Galápagos Islands

Within the theory of evolution , the concept of adaptive radiation (evolutionary development of several species from a single parental stock) has had as its prime example, a group of birds known as Darwin's finches. Charles Darwin discovered and collected specimens of these birds from the Galápagos Islands in 1835 on his five-year voyage around the world aboard the HMS Beagle. His cumulative experiences, copious notes, and vast collections ultimately led to the publication of his monumental work, On the Origin of Species, in 1859. The Galápagos Islands and their unique assemblage of plants and animals were an instrumental part of the development of Darwin's evolutionary theory.

The Galápagos Islands are located at 90° W longitude and 0° latitude (the equator), about 600 mi (965 km) west Ecuador. These islands are volcanic in origin and are about 10 million years old. The original colonization of the Galápagos Islands occurred by chance transport over the ocean as indicated by the gaps in the flora and fauna of this archipelago compared to the mainland. Of the hundreds of species of birds along the northwestern South American coast, only seven species colonized the Galápagos Islands. These evolved into 57 resident species, 26 of which are endemic to the islands, through adaptive radiation. The only native land mammals are a rat and a bat. The land reptiles include iguanas, a single species each of snake, lizard, and gecko, and the Galápagos giant tortoise (Geochelone elephantopus ). No amphibians and few insects or mollusks are found in the Galápagos. The flora has large gaps as well—no conifers or palms have colonized these islands. Many of the open niches have been filled by the colonizing groups. The tortoises and iguanas are large and have filled niches normally occupied by mammalian herbivores. Several plants, such as the prickly pear cactus, have attained large size and occupy the ecological position of tall trees.

The most widely known and often used example of adaptive radiation is Darwin's finches, a group of 14 species of birds that arose from a single ancestor in the Galápagos Islands. These birds have specialized on different islands or into niches normally filled by other groups of birds. Some are strictly seed eaters, while others have evolved more warbler-like bills and eat insects, still others eat flowers, fruit, and/or nectar, and others find insects for their diet by digging under the bark of trees, having filled the niche of the woodpecker. Darwin's finches are named in honor of their discoverer, but they are not referred to as Galápagos finches because there is one of their numbers that has colonized Cocos Island, located 425 mi (684 km) north-northeast of the Galápagos.

Because of the Galápagos Islands' unique ecology , scenic beauty and tropical climate , they have become a mecca for tourists and some settlement. These human activities have introduced a host ofenvironmental problems, including introduced species of goats, pigs, rats, dogs, and cats, many of which become feral and damage or destroy nesting bird colonies by preying on the adults, young, or eggs. Several races of giant tortoise have been extirpated or are severely threatened with extinction , primarily due to exploitation for food by humans, destruction of their food resources by goats, or predation of their hatchlings by feral animals. Most of the 13 recognized races of tortoise have populations numbering only in the hundreds. Three races are tenuously maintaining populations in the thousands, one race has not been seen since 1906, but it is thought to have disappeared due to natural causes, another race has a population of about 25 individuals, and the Abingdon Island tortoise is represented today by only one individual, "Lone-some George," a captive male at the Charles Darwin Biological Station. For most of these tortoises to survive, an active capture or extermination program of the feral animals will have to continue. One other potential threat to the Galápagos Islands is tourism. Thousands of tourists visit these islands each year and their numbers can exceed the limit deemed sustainable by the Ecuadoran government. These tourists have had, and will continue to have, an impact on the fragile habitats of the Galápagos.

See also Endemic species; Ecotourism

[Eugene C. Beckham ]

RESOURCES

BOOKS

Harris, M. A Field Guide to the Birds of the Galápagos. London: Collins, 1982. Root, P., and M. McCormick. Galápagos Islands. New York: Macmillan, 1989.

Steadman, D. W., and S. Zousmer. Galápagos: Discovery on Darwin's Islands. Washington, DC: Smithsonian Institution Press, 1988.