Biogeographic realm

A biogeographic realm is the largest way to divide the Earth's land surface. These realms group living things together based on where they are found. Scientists use these divisions to show patterns in how organisms are distributed across the globe.

Within these large realms, there are smaller areas called bioregions. These are then broken down even further into smaller units called ecoregions. Sometimes, people use the term "ecozone" to talk about these realms. This helps researchers organize the vast complexity of life on our planet.

How do these realms form? They develop because organisms evolve in relative isolation for very long periods. Natural barriers prevent animals and plants from migrating to new areas. These barriers might be wide oceans or very large deserts. High mountain ranges can also act as walls that stop movement. Because of these barriers, life in one realm develops a unique evolutionary history. This makes the organisms in one realm different from those in another, even if they live in similar weather.

It is important to distinguish realms from biomes. A biome is a major habitat type. Biomes are defined by life forms and how they adapt to specific conditions. These conditions include climate, soil types, and other environmental factors. Biomes are often characterized by their climax vegetation, which is the stable plant community in an area. A single biogeographic realm can contain many different biomes. For example, a tropical moist broadleaf forest in Central America and one in New Guinea might share the same biome type. However, they belong to different realms because their plants and animals have very different evolutionary histories.

History shows how our understanding of these areas has grown. In 1872, Alfred Russel Wallace developed a system of zoogeographic regions. He was an ornithologist, which is a scientist who studies birds. He expanded upon an earlier system created by Philip Sclater. Sclater had originally defined six regions. Later, Miklos Udvardy created a system of biogeographic realms based on taxonomic composition. Taxonomy is the science of naming and classifying living things. The World Wildlife Fund (WWF) also uses a scheme that is broadly similar to Udvardy's work. The main difference is how they define the Australasian realm.

There are several major terrestrial realms to identify. The Palearctic realm covers most of Eurasia and North Africa. The Nearctic realm includes Greenland and most of North America. Sometimes, scientists group the Palearctic and Nearctic together as the Holarctic realm. The Afrotropic realm includes Trans-Saharan Africa, Madagascar, and Arabia. In South America, the Neotropic realm covers the continent, Central America, the Caribbean, South Florida, and the Falkland Islands.

The Indomalaya realm includes the Indian subcontinent, Southeast Asia, and southern China.

The Australasia realm is a large area that includes Australia, New Zealand, and many islands like New Guinea and New Caledonia. The way this realm is drawn depends on which system you use. In Udvardy's system, New Zealand was part of the Antarctic realm. In the WWF system, it is part of Australasia. The Antarctic realm includes Antarctica and several surrounding islands like South Georgia.

There are also transition zones where different realms meet. Examples include the Mexican transition zone and the Chinese transition zone. These zones sit between two different realms.

The distribution of life across these realms is tied to the movement of the Earth. Plate tectonics, the study of how Earth's large plates move, has shaped the distribution of landmasses over geological history. As landmasses move, they create or destroy the barriers that define realms. This connection shows that the history of the Earth's physical structure is directly linked to the history of life. By studying biogeographic realms, we can see how the physical world and the biological world change together over millions of years.