected sources. Bat mortalities at wind turbine sites, for instance, are common; the animals may be struck lethal blows by turbine blades. Alice C. Hughes, who studies the conservation and biogeography of Southeast Asian bats, reports: “Bat species worldwide are endangered by a variety of different threats, ranging from hunting, habitat and roost loss [to] pollution, pesticides, persecution, climate change, and disease . . . On a worldwide basis, our lack of knowledge or even the identity of some bat species often makes targeting conservation measures and assessing population trends difficult or even impossible.”

Bats, which first evolved at least 50 million years ago—perhaps even 100 million years ago—may have shared their world with dinosaurs and been present at their extinction. Having shared our world with bats for eons, are we going to be present at their extinction? Are we going to cause their extinction?

Tourists come to the Congress Avenue Bridge in Austin, Texas, to see the Mexican free-tailed bats (seen flying in the foreground and in the distance).

Flickr/MrLaugh

By purchasing bat boxes or sampling roasted bats, tourists are not just encouraging the trade in wildlife, they are also doing their part, albeit a small one, to generate ecosystem imbalance. The taxidermy is emblematic of environmental decline. Tourists and tourism, however, are not always bad for bats. The potential exists to conserve bat populations while also providing benefits to local communities. One of the best examples can be found at the Congress Avenue Bridge in Austin, Texas [see image above], which is home to a 1.5-million-strong Mexican free-tailed bat colony that generates $10 million in tourism revenue from more than 100,000 visitors every year. Of course, the city also benefits from the estimated 15 tons of insects the bats eat each night.

All things considered, we must weigh the threat of zoonotic diseases hosted by bats against the invaluable role they play in ecosystems around the world. The Food and Agricultural Organization of the United Nations has put out a manual that emphasizes this dichotomy in thinking about bats, but comes to the emphatic conclusion that “the destruction of bats and their habitats represents a far greater risk to human health than the existence of pathogens carried by bats.” Given the benefits to our environmental and economic health provided by these flying wonders, isn’t it time for bats to be recognized as superheroes and not as pests or disease-riddled villains of the night?

--DS

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