White-nose Syndrome is likely to kill 86 percent of Indiana myotis within nine years, as this endangered eastern U.S. bat species become functionally extinct across much of its range, according to a highly sophisticated computer model.
|Indiana myotis (Myotis sodalis) Photo © J. Scott Altenbach / BCI
This bleak prediction for Myotis sodalis, which was listed as endangered in 1967, emerged from a team of scientists led by Wayne E. Thogmartin of the U.S. Geological Survey’s Upper Midwest Environmental Sciences Center in Wisconsin. The study was published in the journal Biological Conservation.
Indiana myotis are found throughout the Eastern United States from Midwest and upper New England states and into northern Florida. The largest hibernating populations occur in just three states: Kentucky, Missouri and Indiana. Human disturbance and alteration of hibernation caves, loss of summer habitat due to deforestation and pesticide poisoning all contributed to the decline of the Indiana myotis before the arrival of White-nose Syndrome.
The computer model incorporated a range of environmental and population variables to help predict the probability of the endangered species’ survival in the face of WNS. It focused only on female bats. The study began with a range-wide population estimate of 211,000 Indiana myotis females. By the year 2022, the model predicts, only about 29,000 will remain, although numbers are predicted to climb slowly from that low point.
A crucial concept in the study is the “extinction threshold,” which concludes that a hibernating population of fewer than 250 female bats is too small to maintain itself. By 2022, the scientists said, “only 12 of the [species’] initial 52 wintering populations were predicted to possess the breeding females necessary to sustain their populations.”
“We estimated that more than 90 percent of current wintering populations [of Indiana myotis] are predicted to experience White-nose Syndrome within the next 20 years,” Thogmartin wrote.
For 50-year persistence predictions, the team examined two possibilities: a “best-case scenario” in which surviving bats become immune to WNS, and another in which the disease continues unabated. (Acquired immunity has been suggested, but has not been documented.)
If bats do become immune to the fungus that causes WNS, the study predicts that after half a century, 3.7 percent of wintering populations would be above the 250-female threshold following a 69 percent population decline among female Indiana bats – to less than 65,000 from the current 211,000. Without acquired immunity, just 43,000 female Indiana myotis would exist in 50 years.
The authors stress that their projections are somewhat tentative, given a number of unanswered questions about both the bats and the disease. Even the current number of Indiana myotis is somewhat uncertain, as is the bats’ movement – or lack of movement – among different populations. A key question is exactly how shrinking populations will affect the survival of Indiana myotis, which typically hibernate in very large colonies.
The study ended on a moderately upbeat note: “Our predictions are that Indiana bats will persist for at least the next half century, albeit at greatly reduced numbers. Whether the species can recover from WNS is predicated upon whether populations can grow and fill in depleted portions of the range.”
The future is far from certain, but post-WNS recovery efforts will be critical to the survival of this beleaguered species.
Other authors of the study include: Carol A. Sanders-Reed and Patrick C. McKann, IAP World Services; Jennifer A. Szymanski, Lori Pruitt and R. Andrew King, U.S. Fish & Wildlife Service; Michael C. Runge and Robin E. Russell, U.S. Geological Survey.