Biogeography: The Distribution of Life on Earth
Mapping Life
Biogeography is the study of the distribution of living organisms across the Earth’s surface and the processes that create and maintain these patterns. It asks fundamental questions: Why are certain species found in some places but not others? How did the current distribution of life come to be? How will distributions change in response to environmental change?
Biogeography integrates geography, biology, ecology, and evolutionary science. It is essential for understanding biodiversity, conservation, and the impacts of climate change.
Historical Biogeography
Continental Drift and Plate Tectonics
The theory of plate tectonics revolutionized biogeography. The movement of continents over geological time explains many distribution patterns. Marsupials are concentrated in Australia and South America because these continents were isolated before placental mammals could colonize them.
Vicariance and Dispersal
Vicariance occurs when a population is divided by a geographic barrier, leading to speciation. Dispersal involves the movement of organisms across existing barriers.
Island Biogeography
Islands are natural laboratories for studying biogeographic processes. The equilibrium theory of island biogeography, developed by Robert MacArthur and E. O. Wilson, predicts that species richness on islands reflects a balance between immigration and extinction rates.
Ecological Biogeography
Biomes
Biomes are large-scale ecological communities characterized by their dominant vegetation and climate. Major terrestrial biomes include tropical rainforest, savanna, desert, temperate forest, taiga, and tundra. Aquatic biomes include freshwater and marine ecosystems.
Patterns of Biodiversity
Biodiversity is not distributed evenly across the Earth. Species richness generally increases toward the tropics, a pattern known as the latitudinal diversity gradient. Explaining this pattern is a major focus of biogeographic research.
Biogeographic Regions
The Earth can be divided into biogeographic realms based on shared evolutionary history. Alfred Russel Wallace first proposed these divisions. The major realms are Palearctic, Nearctic, Neotropical, Afrotropical, Indomalayan, Australasian, Oceania, and Antarctic.
Conservation Biogeography
Biogeographic knowledge is essential for conservation. Understanding species distributions, habitat requirements, and the processes that maintain biodiversity informs protected area design, restoration, and management.
Climate change is shifting species distributions, creating new challenges for conservation. Biogeographers model how species ranges might shift and identify areas that will remain suitable for particular species.
FAQ
Why are there more species in the tropics?
Multiple hypotheses have been proposed: greater solar energy and productivity, larger area of tropical land, longer time for speciation in stable climates, and greater habitat heterogeneity.
What is an endemic species?
A species is endemic if it is found only in a particular geographic area and nowhere else. Endemic species are often concentrated on islands and in isolated habitats.
How do species move across barriers?
Species move across barriers through dispersal—by flying, swimming, rafting on vegetation, being carried by wind or currents, or being transported by humans. Long-distance dispersal events are rare but can be evolutionarily significant.
How will climate change affect species distributions?
Climate change is causing many species to shift their ranges toward the poles or to higher elevations. Species that cannot move fast enough or that have nowhere to go face extinction.
Conclusion
Biogeography reveals the patterns and processes that determine where life exists on Earth. Understanding these patterns is essential for conserving biodiversity in a rapidly changing world. For further reading, see physical geography and the study of climatology.