A biome is a major biotic community consisting of different ecosystems in different parts of the world which share common characteristics. A good example is the desert biome. This biome is characterised by low rainfall and there are deserts in all parts of the world. Each has unique plants and animals because they are physically isolated from other desert regions but they all share numerous characteristics that distinguish desert ecosystems from other biomes such as grasslands.
Broadly the world’s major biotic communities are the tundra biome, the boreal forests, temperate forests, tropical forests, temperate and tropical grasslands, deserts and oceans (which can be further subdivided into tropical reef biomes, open and deep ocean biomes and a few others). The two arctic biomes, tundra and taiga (boreal forest) are relatively continuous, sweeping around the globe across Canada, Russia, northern Europe and Alaska with the only breaks created by oceans and glaciers. They are not only similar in climate but also share many of the same species and so are easy to differentiate from other biomes.
Biomes like the desert biome are much more physically separated and so differ greatly in plants and animals from one place to another. Death Valley in California, the Great Sandy Desert in Australia and the Sahara in Africa are much more separated from one another than the Tundra ecosystems of Siberia and Canada. Each of these deserts has unique flora and fauna because the different deserts evolved separately but they share all the common characteristics of deserts: low rainfall, blistering heat and winds, plants adapted to water conservation, animals adapted to survive without drinking. Even though they are separated by great distances, it immediately obvious that they are related and so the term desert biome is used to describe the commonality shared by all deserts.
The same is true of rainforests. The great Amazon jungle and the rainforests of Africa and Asia all share common characteristics. They occur in the tropics, with warm conditions all year round. They have only two seasons, a wet and a dry, rather than the four seasons experienced by temperate ecosystems. They are characterised by a huge diversity of plants and animals, which varies from place to place but overall one can see that the similarities are as important as the regional differences. There is no way one would link deserts and rainforests: they are two distinct biotic communities or biomes.
The world’s biotic communities can be understood better by looking at the hierarchy that exists. First there are the biomes, major biotic communities that develop in similar ways when conditions are similar. Then within a biome there are ecosystems, the individual areas within the biome that develop separately due to geographic isolation. Each desert has unique plant species, its own herbivores and carnivores but all fill similar roles as those found in other deserts. Below the ecosystem is the habitat where varying conditions allow for different species to adapt and live. The tundra ecosystem looks monotonously similar from the air but at ground level there are different habitats with different plants and animals. Ice wedges cause little ridges and depressions to develop. The ridges are dry and windswept while the depressions are wetter and more protected. Within each habitat, different plants and animals can be found. Within a habitat, there can be microhabitats, which even more specialised organisms can find a place to live and flourish.
At the top stand the biomes. These broad ecological categories describe the basic biotic communities that have developed to fit the conditions in the major regions of planet Earth.
