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Indiana Bats


by
Ronald D. Drobney
National Biological Service
Richard L. Clawson
Missouri Department of Conservation
Fig. 1. Range of the Indiana bat and locations of Priority 1 hibernacula (see text for definitions).
The Indiana bat (Myotis sodalis) is an endangered species that occurs throughout much of the eastern United States (Fig. 1). Although bats are sometimes viewed with disdain, they are of considerable ecological and economic importance. Bats consume a diet consisting largely of nocturnal insects and thereby are a natural control for both agricultural pests and insects that are annoying to humans. Furthermore, many forms of cave life depend upon nutrients brought into caves by bats in the form of guano or feces (Missouri Department of Conservation 1991).
Indiana bats use distinctly different habitats during summer and winter. In winter, bats congregate in a few large caves and mines for hibernation and have a more restricted distribution than at other times of the year. Nearly 85% of the known population winters in only seven caves and mines in Missouri, Indiana, and Kentucky, and approximately one-half of the population uses only two of these hibernacula.
In spring, females migrate north from their hibernacula and form maternity colonies in predominantly agricultural areas of Missouri, Iowa, Illinois, Indiana, and Michigan. These colonies, consisting of 50 to 150 adults and their young, normally roost under the loose bark of dead, large-diameter trees throughout summer; however, living shagbark hickories (Carya ovata) and tree cavities are also used occasionally (Humphrey et al. 1977; Gardner et al. 1991; Callahan 1993; Kurta et al. 1993).
As a consequence of their limited distribution, specific summer and winter habitat requirements, and tendency to congregate in large numbers during winter, Indiana bats are particularly vulnerable to rapid population reductions resulting from habitat change, environmental contaminants, and other human disturbances (Brady et al. 1983). Additionally, because females produce only one young per year, recovery following a population reduction occurs slowly. Concerns arising from the high potential vulnerability and slow recovery rate have led to a long-term population monitoring effort for this species.

Bat Census Design
The first rangewide census of wintering Indiana bats was made in 1975. All subsequent population data were gathered according to standardized cave census techniques established by the Indiana Bat Recovery Team in 1983 (Brady et al. 1983). Data presented in this article are based upon counts made at 2-year intervals at Priority 1 hibernacula, which are caves where winter populations exceeding 30,000 bats have been recorded. We chose to use data only from Priority 1 caves because they contain the majority of bats in the population. During midwinter cave censuses, bats hanging singly and in small clusters of up to 25 were counted individually. The number of bats in larger clusters was determined by multiplying the surface area of the cluster by bat density (Fig. 2).

Fig. 2. Hibernating cluster of Indiana bats. Courtesy R. L. Clawson

Bat Populations: Trends and Recovery Prospects
Before the 1970's, the population status of Indiana bats was poorly understood because the locations of many of their winter hibernacula were unknown, and the counts that were conducted were made irregularly and inconsistently. The 1975 census established a benchmark of nearly 450,000 bats using Priority 1 hibernacula. Since 1983 the number of bats tallied has declined significantly, reaching a low of 347,890 during the most recent census in 1993 (Fig. 3).

Fig. 3. State and national trends for Indiana bats, 1983-93.
Although the national trend indicates a 22% decline during the past 10 years, this decrease has not been consistent across the species' winter range (Fig. 3). Most of the decrease in the 10-year national census results can be accounted for by a precipitous 34% decline in the number of bats counted in Missouri. A more favorable pattern has been noted in Indiana, where numbers have increased, and in Kentucky, where the population has remained relatively stable.
Recovery efforts have included placing gates or fences across cave entrances to eliminate disturbances to hibernating bats. These exclusion devices have not halted population declines, suggesting that other factors are negatively influencing bat populations.
Another potential threat is the loss of habitat used by maternity colonies. Maternity roost sites in dead trees exposed to sunlight and located in upland forests and near streams are particularly important. Losses of these sites through streamside deforestation and stream channelization pose significant threats to population recovery.
Pesticides and other environmental contaminants represent additional hazards. Indiana bats are exposed to lingering residues of chlorinated hydrocarbon pesticides such as aldrin and heptachlor. These products have been banned since the 1970's, but persist in the soil and in insects upon which bats feed. Potential detrimental effects of the new generation of pesticides, including organophosphates, are unknown.
The long-term prognosis for Indiana bat populations is uncertain. The fact that wintering populations appear to be increasing in Indiana and are remaining relatively stable in Kentucky provides the basis for some optimism. A better understanding of their summer habitat requirements and factors affecting survival and reproduction is needed so that more effective recovery efforts can be formulated. It is important to recognize, however, that even if the factors that are negatively influencing Indiana bat populations are removed, recovery will occur slowly because this species has a low reproductive rate.
For further information:
Ronald D. Drobney
National Biological Service
Missouri Cooperative Fish and Wildlife Research Unit
University of Missouri
112 Stephens Hall
Columbia, MO 65211
References
Brady, J.T., R.K. LaVal, T.H. Kunz, M.D. Tuttle, D.E. Wilson, and R.L. Clawson. 1983. Recovery plan for the Indiana bat. U.S. Fish and Wildlife Service, Washington, DC. 94 pp.

Callahan, E.V. 1993. Indiana bat summer habitat requirements. M.S. thesis, University of Missouri, Columbia. 74 pp.

Gardner, J.E., J.D. Garner, and J.E. Hofmann. 1991. Summer roost selection and roosting behavior of Myotis sodalis (Indiana bat) in Illinois. Final report. Illinois Natural History Survey, Illinois Department of Conservation, Champaign. 56 pp.

 Humphrey, S.R., A. R. Richter, and J.B. Cope. 1977. Summer habitat and ecology of the endangered Indiana bat, Myotis sodalis. Journal of Mammalogy 58:334-346.

Kurta, A., D. King, J.A. Teramino, J.M. Stribley, and K.J. Williams. 1993. Summer roosts of the endangered Indiana bat (Myotis sodalis) on the northern edge of its range. American Midland Naturalist 129:132-138.

Missouri Department of Conservation. 1991. Endangered bats and their management in Missouri. Missouri Department of Conservation, Jefferson City. 8 pp.


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