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Human Use of Caves

Bats and Caves


Karst and Nonkarst Watershed Models

Cave Creations

Bat Babies

Life in the Dark


Based on an article in
Science & Children Magazine,
Published by the National Science Teachers Association, October 2002

Cave Ecosystems

Bats: Essential to Healthy Ecosystems

Bats are among the most beneficial and necessary animals on Earth. These flying mammals comprise nearly a quarter of all mammal species and live in almost every habitat. They are primary predators of vast numbers of insect pests, including beetles, moths, leafhoppers, and other insects which, if not for bat predation, would cost farmers and foresters billions of dollars annually in lost crops. Some bats also consume mosquitoes, which are at best an annoyance to humans, and at worst, disease carriers. Bats also pollinate flowers and disperse seeds in rainforests and deserts. In any of their habitats, bats are critical elements of the ecosystem.

There are 45 species of bats in the United States (out of 1,000 worldwide), and they exhibit surprising physical variety. Most cave-dwelling species, which represent by far the largest bat colonies, range in length from 6 to 12 cm, with wingspans from 19 to 41 cm and weights of 30 g or less. Bat species are divided into two suborders, the Megachiroptera, large, long-faced, non-carnivorous species that include flying foxes and fruit bats, and Microchiroptera, smaller, pug-faced species that are typically carnivores. (Chiroptera is Latin for "hand wing.") Most of the former species feed on fruit and nectar, and most of the latter feed on insects.

Bats can live up to 30 years of age, and most have only one baby, called a pup, each year. As with all mammals, pups are fed milk by their mothers, which are able to recognize their young by voice and scent. (Contrary to popular belief, bats are not blind; they are able to see as well as most other mammals.) For warmth and protection, females often live together in dense "bat nurseries," roosting in caves or other structures, such as buildings or abandoned underground mines, during the day. Some species live in trees, on cliff faces, or in narrow rock crevices. Roosting bats hang upside down with their wings folded at their sides or around their bodies. While some bat species migrate to southern climes during the winter, many American species hibernate until spring, surviving on stored fat reserves.

Insect-eating bats feed at night, catching many pests such as mosquitoes, moths, and flying beetles. They emit high-frequency sound pulses that bounce off objects as echoes. Bats' sensitive ears enable them to use these echoes to locate, identify, and capture moving prey while flying through the dark. This mechanism is known as echolocation. It is so effective that bats can hear the footsteps of walking insects and can detect objects as fine as a human hair. Echolocation also permits bats to navigate through deep caves in total darkness.

Though the Pallid bat and big brown bat are commonly found in buildings, America's largest bat colonies are found in caves. For example, Bracken Cave in central Texas is the summer home of 20 million Mexican free-tailed bats. During their annual residency period, thousands of square meters of cave wall are covered by 240 tons of roosting bats, with a body density of more than 5,000 per m 2 . On each of their nightly forays, these bats eat more than 200,000 kg of insects over surrounding towns and farmlands. They spend their days in total darkness more than 3 km inside the cave; how they know when it is time to emerge each evening remains a mystery. Bats are extremely susceptible to disturbance of their habitats. For example, thousands die each winter when inadvertently wakened by noise or other upheaval, which forces them to waste precious energy reserves. Many more bat colonies are affected by chemical pollution, cave vandalism, and other habitat destruction. The loss of bats increases reliance on chemical pesticides, which can negatively affect human and environmental health. Populations of native plants and the animals that depend on those plants also decline when bat populations drop.

The saguaro, organ pipe cactus, and agave are a few such bat-dependent plants. So important is the long-nosed bat to the agave's reproductive process, that if its flowers are not visited by the bats, the odds of successful seed production are one three-thousandth of normal. Yet that bat species is now endangered, with only two nursery colonies known to remain in the United States.

Bat guano also provides the primary nutrient source for entire ecosystems of cave life. It is so rich in nutrients that a single tablespoonful can contain hundreds of species of bacteria of great value. The Alabama cave fish, for example, lives beneath a bat roost in only one cave. Loss of the bat colony and the guano it produces could lead to the fish's extinction. Recent tests have shown that some of the bacteria in guano also produce enzymes that have potential for use in detoxifying industrial wastes, improving detergents, and producing gasohol and antibiotics.

Many colonies of cave-dwelling bats have already been lost because of the impact of human activities. Of the bat species living in the United States and Canada, more than half are endangered or are candidates for such status. Conservation efforts, including some undertaken jointly by BLM and Bat Conservation International, have proven successful in halting some bat population declines. For example, several modern bridge designs have incorporated long, vertical crevices to pro-vide habitat for millions of bats. A number of formerly closed abandoned mines in northern California have been reopened and gated to provide new bat habitat for Townsend's big-eared bats. Other successful conservation initiatives include the use of artificial bat houses and the gating of caves essential to the survival of particular bat colonies.