Ice Age Refuges
Ice Age Refuges
The ice ages that occurred between 2.4 million and 10,000 years ago had a dramatic effect on the climate and life on Earth. During each glacial period the tropics became both cooler and drier, turning some areas of tropical rain forest into dry seasonal forest or savanna. Local topography, geography, and climate allowed some areas of forest to escape the dry periods and act as refuges for forest biota. During subsequent interglacial periods, when humid conditions returned to the tropics, the forests expanded and were repopulated by plants and animals from the species-rich refuges.
Ice age refuges today correspond to present day areas of tropical forest that typically receive a high rainfall and often contain unusually large numbers of species, including a high proportion of endemic species. These species-rich refuges are surrounded by relatively species-poor areas of forest. Refuges are also centers of distribution for obligate forest species such as the gorilla, with a present day narrow and disjunctive distribution best explained by invoking past episodes of deforestation and reforestation. The location and extent of the forest refuges have been mapped in both Africa and South America. In the African rain forests there are three main centers of species richness and endemism recognized for mammals, birds, reptiles, amphibians, butterflies, freshwater crabs, and flowering plants. These centers are in Upper Guinea, Cameroon and Gabon, and the eastern rim of the Zaire basin. In the Amazon basin more than 20 refuges have been identified for different groups of animals and plants in Peru, Columbia, Venezuela, and Brazil.
The precise effect of the ice ages on biodiversity in tropical rain forests is currently a matter of debate. Some have argued that the repeated fluctuations between humid and arid phases created opportunities for the rapid evolution of certain forest organisms. Others have argued the opposite—that the climatic fluctuations resulted in a net loss of species diversity through an increase in the extinction rate. It has also been suggested that refuges owe their species richness not to past climate changes but to other underlying causes such as a favorable local climate, or soil.
The discovery of centers of high biodiversity and endemism within the tropical rain forest biome has important implications for conservation biology. A refuge rationale has been proposed by conservationists, whereby ice age refuges are given high priority for preservation because this would save the largest number of species, including many unnamed, threatened, and endangered species, from extinction.
KEY TERMS
Biodiversity —The biological diversity of an area as measured by the total number of plant and animal species.
Endemic —Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.
Interglacial period —A period of time between two glacial periods during which Earth’s average annual temperature is significantly warmer than during the two glacial periods.
Tropical rainforest —A tropical woodland marked by large amounts of rainfall and an abundance of plant and animal species.
Because refuges survived the past dry-climate phases, they have traditionally supplied the plants and animals for the restocking of the new-growth forests when wet conditions returned. Modern deforestation patterns, however, do not take into account forest history or biodiversity, and both forest refuges and more recent forests are being destroyed equally. For the first time, future tropical forests which survive the present mass deforestation episode could have no species-rich centers from which they can be restocked.
Resources
BOOKS
Barnosky. A., ed. Biodiversity Response to Climate Change in the Middle Pleistocene: The Porcupine Cave Fauna from Colorado. Berkeley, CA: University of California Press, 2004.
Wiens, J.A., and M.R. Moss, eds. Issues and Perspectives in Landscape Ecology. Cambridge, United Kingdom: Cambridge University Press, 2005.