The science and uncertainty of estimating the impacts of white-nose syndrome in North American bat populations.

Eastern small footed bat with WNS, credit Ryan von Linden/NYDEC

Recently, the U.S. Fish and Wildlife Service released an updated estimate for the number of bats that have died as a result of white-nose syndrome. This estimate, that at least 5.7 to 6.7 million bats have been lost to this terrible disease, represents a considerable and alarming increase from the previous estimate. But sadly, to many us who have been working on WNS for the past few years, I don’t think the number came as a surprise.

The last time the scientific community got together to assess how many bats had died from WNS was 2009, just three years after the disease was discovered in New York. Significant mortality events had been documented in New York and western New England, but the disease had yet to spread into most of the now-affected area.

Healthy gray bats, credit Ann Froschauer/USFWS

Since that time, we have been through two additional hibernation seasons with extensive geographic spread and mortality events. And as we head into yet another winter, the outlook isn’t good for bat populations across the affected area. From Ontario and New Brunswick south to North Carolina, Tennessee, Ohio, Indiana, and Kentucky, early reports of new sites and bats exhibiting the telltale signs of WNS have already been coming in. Across the rest of the already affected area, the disease continues to spread and affect bats that have managed to thus far avoid the fate of the missing and the dead among their ranks.

It is difficult to wrap our brains around a million of any one thing, let alone 5.5 million, and it is particularly difficult if that something is as elusive and mysterious as a bat. I’ve been working with bats for about seven years, and it wasn’t until my first trip last year to a grey bat cave with over 200,000 hibernating bats that I could even visualize what “a million” hibernating bats could look like. And unfortunately, a visit to Aeolus Cave in Vermont showed me the devastating aftermath of what losing 300,000 bats in one cave to white-nose syndrome looks like, too.

Bat remains in Aeolus Cave, credit Ann Froschauer/USFWS

The reality is that we don’t know much about our hibernating bats. There are hundreds of thousands of caves and mines in North America, the majority of which have never been surveyed for bats. Unfortunately, in the east WNS moved so quickly we will never know exactly how many bats there were. But what we do have is solid population data from many sites and the expert opinions of state and federal bat biologists and bat researchers- some with over 30 years of experience working with bat populations- to help us understand what the pre-WNS hibernating bat population likely was.

The question “how many bats have died?” is a vexing one, so I asked Dr. Jeremy Coleman, the National White-Nose Syndrome Coordinator, to explain- from the scientific perspective- the processes and challenges to estimating the mortality of bats from this terrible disease.

WNS mortality estimates, January 2012.

White-nose syndrome (WNS) is an infectious disease responsible for unprecedented mortality in hibernating bats in eastern North America since its discovery in 2007. In 2009 WNS was known to exist in nine states, with mass mortality largely restricted to portions of New York, Vermont, Massachusetts, and Connecticut. At that time, it was estimated that losses from WNS exceeded one million bats (BCI 2009). Since 2009 the disease has continued to spread, and WNS, and/or the causative fungus Geomyces destructans (Lorch et al. 2011), has now been detected in 19 states and 4 provinces. Population declines have continued at most affected sites, with 85-100% losses reported at many winter hibernacula. There are some hibernacula and maternity colonies in the affected area where remnant populations of bats continue to be observed. We are working to improve our understanding of the nature of these populations and to document their numbers. As the majority of these animals are not banded or marked, however, it is premature to infer that these bats are likely to survive with WNS, or that they will be capable of maintaining viable populations that could eventually lead to the recovery of affected populations.

Decades of targeted monitoring of federally listed species has bestowed confidence in the consistency of our assessments of Indiana bats (Myotis sodalis) and other listed bats, however we are limited in our knowledge of other cave-hibernating bat species at the landscape-scale. Largely because of the monitoring efforts for listed species, we do have consistent counts of non-listed bats at many hibernacula in the eastern United States. These historic data have allowed us to assess population declines at specific WNS-affected hibernacula, and thus to ascertain the relative impacts of WNS on bat populations by species (see Turner et al. 2011). Assessing total population losses in terms of numbers of individual bats, however, poses a considerable challenge given the relative lack of pre-WNS count data for the bat species that were, until recently, considered “common.” Additionally, there are tens of thousands, possibly hundreds of thousands, of abandoned mines and natural caves in the affected region that have never been surveyed. There are also several known hibernacula that have not been surveyed post-WNS in order to minimize disturbance to infected bats. Unfortunately, it was with the arrival of WNS that several little brown bat (Myotis lucifugus) hibernacula of considerable size were discovered, when dead and dying bats appeared on the landscape in large numbers. We cannot know the number of bats that inhabited such sites, or the number of sizeable hibernacula that have gone undetected. For these reasons, efforts to determine pre-WNS bat populations and/or the number of bats that have died from WNS are rife with uncertainty.

In hopes of better understanding the greater ecological impacts of WNS, and to convey the gravity of the population-level effects of this disease on affected bat populations, on 12 January 2012 we convened a group of approximately 20 bat biologists (state and federal agency personnel and academic researchers) to update the estimates of total mortality to date. We employed 2 different methods to estimate total mortality, and both are reflected in the numbers we released on 17 January of 5.7-6.7 million bats lost to WNS through 2011. The first estimate was generated using the same general methods used in 2009, when we estimated that over one million bats had died. This approach is based on known losses of bats at hibernacula, and the expert knowledge of state bat biologists regarding the proportion of the total bat population captured by their assessments. Estimates of mortality in 10 states were tallied to derive a total estimate for WNS-affected states in the northeastern U.S. The second method involved estimating the total bat population in affected states based on published estimates of bat populations by species. Observed declines reported by Turner et al. (2011) were then used to estimate total losses by species, which were then tallied. Lastly, in consultation with Canadian partners, we estimated bat losses in affected provinces based on estimates from WNS-affected states and the assumption that bat densities in the affected region of Canadian provinces are comparable to those in bordering states. This assumption is based on summer capture data and knowledge of wintering populations and habitat.

The estimates of bat mortality released by the U.S. Fish and Wildlife Service on 17 January 2012 are based on the best available data and derived by experts in the field. Given the uncertainties inherent in the development of this assessment, as detailed above, we felt it was important to both replicate the methods employed in 2009 for consistency, but also to employ an alternate approach to provide a range of values. The methods we used for this exercise preclude the generation of confidence estimates, but it suffices to say that all involved were generally satisfied by the relative agreement of the estimates generated by the different methods. We are preparing a manuscript for publication in a peer-reviewed journal to provide a detailed treatment of the data and a refinement of the methodology. As part of the national response to WNS, we are also developing guidance for a national bat population monitoring strategy that will help to standardize data collection and improve our ability to track the impacts of WNS on bat populations as the disease continues to spread.

Literature Cited:

Bat Conservation International (BCI). 2009. White-nose Syndrome Science Strategy Meeting II, May 27-28, 2009, Austin, Texas. Bat Conservation International. Austin, Texas., accessed 2 February 2012.

Lorch, J. M., C. U. Meteyer, M. J. Behr, J. G. Boyles, P. M. Cryan, A. C. Hicks, A. E. Ballmann, J. T. H. Coleman, D. N. Redell, D. M. Reeder, and D. S. Blehert. 2011. Experimental infection of bats with Geomyces destructans causes white-nose syndrome. Nature 480:376-378.

Turner, G. G., D. M. Reeder, and J. T. H. Coleman. 2011. A five-year assessment of mortality and geographic spread of white-nose syndrome in North American bats and a look to the future. Bat Research News 52:13-27.