Polar bear

Why do polar bears live only in the Arctic and most penguins only in the Antarctic? Why are there so many hummingbirds in South America and so many sunbirds in Africa? Is it true that horses and camels (and perhaps rhinoceroses) evolved in North America, only to die out there? And why do the biggest tortoises live on islands?

These are the kinds of questions a biogeographer might ask. Biogeography is the study of the distribution of life. It overlaps with such disciplines as zoology, botany, ecology, evolution, and paleontology to explain why various living things live where they do.

Biogeographic Realms ˆ

Physical geographers divide Earth’s land areas into continents. However, humans don’t always respect boundaries, making human geography more complex. While terrestrial plants and animals may have a hard time migrating across the sea, they, too, have little respect for imaginary lines separating continents. Though we think of the lion as an African animal, lions also live in India. In fact, they were widespread across Eurasia in relatively recent times and lived in North America during the Ice Age. Similarly, pumas and jaguars live in North America and South America both.

Nevertheless, there are significant differences between different regions, and these differences nurture the evolution of distinctive communities of living things. Scientists have even mapped the biosphere, dividing Earth’s land areas into eight biogeographical realms.

The New World is relatively simple, with North America and South America renamed the Nearctic and Neotropical Realms. The division isn’t precise, however. The Neotropical Realm includes southern Mexico, Central America, and the Caribbean.

The Old World is more complex. Most of Eurasia, along with northern Africa (primarily the Sahara Desert) makes up the Palearctic Realm. The rest of Africa makes up the Afrotopical Realm (formerly the Ethiopian Realm), while southern Asia is called the Indo-Malay Realm (formerly the Oriental Realm).

Map of biogeographic realms

The Australasian Realm consists of Australia and a variety of nearby islands. The least diverse realm by far is the Antarctic Realm, consisting primarily of Antarctica.

The eighth terrestrial realm is the Oceanian, which consists of islands in the Pacific Ocean.

The Afrotropical and Indo-Malay realms combined are often referred to as the Old World tropics, while the Nearctic and Palearctic realms are combined into the Holarctic. The latter two are very similar, with wolves, moose, and blueberries among the species living in both northern North America and northern Eurasia.


  • Nearctic—Wolves, moose, caribou, grouse, ravens, willows, conifers, and blueberries are characteristic species. The world’s only venomous lizards also live here.
  • Palearctic—Wolves, moose, caribou, grouse, ravens, willows, conifers, and blueberries are characteristic species. The Palearctic Realm is also home to the famous Siberian tiger.
  • Nearctic—Fabulously diverse, the Narcotic Realm boasts more species of birds, mammals, reptiles, amphibians, and freshwater fishes than any other realm. As you might guess, South America. Is home to more species of mammals than any other continent, most of them rodents and bats.
  • Afrotropical—Tropical Africa is renowned for its extraordinary diversity of hoofed mammals and large predators, including the biggest land mammals—the African elephant, hippopotamus, two species of rhinoceroses, and the giraffe. Africa is also the home of chameleons and man’s closest living relative, the chimpanzee.
  • Indo-Malay—This fabulously diverse realm is home to tigers, orangutans, Komodo dragons, and exotic hardwoods.
  • Australasian—Its splendid isolation gives this realm some truly unique flora and fauna, including most of the world’s marsupials and the only surviving egg-laying mammals.
  • Antarctic—Penguins!

Oceanic Realms ˆ

Mapping marine biogeographic realms is a little trickier. After all, all the world’s oceans are connected into one world ocean.

Nevertheless, there are various environmental factors, notably temperature and salinity, which influence the distribution of marine life.

Actually, the most important factor is sunlight, which can only penetrate water to a certain depth. Believe it or not, the world ocean is essentially an enormous desert except for the continental shelves, which support an amazing diversity of life.

World Wildlife Fund divides the seven seas into twelve biogeographic realms:

  1. Arctic
  2. Temperate Northern Atlantic
  3. Temperate Northern Pacific
  4. Tropical Atlantic
  5. Western Indo-Pacific
  6. Central Indo-Pacific
  7. Eastern Indo-Pacific
  8. Tropical Eastern Pacific
  9. Temperate South America
  10. Temperate Southern Africa
  11. Temperate Australasia
  12. Southern Ocean

However, other classification schemes may recognize twice as many realms.

On Your Knees ˆ

Telling someone that ibex live in the Palearctic Realm is about as helpful as telling them that Microsoft is a corrupt corporation based in North America. Do ibex live in Scandinavia? The Sahara Desert? Do they live in trees, or underground?

Any self-respecting naturalist has spent lots of time on his or her hands and knees peering at Nature’s mysteries. To track down those elusive ibex (ibexes?), we’re going to have to divide biogeographic realms into biomes, bioregions, ecoregion, and habitats. Let’s take a closer look at ecohomes.

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