Animal Distribution (Zoogeography)
Animal Distribution
(Zoogeography)
The study of zoogeography tries to explain why animals are distributed as they are, their patterns of dispersal, and the factors responsible for their dispersal. Most animals typically occupy limited geographic areas. Humans, however, and creatures such as house mice and cockroaches that share human habitations, are able to live almost anywhere on earth. It is not always easy to explain why animals are distributed as they are, since similar habitats on separate continents may be occupied by quite different kinds of animals. A particular species may be absent from a region that supports similar animals because of barriers that prevent it from getting there or because established populations of other animals prevent it from colonizing.
Thus we would like to discover why animals are found where they are or are not found where one thinks they ought to be. Usually this means studying the past. The fossil record plainly shows that animals once flourished in regions from which they are now absent. Extinction has played a major role, but many groups left descendants that migrated to other regions and survived. For example, ancestors of camels originated in North America, where their fossils are found. They spread during the Pleistocene epoch by way of Alaska to Eurasia and Africa, where they are represented today by true camels, and to South America, where their descendants survive as llamas, alpacas, guanacos and vicuñas. (The Pleistocene began about 1.7 million years BP and ended about 11 thousand years BP; see the geological time table on the back inside-cover.) Then camels became extinct in North America about 10,000 years BP at the close of the Ice Age. Thus the history of an animal species or its ancestor must be known before one can understand why it lives where it does. The earth’s surface is undergoing constant change. Many areas that are now land were once covered with seas; fertile plains may be claimed by advancing desert; impassable mountain barriers may arise where none existed before; or inhospitable ice fields may retreat before a warmer climate to be replaced by forests. Geological change has been responsible for much of the alteration in animal (and plant) distribution and has been a powerful influence in shaping organic evolution.
The study of zoogeography tries to explain why animals are distributed as they are, their patterns of dispersal, and the factors responsible for their dispersal. Most animals typically occupy limited geographic areas. Humans, however, and creatures such as house mice and cockroaches that share human habitations, are able to live almost anywhere on earth. It is not always easy to explain why animals are distributed as they are, since similar habitats on separate continents may be occupied by quite different kinds of animals. A particular species may be absent from a region that supports similar animals because of barriers that prevent it from getting there or because established populations of other animals prevent it from colonizing.
Thus we would like to discover why animals are found where they are or are not found where one thinks they ought to be. Usually this means studying the past. The fossil record plainly shows that animals once flourished in regions from which they are now absent. Extinction has played a major role, but many groups left descendants that migrated to other regions and survived. For example, ancestors of camels originated in North America, where their fossils are found. They spread during the Pleistocene epoch by way of Alaska to Eurasia and Africa, where they are represented today by true camels, and to South America, where their descendants survive as llamas, alpacas, guanacos and vicuñas. (The Pleistocene began about 1.7 million years BP and ended about 11 thousand years BP; see the geological time table on the back inside-cover.) Then camels became extinct in North America about 10,000 years BP at the close of the Ice Age. Thus the history of an animal species or its ancestor must be known before one can understand why it lives where it does. The earth’s surface is undergoing constant change. Many areas that are now land were once covered with seas; fertile plains may be claimed by advancing desert; impassable mountain barriers may arise where none existed before; or inhospitable ice fields may retreat before a warmer climate to be replaced by forests. Geological change has been responsible for much of the alteration in animal (and plant) distribution and has been a powerful influence in shaping organic evolution.
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