Biomes are divisions of the biosphere into regions with climatic conditions distinct enough to produce characteristics that are evident in vegetation and animal species adapted to them.
Biomes may be variously defined, such as leaf type, vegetation density, climate, and patterns of succession.
The high latitudes, but not polar regions, where sunlight is weak and winters harsh, with short daytimes, leads to a biome distinction in its lack of large vegetation, namely trees.
The dominant vegetation of the Tundra is grass, moss, lichen, with the occasional shrub.Tundra may be sub-divided into Arctic, Alpine and Antarctic tundras. As well as latitude, altitude creates ecological boundary regions (ecotones) in a treeline, at the changeover point between tundra and forest.
The Savanna, or grasslands, is a biome characterised by large swathes of open grassland, with scattered pockets of trees.
The density of trees can vary greatly in different savannas, but the open canopy permits richer ground-level ecosystems to develop.
The Savanna is often home to migratory herds of herbivores, such as the wildebeest and zebra in Tanzania. The migratory tendencies are due mainly to the seasonal precipitation patterns, and often there is a single 'wet season'.
The Rainforest is a biome characterised by dense vegetation and extremely high biodiversity. Rainfall is high, and the monsoon is a critical factor in the development and maintenance of the tropical rainforest.
Rainforests can be tropical or temperate. As much as 60% of all species have their habitat in the rain forests. Rain forests provide services for the global environment, such as the carbon sink, and microorganisms for the pharmaceutical industry.
Rainforests are being depleted at an alarming rate, and as a result anthropological climate change is being accelerated, as well as being a major cause of the current biodiversity catastrophe.
Deserts are areas with a higher rate of evapotranspiration than precipitation. Deserts have less than 250mm of rainfall annually, although some deserts, such as in Chile, receive considerable amounts of moisture in the form of fog.
Semi-deserts, or steppes, have up to 500mm of precipitation, and may have grass covering.
The desert has far lower biodiversity levels than the savanna, and present extremely harsh conditions for survival, resulting in highly specialised species.
Deserts are usually hot and sandy, however there are cold deserts, such as in polar regions.
Aquifers may bring water from mountain sources many hundreds of kilometres away. When these reach the surface, usually due to a fault in the subterranean rock strata, it may form an oasis, a micro-climate which forms an 'island' of thriving life in the middle of desolation.
Those who live in northern Europe, and many other temperate zones, will be familiar with the deciduous forest: oak, elm, beech, maple attest a greater biodiversity than the Taiga.
Since deciduous trees lose their leaves in autumn, the canopy coverage seasonally changes dramatically. During canopy cover, species adapted to low-light conditions have adopted a wide range of survival strategies. Many tree species specialise in adverse conditions to out-compete more common species.
Typical of northern Russia and Canada, the Taiga, or boreal forest, is a biome with coniferous forests (primarily larch, spruce, and pine).
Temperatures can be extremely low, and growth periods limited to less than 100 days a year. Trees are in competition for the low-intensity sunlight, so grow straight and tall, except where peat and water-logged terrain impedes growth.
Taiga soils are poor quality, due mainly to the cold. The Brown Bear is a dominant omnivore species of the Taiga.
Nearly all life is found in the oceans (maximum depth of 11 km), and within about a kilometre beneath the Earth's surface. Life has adapted to survive in practically all places on Earth, whether at the poles, or in arid desert at the equator, high in the mountains, or underground. This thin veneer of the surface of the planet is the biosphere.
Technically, the biosphere is the sum of all ecosystems, and materially is a closed system: life does not normally go walk-about around the cosmos. However, life is totally dependent on the Sun's energy and light. And the Earth's surface benefits from heat created by nuclear decay of uranium, thorium, and a few other radioactive substances, in its interior.