They’re reclusive by day and highly mobile by night. So how do we go about counting bats, let alone assessing how many have died due to a mysterious disease?
It’s not easy, says Carthage College professor Deanne Byrnes, recently named chair of the board of directors of the North American Society for Bat Research. To identify bats in the wild, scientists rely on two primary methods: acoustic monitoring, and trapping and marking. Both suffer from shortcomings that leave serious gaps in information; one is too broad in scope, the other too narrow. In the end, Byrnes said, we have no accurate numbers on how many bats exist in the country. “Our estimates have huge error bars around them.… It’s a problem. It’s a big problem.”
The problem isn’t unique to bats. When it comes to surveying wildlife populations, biologists are often making no more than educated guesses. And the implications of those guesses can be immense, influencing important conservation and development decisions.
While not all animals are as difficult to track as bats, many pose unique challenges of their own. Take the desert tortoise, a shy creature that spends about 95 percent of its life in underground burrows. In the California desert, where the rush to develop large-scale solar energy has resulted in habitat loss, accurate population counts are crucial. The species’ long-term survival may depend on it.
Just nine adults – that’s how many mature desert tortoises federal and state agencies anticipated would be killed during the three-year construction of BrightSource’s Ivanpah solar plant. Located on 5.6 square miles of intact habitat just beyond the northeastern border of California’s Mojave National Preserve, the project would also result in the death of 35 juveniles, 139 eggs, and the capture or harassment of another 32 adults.
The plant, slated to be the largest of its kind in the world, was permitted based on these numbers. Yet no one had any idea how many tortoises were really out there, nor how many would be harmed by construction. The numbers were mere speculation based on the best data available.
Within six months of the project’s October 2010 groundbreaking, it became clear that impacts on the tortoise would be far greater than expected. Last April the Bureau of Land Management (BLM), which owns the land, halted construction and commissioned a new biological opinion. The results were shocking, particularly for harder-to-spot juveniles. Instead of 35, as many as 700 young tortoises – up to 90 percent of the projected total – could be killed during construction. The remaining 10 percent, plus 160 adults (about five times as many as originally thought) would have to be transported off-site, potentially resulting in more deaths.
Beware the Bends
While white-nose syndrome poses a risk to many species of cave-dwelling bats in the Northeast, migratory tree bats across the nation face a different type of threat: wind turbines.
Unlike their hibernating cousins, migratory tree bats – which include the hoary bat, the silver-haired bat, and the eastern red bat – do not inhabit caves. Instead they roost in trees and seasonally travel long distances following warmer weather, much like birds. Over the past several years, migratory bats have been found dead by the hundreds and even the thousands under wind turbines during their peak mating and migrating seasons. According to the US Geological Survey, migratory tree bats comprise
approximately three-quarters of the bat fatalities observed at wind turbine sites.
Amy Fesnock, BLM’s chief wildlife biologist in California, who worked on both estimates, attributes the discrepancy to differing methods of assessing desert tortoise populations. For the first survey, biologists walked the property along a series of transects, recorded the number of individuals sighted, then applied a simple ratio of observed to projected individuals to arrive at a final number. Unfortunately, this didn’t properly account for the fact that the animals spend most of their lives underground. Nor did it consider that it was a drought year, meaning that more tortoises than usual were secreted away in their burrows.
The second Ivanpah survey was a bit more sophisticated, relying upon a “life table” to determine how many individuals at different ages the site likely contained based upon the numbers and approximate ages of observed tortoises. It also employed an algorithm to figure in the low probability of being able to detect a tortoise when one happened to be present.
So are the new, higher numbers accurate? Not really – especially for juveniles. “There is no reliable way of estimating the number of animals that are on your site,” Fesnock said. “And that continues to be a problem. That’s where the science is right now. With the science that we have, we cannot provide a reliable estimate of those age classes.”
A number of other solar plants built on desert tortoise habitat, including First Solar’s six-square-mile Desert Sunlight Solar Farm, have been approved using the same flawed processes. Until biologists employ more accurate methods, like using trained dogs to sniff them out – which has been shown to vastly improve detection of young tortoises – the scope of our impacts on these vulnerable animals will remain but an educated guess.