USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Wildlife & Fish

Developing Quantitative Methods for Assessment of Bat Populations

Picture of a silver hair bat

Bats are important components of biodiversity and, as primary predators of nocturnal insects, play an important role in ecosystem function, especially in forests. Nevertheless their contributions are often overlooked because their activities take place out of sight from humans: they roost in inconspicuous places and emerge from them at night to forage under the cover of darkness.

Uncovering the mysteries of bat ecology often requires the use high-tech tools such as echolocation detectors to determine their activity patterns and radio-transmitters as small as 0.33 g (0.01 oz) to gain information about roost locations and foraging areas.

In addition, the activity patterns of bats are anything but regular or predictable. Bats in forests switch roosts every 2-3 days and alter the timing and location of their foraging activities on a nightly basis in response to weather conditions and availability of insects. Yet, against the backdrop of these challenges, Bill Zielinski and Ted Weller found that "a growing number of government agency personnel, and others, require more information about the species of bats that occur in their area", their conservation needs, and methods that can be used to assess the status of their populations.

Picture of setting up mist net over streamStandardizing Inventory Methods

Accurately assessing patterns of bat activity or species presence is challenging because of uncertainties in the system (e.g., spatio-temporal variability in activity patterns and their relation to environmental variables) and the methods (e.g., mist nets, acoustic surveys) used to achieve these objectives.

Because variability in the system is an intransigent (and fascinating!) component of bat ecology, our goal has been to help minimize, or at least account for, variability on the methodological side of the equation. The Faunal Diversity unit at PSW's Redwood Sciences Laboratory has a long history in development of standardized methods for the study of wildlife populations in forests and our work with bats continues in that tradition.

One component of our research explores how variability in methods used to study bats affects conclusions about their distributions and use of habitat. For example, we have noted how a lack of standardized procedures in the interpretation of sounds that biologists use to infer foraging activity of bats (Weller et al. 1998), how standardizing the height and angle at which bat detectors could improve comparability among detectors placed in different locations (Weller and Zabel 2002), and how measurement of a couple simple characters can be used to reliably distinguish between two species which are otherwise nearly impossible to tell apart in the field (Weller et al. 2007). The common thread in these studies was that lack of standardized, quantitative methods impeded the ability to reliably compare results among studies. Our approach has been to call attention to the potential consequences of inconsistent field methods while recommending relatively simple changes in methodology to help improve the rigor of individual studies and increase comparability among studies. Box plot of site occupancy by species

What Species Occur Here?

Our ultimate goal is to devise methods that accurately assess the status of bat populations. In many cases this starts with the seemingly simple question "What species occur here?" (Weller and Zielinski 2006) and our work has looked at the type and amount of survey effort necessary to document the presence of both rare and common species of bats in an area. As with many animal groups, a single survey is unlikely to detect all the species of bats that use a particular site or area. So a more relevant question is really "How many and what type of surveys are necessary to determine which species occur here?" Clearly this varies according to how common the species are and how susceptible they are to detection via a particular survey method. For instance we looked at 5 years of mist net data from a forested watershed in Northern California and found that, depending on the species, it could take anywhere from 2-45 nights of mist netting to ensure that we captured each species (Weller and Lee 2007).

Assessing Population Status of Bats

Map by number of bat species detected at sample units throughout the Northwest forest Plan area, 2003-2004 Around the same time we became interested in how we might be able to use presence/not-detected information as an inventory and monitoring tool for populations of forest bats (Weller 2007). For instance we participated in a USGS project in Hawaii which used ANABAT echolocation detectors to evaluate design considerations for inventory and monitoring the endangered Hawaiian Hoary Bat (Gorresen et al. 2008). Doing so required the use of site-occupancy analysis methods which account for the fact that the chance of detecting a species during a single survey is less then 100%.

More recently we used results from surveys in Northern California, Oregon , and the Olympic Peninsula of Washington to evaluate the use of a site-occupancy approach for evaluating the effectiveness of the Northwest Forest Plan for protecting bat habitat at landscape scales (Weller 2008). Our current work examines how detection probabilities for bats may vary across the northwestern U.S. An important goal of this work is to determine which species may be best suited to population assessment using the most recent field methods and data analysis tools.