It is generally believed that bat populations have declined worldwide in recent decades. Because bats have low reproductive rates, populations are very susceptible to elevated mortality or depressed recruitment. There is developing concern about the conservation status of bats as many species of bats are increasingly affected by multiple actions of humans. Bats face multiple threats of ignorance, suspicion, pesticide poisoning, roost destruction and closure, habitat loss, over-exploitation, and extermination as pests. Among mammals, bats are the second most diverse order (after rodents) and occur on all continents except Antarctica. Bats often range widely in their foraging activities and habitat modifications such as urbanization, agriculture, and other land use practices may affect local plant and insect populations and thus the food resources of bats. At present, however, virtually nothing is known of the specific effects of global warming on bats although certain predictions can be made based on the biology of bats.

Figure 1. Mexican big-eared bat, Idionycteris phyllotis. Largely a forest dweller inhabiting caves in pine-oak forests. (Photograph by Dr. J. Scott Altenbach. Department of Biology, University of New Mexico).

Bats seem to have evolved as moderately long-lived (e.g., 5-30 years), intelligent creatures that are acutely in synchrony with global climates. Worldwide, bats are known to depend on a variety of natural resources. Many tropical species are dependent on nectar, pollen, and flowers and fruits of plants and are known to "track" the development of the plant resources upon which they depend. Not surprisingly, bats are frequent and important pollinators of plants, especially in tropical areas and on island ecosystems in the Pacific. Additionally, many bats rely on plants (especially trees) as roosting sites for varying periods of time. Disturbances to climate that interrupt or alter the phenology of plants, or greatly alter plant species occurrence or distribution, can be expected to affect bats. For example, a mean global warming of 3 C will change climates sufficiently so that 7-11% of vascular plants in North America will no longer occur within the appropriate climate "envelope." This will require that such plants will have to adapt to the change, move to stay within the appropriate climate, or become extirpated. Such disruptions will likely affect bats.

Other species of bats, especially in temperate zones, are insectivorous and collectively consume large quantities of insects. Just as pollinating activities of bats are important to plant ecology, the insectivorous food habits of bats play an important role in maintaining a balance among insect populations. Although studies of bats have demonstrated some flexibility in food habits over time, most species appear to be specialized to pursue and capture selected kinds or categories of insects. Furthermore, bats are dependent upon a reliable and consistent "supply" of prey, even though specific insect populations grow and disappear over the course of a summer season. Changes in worldwide insect population occurrence or distribution can be expected to affect numbers and species diversity of bats.

In temperate latitudes, both northern and southern, bats avoid seasonal food shortages by either hibernating, often in caves or mines, or by migrating to regions where food is still available. We suspect that nearly all attributes of hibernation or migration are mediated by combinations of changes in ambient light regimes, temperature, and food resources. The dependence of temperate-zone bats on the interplay of these factors ultimately revolves around the bats' ability to acquire sufficient energy (in the form of food) to either last them through a hibernation sequence or through the rigors of (sometimes) long-distance migration. Temperature changes that would affect the supply of food to bats or otherwise upset an energy balance that has evolved over millenia should have significant consequences for bats. Also, climate changes that would lead to changes in the internal temperatures of roosts that have been used by bats for decades will force bats to locate and use new or different roosts.

Figure 2. Range of the Mexican big-eared bat (Note: Click the Figure to view a larger image).

Ecologically, bats exhibit distributional (biogeographic) patterns that reflect preferences for certain life zones or habitats. The majority of bat species occur in tropical and subtropical ecosystems but a significant proportion live in temperate zones that may be more affected by global warming. Some temperate species prefer high-elevation habitats that are more mesic and cooler; other species are adapted to live in more arid areas. Bats whose current ranges are limited altitudinally or latitudinally (toward the poles) by cool temperatures may be displaced as the earth warms. Bats restricted to cooler climates, either latitudinally or altitudinally, may be jeopardized by global warming and if unable to adapt or persist will become extinct. Bats already adapted to warm climates may increase both in numbers and geographic ranges. To the extent that global warming alters the zonation of plants and animals (especially insects) around the world, we can expect bats to respond to such forces.

As a part of monitoring effects of global warming, it is particularly critical to continue to assemble information on the basic biology of bats, as well as the status of existing colonies of bats and on trends in numbers over time. Site- and species-specific data may help avoid controversies about the status of species that can arise as a result of decisions based on incomplete information. In terms of impacts of global change on bat populations it will be important to document the change in physical and biotic factors that are important to bats, such as temperature (both ambient and within important roost sites), precipitation, loss or expansion of available habitats, and change in insect and fruit crops. Such monitoring should begin now.

Additional Reading:

Barbour, R. W., and W. H. Davis. 1969. Bats of America. University Press of Kentucky, Lexington. 286 pp.

Clark, D. R., Jr. 1981. Bats and environmental contaminants: a review. U.S. Fish and Wildlife Service Special Scientific Report--Wildlife 235: 1-27.

Cockrum, E. L. and Y. Petryszyn. 1991. The long-nosed bat, Leptonycteris: an endangered species in the southwest ? Occasional Papers, The Museum, Texas Tech University, 142: 1-32.

Fish and Wildlife Service. 1994a. Endangered and threatened wildlife and plants. 50 CFR 17.11 and 17.12. 42 pp.

Fish and Wildlife Service. 1994b. Endangered and threatened wildlife and plants; animal candidate review for listing as endangered or threatened species; proposed rule. Federal Register, 59:58982-59028.

Fish and Wildlife Service. 1996. Endangered and threatened species, plant and animal taxa; proposed rule. Federal Register, 61:7595-7613.

Hall, E. R. 1981. The mammals of North America. Volume I: 1-600 + 90 pp. John Wiley and Sons, Inc., New York.

Hill, J. E., and J. D. Smith. 1984. Bats, a natural history. University of Texas, Austin. 243 pp.

Gillette, D. D. and J. D. Kimbrough. 1970. Chiropteran mortality. Pp. 262-283 in B. H. Slaughter and D. W. Walton, (eds.). About bats: a chiropteran biology symposium. Southern Methodist University Press, Dallas, TX. 339 pp.

Jones. J. K., Jr., R. S. Hoffman, D. W. Rice, C. Jones, R. J. Baker, and M. D. Engstrom. 1992. Revised checklist of North American mammals north of Mexico, 1991. Occasional Papers, The Museum, Texas Tech University, 146:1-23.

Kunz, T. H. 1988. Ecological and behavioral methods for the study of bats. Smithsonian Institution Press, Washington, DC. 533 pp.

Kunz, T. H., and E. D. Pierson. 1994. Bats of the world: An introduction. Pp. 1-46 in R. M. Nowak. Walker's bats of the world. Johns Hopkins University Press, Baltimore. 287 pp.

Racey, P. A., and S. M. Swift (eds.). 1995. Ecology, evolution and behaviour. Oxford University Press, New York. xxi + 421 pp.

Schmidly, D. J. 1991. The bats of Texas. Texas A & M University Press, College Station. 188 pp.

Tuttle, M. D. 1988. America's neighborhood bats. University of Texas Press, Austin. 96 pp.

Wilson, D. E. 1997. Bats in question. Smithsonian Institution Press, Washington, DC. 192 pp.

Web Sites of Interest:

Bat Work-Foraging Echolocation in Bats

Museum of Southwestern Biology-UNM

Pacific Northwest Bat Call Library

Midcontinent Ecological Science Center

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