Value of Bats in the Ecosystem
Temperate insectivorous bats serve important roles in many ecosystems, providing concomitant ecosystem services of benefit to humans.
- Insectivorous bats are very effective at suppressing populations of nocturnal insects, and some authors estimate the value of bats to the agricultural industry in the U.S. at roughly $22.9 billion each year through the suppression of insect pest species.
- Insectivorous bats are effective top-down predators of forest insects.
- In nutrient-poor environments bats can serve a nutrient “resets,” feeding intensely on aerial insects in nutrient-richer areas (e.g., riparian corridors, ponds, agricultural fields, etc.) and then transporting and depositing nutrient-rich material, in the form of guano, in nutrient-poorer upland roost sites or in caves. In some cases, bat guano may be the sole source of nutrient input for entire cave ecosystems.
Potential declines in populations of bats could have far-reaching consequences across ecosystems and biological communities.
Threats to Bats
Established threats to bats have traditionally included human destruction and modification of hibernacula and other roost sites as well as pesticide use and loss of important foraging habitats through human development and habitat conversion. However, recent emerging threats (white-nose syndrome [WNS] and wind-energy development) have impacted populations of bats at levels without precedent, eclipsing these traditional threats in at least the eastern United States. WNS, first observed in a hibernation cave near Albany, New York, in 2006, has been identified as a major threat to multiple bat species. The disease has swept northeast into Canada and south and west, first along the Appalachian Mountains and then into the Midwest, affecting most major bat hibernation sites east of the Mississippi River and killing an estimated 5.5–6.7 million bats in seven species. Documented declines of heavily impacted populations in the Northeast exceed 80 percent. How the disease will affect western bat species is uncertain. WNS is considered one of the greatest wildlife crises of the past century with many once common bat species at risk of significant declines or even extinction. Wind-energy development is expanding rapidly across the western U.S., and unprecedented mortality rates of bats have occurred recently at many of these facilities; however, widely accepted estimates remain elusive. Despite recent focus on emerging threats, direct impacts to hibernacula by humans remains the single most important conservation concern for bat populations in many areas.
Bat Monitoring on the INL Site
Over the past several decades, research and monitoring of bats have been conducted on the INL Site by contractors of DOE-ID in a somewhat ad hoc fashion. During that time, four theses, three reports, and one publication have been produced by contractors, university researchers, and graduate students. The majority of that research and monitoring occurred in the late 1980s and early 1990s.
Of the 14 confirmed species of bats that reside in the state in Idaho, 11 of those species are documented to occupy the INL Site during some part of the year (Table 1). All 11 of these species may be detected at the INL Site in appropriate habitats throughout the summer season. Three of them are year-round residents and have been documented hibernating in INL Site caves; two of the species are long-distance migrants with increased numbers detectable during fall migration (Table 9-1). An additional two species (western red bat [Lasiurus blossevillii] and Brazilian free-tailed bat [Tadarida brasiliensis]) are not listed as occurring in the state of Idaho and are possible vagrants at the INL Site (Table 9-1). Several bat species detected at the INL Site are considered for different levels of protection by the FWS, Bureau of Land Management, Western Bat Working Group, and other conservation organizations (Table 9-1).
ESER Bat Monitoring Program
Pasive Accoustical Monitoring
To assess bat activity and species occurrence at critical features, a program of passive acoustic monitoring of bat calls was initiated by ESER in 2012. In 2015, ESER continued monitoring bat activity using acoustical detectors set at hibernacula and other important habitat features (caves and facility waste water ponds) used by these mammals (Figure 1). Preliminary analysis of a pilot data set was conducted during 2015 (Figure 9-2). Over one million recorded files of bat calls were analyzed in this effort. Initial species review of these data indicate:
- Summer resident bat community appears to consist predominantly of western small-footed myotis (Myotis ciliolabrum), Townsend’s big-eared bat (Corynorhinus townsendii), big brown bat (Eptesicus fuscus), and western long-eared myotis (Myotis evotis) with some little brown myotis (Myotis lucifugus) and silver-haired bat (Lasionycteris noctivagans) detected at moderate levels at a few locations. Low levels of summer activity of hoary bat (Lasiurus cinereus) were detected through the summer at many features.
