White Nose Syndrome, a fungal infection that kills bats by interfering with their hibernation cycle, was first spotted in a cave in New York in 2006. In just four years, it has spread over 1,200 km through the US and Canada, reaching from Quebec to Missouri, and killing off as many as 90 percent of the bats in infected areas. Those precipitous declines would seem to be unsustainable, and a new study in Science indicates that they are indeed: even in many scenarios where the virulence of the disease tails off, a common species of bat appears headed for regional extinction, perhaps in as little as 15 years.

The species in question is the little brown bat (Myotis lucifugus), which is among the most common in North America, and lives in the areas being ravaged by WNS. At least six other species of bats have been affected by WNS, but the little brown bat has an advantage for scientific studies: there are 30 years of data on the animal's numbers at major hibernation spots, and another 16 of breeding and mortality data from a single site. These can be combined to give decent population estimates, and integrated into a model of the impact of WNS.

Prior to the disease's appearance, all data indicate that little brown bat populations were benefitting from conservation measures, as most populations were either stable or expanding. Things changed rapidly late in this decade, as the fraction of infected hibernation spots shot up from five percent to nearly 60 percent in only three years. This trajectory of spread was also incorporated into the population model, as were the mortality rates generated from the first few years of studying WNS.

Even assuming a reasonably good case—population declines remain static at the most recent year's average of 45 percent—the population declines will be radical. In 99 percent of the model runs performed by the authors, regional populations of bats went extinct within 16 years.

Even assuming that the rates of WNS-induced killing declines, the situation may end up grim for the little brown bats. If death rates decline to only 10 percent over time, there's still a greater than 90 percent chance that there will be regional extinctions; it'll just take longer (within 65 years). To actually significantly reduce the chances that regional extinctions will hit, mortality will have to drop below five percent. Even under that scenario, the population will have declined from about 6.5 million bats to about 65,000.

That alone could have radical effects on the Northeast's ecosystems, given the role of bats in cutting down the insect population. This is especially true given that the other species that live in the same range as the little brown are experiencing similar declines.

The authors also consider what this pattern of infection and mortality says about the fungus that causes WNS, the ominously named Geomyces destructans. They conclude that this sort of spread suggests a pathogen that has only recently been introduced to North America, and refer to a report of the fungus having been found in Europe to indicate that it probably travelled via a human vector. To prevent it from spreading further, the US government is planning on ending human access to caves that are frequented by bats.

It's difficult to predict how accurate the models will end up being. It's possible that a pool of Geomyces destructans resistant bats already exists in the North American population, and will become apparent in the form of lower rates of mortality in affected regions. But such a pool had better appear quickly, given the pace of current declines. Another key issue will be the geographic range within which Geomyces destructans thrives. That will determine whether there are significant bat populations in nearby regions that can recolonize the area when WNS dies down.

Science, 2010. DOI: 10.1126/science.1188594 (About DOIs).

Listing image by Ryan von Linden