Biogeography
Why do different species occur in the places they do? Biogeography is the study of why animal species (and also plants) live in different regions on Earth. This includes both organisms alive today as well as those that have become extinct. Any particular animal species is found where it is because that species either evolved and originated there or came there from some other place. The two divisions of biogeography reflect these two ways that animals come to occupy an area. Biogeography can be broken down into historical biogeography, which studies the past history and evolution of a species, and ecological biogeography, which studies the environment of a species.
Ecological Biogeography
Ecological biogeography studies how animal species are distributed in relation to the environment. The environment that influences what animals are present in a region includes both nonliving, abiotic factors (such as climate or soil composition) as well as living, biotic factors (such as other plants and animals). Earth is divided into major ecological areas called biomes. Biomes are regions of distinct climate and plant life. There are several kinds of biomes. Examples include the dry, hot desert in which cactuses and other plants are adapted to low water conditions, and the tropical evergreen forest with heavy year-round rainfall and lush plant life.
Dispersal occurs when an animal moves away from the area in which it was born and lives in another area. Dispersal increases the biogeographic range of a species, spreading the population. However, the extent to which an animal can disperse may be limited by ecological factors. Animals that disperse into areas for which they are not adapted will not survive. For example, alligators cannot disperse into central North America because it is too cold during the winter. These ecological limits to dispersal help determine the range of an animal species.
Historical Biogeography
Historical biogeography is the study of how animals that are present in a geographical region today relate to the animals that lived there in the past. A major factor explaining why a species is present in a region today is the presence of the same species in the past, or the presence of a closely related species that once lived there and from which the current species has descended. That is to say, a species is located somewhere because it was there in the past, or because an ancestor of the species lived there.
Continental drift is a major factor in determining current species distributions. All the continents on Earth were once part of one single land mass called Pangaea. About 200 million years ago, this landmass began to drift apart to form the continents of today. There are correspondingly six major biogeographic regions. They are the Neararctic, covering North America; the Neotropical, covering South America; the Ethiopian, covering Africa; the Oriental, covering India and southeastern Asia; the Palearctic, covering Europe and northern Asia; and the Australian, covering Australia.
Each of these regions has a group of animals that are more closely related to each other than to animals in other biogeographic regions. This is because of local diversification by speciation (the forming of new species) and the radiation (spread) of species within a biogeographical region; animals in a region are descendants from the ancestors that were previously there. The same is true for plants. Many animal species that are closely related stay in the same biogeographical region because it is hard to disperse or move between these regions. These regions are isolated from one another by an ocean or a very large mountain range, or are connected by only a narrow landmass (an isthmus). This isolation serves as a barrier to dispersal; most animals simply can not swim across the ocean to colonize another continent. Likewise, most animals that live in the Pacific Ocean cannot cross the land bridge that joins North and South America to reach the Atlantic Ocean, and vice versa.
Sometimes a population of animals is split into two populations by the sudden appearance of a physical barrier across which no individual can disperse; this is called a vicariant event. These two populations can become separate species over time because of isolation. An example of a natural vicariant event is an earthquake making a new canyon that is too wide for mice on either side to disperse across. Humans create obstacles that can also cause vicariance, such as highways that would stop mice from dispersing.
Humans can help promote dispersal. As technology has increased worldwide travel and transportation in the nineteenth and twentieth centuries, some animals have been able to disperse into new biogeographic regions on boats, trucks, or planes. How all the organisms in one place interact with each other and their environment is called the community ecology of an area. Biogeographic regions strongly determine the community ecology of an area. As a consequence, species that successfully disperse to new biogeographic areas can cause huge ecological impacts. For example, the brown tree snake began invading Pacific islands late in the twentieth century. The local animals, especially birds, are easy prey for brown tree snakes because they have not adapted to snake predators. The snakes can quickly wipe out the bird populations that can not adapt fast enough. SEE ALSO Living Fossils.
