Bats in Forests: Conservation and Management

10. ASSESSING POPULATION STATUS OF BATS IN FORESTS: CHALLENGES AND OPPORTUNITIES

10 Interest in bats has increased during the past two decades in scientific communities, land management agencies, and the general public (Fenton 1997). Growing knowledge of the interdependence between bats and forests in concert with concern over human-induced changes to forested ecosystems has spurred the need for a greater understanding of the ecology of forest-dwelling bats (Barclay and Brigham 1996). Increased understanding has been facilitated by technological advancements (e.g., miniature radiotransmitters and affordable bat detectors) that have allowed exciting insights into many formerly unknown aspects of bat ecology . Prior to these advances, scientific and public interest in bats was focused on roosts where bats aggregated in large numbers and could be easily viewed, such as caves and buildings (Brigham, chap. 1 in this volume ). The advent of affordable bat detectors meant an increasing number of people could eavesdrop on the formerly silent world of bats for a variety of purposes ranging from nature walks to in-depth studies of echolocation. Similarly, radiotransmitters small enough to be carried by bats have vastly increased our knowledge of bats that use less conspicuous roosts. Nowhere are these gains more evident than for forestdwelling bats: bat detectors have been used extensively to compare habitat use among forest types and ages, and the use of radiotelemetry has determined that many bats in forests tend to roost in small groups and switch between a large number of roosts (Barclay and Kurta, chap. 2 in this volume). With increased interest and recognition come questions regarding the vitality of bat populations. Population declines have been measured for cave-dwelling species in the eastern United States (Tuttle 1979; U.S. Fish and Wildlife Service 1999), but other bat populations may also be declining (Pierson 1998). Concerns about population declines have led to the listing of several species under the U.S. Endangered Species Act, and incorporation on provincial red lists in Canada and other regional lists of sensitive species. As a result, a large number of agency personnel are responsible for managing bats and their habitats (Weller, in prep.). The challenges of monitoring bat populations are myriad (O’Shea and Bogan 2003). Most of these challenges result because bats are small, nocturnal flying animals that roost secretively and produce calls largely outside the hearing range of humans; thus, bats are difficult to detect withASSESSING POPULATION STATUS OF BATS IN FORESTS: CHALLENGES AND OPPORTUNITIES Theodore J. Weller out the use of specialized tools such as bat detectors. Our ability to assess the population status of bats varies according to the ecology of the species and their susceptibility to existing inventory methods. Species that roost in large numbers in relatively accessible locations afford the best opportunities to achieve rigorous estimates of conventional population parameters such as abundance and survival (O’Shea and Bogan 2003; Sendor and Simon 2003; Warren and Witter 2002). However, most species of North American bats exhibit several ecological attributes that render their monitoring difficult, including low detectability, aggregation in low densities, unpredictable movement patterns, and poorly known biology (Clarke et al. 2003; O’Shea and Bogan 2003). Forest-dwelling bats are among the most difficult group to monitor. During the warm season most forest bats roost in cracks, crevices, and cavities, or under the bark of trees (Barclay and Kurta, chap. 2 in this volume ; Kunz and Lumsden 2003). These roosts are difficult to observe because , in general, they are inconspicuous and used by relatively few individuals . Fidelity to these roosts is also much less than for bats that roost in more permanent structures (Lewis 1995). As a result, monitoring individual roost sites to assess population status of forest-dwelling bats presents even greater challenges than for bats that roost more conspicuously .Away from roosts, bats are difficult to capture and activity patterns vary greatly in both space and time (Lacki et al., chap. 4 in this volume). Often just determining whether a given species occurs in a particular forested area can require significant effort (Weller, in prep.), and evaluating more conventional population parameters using existing methods, such as abundance levels, is nearly impossible (O’Shea and Bogan 2003). Despite these obstacles, biologists are asked to assess the status and trend of bat populations, and often for several species simultaneously. Although little specific guidance exists as to how to do this for forestdwelling bats (Resources Inventory Committee 1998;Vonhof 2000),there are several resources that identify important considerations for conducting inventory...