Conservation biology habitat fragmentation and island biogeography

The theory of island biogeography was formulated to explain how rates of colonization and extinction affects species diversity observed on actual islands. Currently, protected areas (such as national parks, to which many threatened mammalian species are increasingly restricted) are beginning to resemble habitat-islands in vast seas of agricultural or even urban development. Therefore, island biogeography is increasingly considered an intellectual tool with which conservation biologists should be familiar.

The basic theory of island biogeography grew out of two empirical observations: (1) larger islands often have more species than small islands, and (2) an island's distance from the nearest continent is inversely related to the island's species diversity. These observations were eventually brought together into the equilibrium theory of island species diversity. Conservation biologists use insight from this theory in the management of fragmented landscapes. In particular they often ask how small a refuge "island" can be, before threshold effects arise and species-extinctions dominate community dynamics.

Basically, if a habitat-patch is too small to include home ranges for a viable population of a mammalian species, then the long-term survival of that species is improbable. Information about extinction rates of small mammals in habitat fragments is difficult to evaluate, in part because biologists lack comparable data from undisturbed habitats to serve as controls. However, two studies on forest fragments provide disturbing evidence that mammalian diversity can decline quickly:

• Short term, Thailand. In Surat Thani Province approximately 100 islands were created in 1986 when

A conservationist with chimpanzees (Pan troglodytes). (Photo by © Yann Arthus-Bertrand/Corbis. Reproduced by permission.)

the Saeng River was dammed to create a hydroelectric reservoir. Rapid changes occurred in the small mammal assemblages on these new islands. Within five years, two of the 12 species (a murid rodent, Leopoldamys sabanus and an insectivore, Hylomys suil-lus) were lost. Further extinctions are likely.

• Long term, Panama. Early in the twentieth century, several forest hilltops were isolated during the damming of the Chagras River during the construction of the Panama Canal. After 80 years of isolation, only one out of 16 rodents species remained on islands smaller than 42.3 acres (17.1 ha). The rate of mammalian species-loss from these small islandfragments was approximately one species per 3-11 years.

The fate of large mammal communities in small habitat-fragments is even grimmer. Most big mammals must have a great deal of space. For instance, the home range of a Southeast Asian rhinoceros (Rhinoceros sondaicus annamiticus) in Cat Tien National Park, Vietnam has been estimated at 1,480-2,470 acres (600-1,000 ha). Some solitary carnivores require areas an order of magnitude larger. A tiger, for instance, might roam across more than 24,700 acres (10,000 ha), and a single wolverine (Gulo gulo) would probably need twice that much room.

These are area-requirements for individual mammals, while of course, viable populations are comprised of many individuals. These populations need even larger patches of habitat. For example, many species of African grazing artio-dactyls can exhibit their natural social behavior only in large groups. Large groups require enormous areas, sometimes with widely separated dry-season and wet-season ranges. The

Wardens fix water pipes in Khaudum Game Reserve, in Namibia, as elephants in the backround wait for water. (Photo by Rudi van Aarde. Reproduced by permission.)

annual migration of east African wildebeest (Connochaetes tau-rinus) covers hundreds of miles (kilometers) and crosses national borders. Of course wildebeest can be kept alive in modest pastures, and tigers can be maintained in zoo-cages. But these conditions are not fully satisfactory and clearly only the largest national parks allow viable populations of most mammals to exist in natural social conditions.

Conserving such large tracts of habitat is often difficult. One approach is to connect habitat fragments by means of corridors, or protected habitat-strips that allow animals to move between patches. In Africa it has been observed that some mammals (as well as reptiles and birds) use corridors as inter-patch bridges. However, some conservation biologists question whether this phenomenon is at all general.

It should be clear that as conservationists contemplate the establishment, enlargement, or maintenance of a refuge, they should be aware of the particular needs of those target organisms that the refuge is designed to shelter. Behavioral ecol-ogists, for example, often gather data on a species' activity patterns, foraging behavior, group size, home range, and ter ritorial behavior. Such information is useful for predicting how a target species will respond to habitat fragmentation, how edge-effects will impact a given species, or whether the species will use habitat corridors.

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