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The major wilderness types follow the changes in latitude and hence in climate. At the pole there is little precipitation (rain, sleet, snow, hail) and what does fall becomes locked up in ice and snow. This creates polar desert conditions. At slightly lower latitudes, where the ground thaws in summer, lies the tundra, an area of low growing plants that in winter are protected under a blanket of snow. Beyond the tundra is a band of coniferous forest, which gives way to deciduous forests of species such as oak and beech as winters become warmer. Closer to Equator, where rainfall is seasonal, there are hard-leaved forests, adapted to conserving water. The most humid tropics and subtropics support evergreen rainforests.
Where high mountains or distance from the sea reduce the amount of moisture reaching the land, vast expenses of grasslands have developed. In the areas where rain is still scarcer, there are deserts. By contrast, some coasts experience very heavy rainfall, which can promote the growth of temperature rainforests as luxuriant as any tropical forests. The largest wilderness of all is the ocean, which covers some 70% of the globe to an average depth of 4 km.
These great wilderness types are by no means uniform. There are thousands of lakes and bogs among the dark coniferous forests, islands of pines and birches invade the temperature grasslands, green oases enliven in deserts. Wilderness areas contain a bewildering variety of plants and animals, a web of life whose components are continually changing as they interact with each other and respond to global changes.
Although wilderness areas across the planet have enormous diversity, certain general types, such as grassland or tundra, can easily be distinguished. These wilderness types are characterized by their dominant vegetation and are called biome. There are 14 basic types of biomes, ranging from forests and grasslands to deserts, tundra, islands and lakes.
The wilderness is not static but is continuously changing. As mountains gradually rise or are eroded, the landscape and climate change, and nature of wilderness alters with it. Plants and animals need to evolve in response to these changes. Some adapt to the new environment, while others migrate to surrounding areas and adopt new ways of life.
Over many millions of years the continents have drifted over the face of the Earth. They moved from one latitude to another, coalescing and fragmenting. In the process of adapting to these changes a huge diversity of living species has evolved.
Species are the basic unit of diversity. A species is a group of similar living organisms that can be interbreed. Individual populations gradually evolve adaptations that make them particularly successful in their local habitat. This can lead to the evolution of subspecies and eventually of new species when the isolated population differs so much from other populations, that it can no longer interbreed with them.
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