Thursday, March 14, 2013

To the Bat-Cave...or not

I thought I'd make me last post about White-Nose Syndrome; even though I mentioned it in an earlier post, I think it's a really important issue and everyone should know about it because it's going to start affecting all of us in the near future. A lot of this text is from a presentation I did a few years ago in Maine, so feel free to skim if it's too long.
There are 45 species of bat in North America, and more than 1,000 in the world. Bats live on every continent except Antarctica. White-Nose Syndrome is caused by the fungal pathogen Geomyces destructans, and is identifiable in hibernating bats by a white, fuzzy fungal growth on the bat's nose, ears, and wing membranes. The fungus can also cause skin ulcers or lesions, and thrives in darkness, high humidity, and low temperature (below 20 degrees Celsius), the conditions found in bat hibernaculums. 

When bats hibernate, they slow their metabolism and lower their body temperature to within a few degrees of ambient cave temperature; in this state, their immune system is weakened, and their inability to fight off infection makes them ideal hosts. Rather than causing the collapse of internal organ systems like most pathogens, Geomyces manifests as a skin infection that becomes a chronic disturbance during hibernation. A bat's wings make up about 85% of the surface area of its body and are used in important procedures like body temperature regulation, blood pressure and gas exchange, as well as flying and feeding. The fungus leeches water out of an infected bat's wing tissue, causing it to wake up early due to thirst. Although bats periodically wake up from hibernation to drink, this process burns up to 90% of a bat's fat reserves. If they have to wake up with greater frequency, they can become emaciated. An infected bat burns all of its fat reserves trying to fight off the fungus, and must either leave the cave to hunt for food before there are enough insects out to sustain it, or starve to death inside.

White-Nose Syndrome was discovered in Howe's Cave in Albany, NY in 2008; within the year it had spread to other caves in New York State, Connecticut, Massachusetts and Vermont with an infection rate of 90%. By 2009 it had reached New Hampshire, Pennsylvania, New Jersey, Virginia and West Virginia, and by March of 2010, it had been found in Tennessee and Oklahoma, all the way up into Ontario, Quebec and New Brunswick. As of this year the disease has killed over one million bats, spread to 19 states and 4 Canadian provinces, and continues to move westward. Howe's Cave is visited by tens of thousands of tourists each year and the disease has grown outwards in a way that suggests Geomyces might be an exotic species recently introduced to the U.S., but researchers don’t know if this is the case or if it was already present and began infecting bats for some other reason. WNS cannot spread to humans, but we are active transmitters of it: the fungus can hitch a ride on caving equipment or visitors’ clothes, but can also be spread by bat species that don’t return to the same cave every winter. The fungus can survive in dead bats on the floor of the cave, and doesn't die with its host like most parasites. Similar fungal growth on bats has been documented in Europe, but without the subsequent mortality levels. WNS could potentially have come from Europe, where Old-World bats had developed an immunity to it over thousands of years and infected New-World bats, who had no resistance to the disease, or the fungus could have been present in North America all along, but only recently mutated to infect bats in this way. 

Some of the species affected include big and little brown bats, eastern pipistrelles, northern long-eared bats, Virginia big-eared bats, Indiana bats, and gray bats. The latter three are federally listed endangered species, but all North American bat populations are vulnerable. Most species have an average lifespan of 10-20 years, with only a single offspring per year; infant mortality rate is very high and around 50% of baby bats die during their first winter. A majority of the hundreds of thousands of hibernating bats in New York state inhabit just five caves, and half of the 52,000 Indiana bats that winter in New York return to just one abandoned mine that is now infected with WNS. In addition to their low rate of reproduction, bats migrate hundreds of miles to their summer ranges and can easily spread the infection to other populations along the way. Bat populations don't fluctuate over long periods of times, and it is unlikely that they will recover from such a drastic decline. 

Why should we care about bats? For starters, insectivorous bats have a huge economic value, saving farmers an average of 74 dollars per acre of cropland in the U.S. that would otherwise be sprayed with expensive and toxic insecticides. A single bat eats its own weight in insects every night - about 3,000 mosquitoes that might otherwise spread diseases like West Nile. Bats suppress insect populations, pollinate flowers and disperse seeds, and are overall crucial to maintaining a healthy ecosystem in both agricultural and forestry sectors. Recently, a study was done on the effect of bats on agriculture in Ohio; if WNS continues to kill bats throughout the U.S., farmers in Ohio could suffer losses of anywhere from $740 million to $1.7 billion, depending on how many bats the state loses. In parts of Texas, the estimated value of bats in controlling cotton pests is $1.7 million per year. The estimated total economic value of bats to U.S. agriculture (and potential losses due to WNS) ranges from a low of $3.7 billion to a high of $53 billion.

In an effort to reduce human transmission of the disease, there has been a voluntary moratorium on caving activities in the infected areas; some unaffected states have closed caves in known bat habitat as a preventative measure. Government funding for research of the disease is quite modest, but scientists are trying to find a cure. Captive breeding might become the only option to preserve species severely affected by WNS. Unless we take action, most of our native cave-hibernating bat species could go extinct within a decade. 

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