- Western small-footed myotis was the most commonly detected bat at all surveyed features.
- Most identified bat species were detected at all features (both facilities and caves). One exception, Townsend’s big-eared bat, was detected at all caves but only at two facilities. The two facilities (Materials and Fuels Complex and RWMC) where Townsend’s big-eared bat was detected are nearer to areas of the INL Site where typical Townsend’s big-eared bat roost habitat (e.g., exposed rock outcrops, caves, and cave-like features) is most common.
- Tree bats (hoary bats and silver-haired bats) were detected more frequently at facilities than caves. Patterns suggest both resident and migrant tree bats occur at INL Site facilities.
The results of our passive monitoring program will provide critical information regarding bat ecology and conservation on the INL Site.
In conjunction with the IDFG, Bureau of Land Management, U.S. Forest Service, and USFWS, the ESER program developed two preliminary active acoustic driving survey transects in 2014 for bats on the INL Site. Survey transects were developed consistent with the North American Bat Monitoring Program, a multi-agency, multi-national effort that is designed to standardize monitoring and management of bat species. Feasibility was assessed and preliminary data were collected on these transects during 2015. High-flying, open-air foragers; big brown bats; and silver-haired bats were detected most frequently on survey routes. We expect that at least one of our driving survey routes will become a North American Bat Monitoring Program participating transect, and that data from these transects can be used by state and federal agencies to better understand regional and nationwide bat population trends.
At least 17 out of 23 caves that are known to exist on the INL Site are used by several species of bats for winter hibernacula, as well as for summer day and night roosts. Lava caves are also an essential habitat during most of the year for three resident species. Much of the historic information concerning bats on the INL Site comes from research that has centered on counting and trapping at caves. In addition to being used as roost and hibernation areas, caves also provide habitat for concentrated patches of insect prey for these mammals. Indeed, in a number of cases, cold-trap crater caves that are too cool during summer to serve as day roosts will have high levels of evening activity as bats focus foraging at these sites. Beyond their use as roosts, caves at the INL Site serve as important habitat features for summer resident bats. Additionally, preliminary surveys indicate that caves may be used as stop-over habitat during fall migrations by previously undocumented forest bats, such as the hoary bat (Lasiurus cinereus) and possibly the western red bat (L. blossevillii). Very little is known about the use of caves by migrating forest bats, and these areas may provide vital resources as bats traverse atypical habitats.
Bat Monitoring Schedule
Currently, monitoring of hibernating bat populations is conducted biennially by ESER wildlife biologists at nine known INL Site hibernacula. Surveys are conducted in coordination with Bureau of Land Management and IDFG surveys conducted across the region. The winter of 2014–2015 was a scheduled survey year with surveys conducted mid-winter during early 2015 when numbers of hibernating bats are presumed highest and most stable. All internal surveys are conducted consistent with OP-8, ESER Cave Protection and Access, and an approved INL Site cave entry permit. The latest USFWS decontamination protocol to avoid the spread of WNS is carefully followed.
Townsend’s big-eared bat is the most commonly counted over-wintering bat species, with western small-footed myotis being the second most common but with far fewer numbers. Trends and numbers of those species have been stable over the past two counts in all nine hibernacula on the INL Site (Figure 9-3). Historically over-wintering big brown bats have been encountered but not during the most recent surveys.
Anthropogenic structures (facilities, bridges, and culverts) are also used as habitat by bats on the INL Site. These areas, and their associated lands, occupy about 0.38 percent of the INL Site. Some of these facilities were constructed in the 1950s and are surrounded by mature landscaping trees and wastewater ponds, which provide bats with vertical-structure habitat, water, and foraging areas. Indeed, during summer, all resident and migratory bat species use anthropogenic structures around facilities and near roads for roost sites. An analysis of passive acoustic data collected at facility ponds indicated high variability in activity across facilities and seasons (Figure 9-4).