What are biomes?
Biomes are regions of the Earth’s surface that share similar geographic and climatic features. They are typically characterized by the dominant abiotic (non-living) and biotic (living) components of the environment, and can therefore be thought of as the world’s broadest classes of ecosystems. Abiotic components include the gases of the atmosphere, light, water, soil and rocks. Biotic components include all living organisms, such as plants, animals, fungi and bacteria. Each of the world’s major biomes consists of a unique assemblage of plants, animals and other organisms, which are well adapted to the area’s physical environment.
Biomes are heavily shaped by climate. Although climate is determined by a number of meteorological conditions, as averaged over a long period of time, the two most important factors are temperature and precipitation. Precipitation includes rain, snow and ice. Very cold areas like Antarctica rarely receive much precipitation because cold air holds less moisture than warm air. Some of the wettest places on Earth are found within the tropics where the air is warm all year around.
All the biomes on Earth may be classified as one of two types: terrestrial or aquatic. Terrestrial biomes are found on land, and aquatic biomes are found in environments dominated by water. Aquatic biomes may be either marine (salt water) or freshwater. Although the oceans cover most of the Earth, making the marine biomes the largest on the planet, terrestrial biomes are the most diverse. The interactions between the atmosphere and the topography of the land helps to create a wide variety of climates and ecosystems. Below we describe a few of the more common global terrestrial biomes found on Earth.
Tropical Rainforest Biome
Around the Earth’s equator, temperatures fluctuate very little throughout the year. Within this region, known as the tropics, it is generally warm all year around. The only exceptions are mountainous areas, such as the Andes in South America. The high temperatures and abundant rainfall supports the growth of dense vegetation. In certain areas this dense vegetation forms large forests known as tropical rainforests. The largest tropical rainforests on Earth include the Amazon Rainforest in South America (drained by the Amazon River), Central Africa around the Congo River, Southeast Asia and Indonesia.
Tropical rainforests are the most biologically diverse regions on Earth. Although the exact numbers are unknown, tropical rainforests are likely home to over a million different species, and probably contain at least half of all species on Earth. Unfortunately, the area covered by rainforest has diminished significantly over the last century. The renowned biologist E.O. Wilson estimated that as many as 50,000 species are driven to extinction every year due to deforestation in the tropics. Furthermore, the World Wildlife Fund (WWF) has estimated that as much as 60 percent of the Amazon Rainforest may be destroyed or severely degraded by 2030.
Desert Biome (Arid & Semi-Arid)
The desert biome, often divided into the sub-biomes based on rainfall and vegetation, is characterized by limited rainfall. The desert biome can either be warm or cool (not all deserts are hot!). Regions in the very northern and southern latitudes (near the poles) , however, are generally not considered “deserts” even if they receive very little rainfall. Therefore, most desert biomes are found in the Earth’s low-to-mid latitudes. An area is generally designated a desert if it receives less than 10 inches (25 centimeters) of rainfall annually. While the lack of rainfall does tend to limit the growth of vegetation, plants that can efficiently absorb and retain water, known as xerophytes, are not uncommon.
The Sahara Desert in northern Africa is the largest desert on Earth. It covers some 9.4 million square kilometers (3.63 million square miles) and contains massive sand dunes up to 180 meters (600 feet) high. Much of the desert has very little vegetation; most plants and animals live around small pockets of water throughout the desert, known as oases. As large and barren as the Sahara is, it does not contain the driest place on Earth. Portions of the Atacama Desert, just west of the Andes Mountains in South America, receive as little as 1 millimeter (0.04 inches) of rainfall per year on average, making it the driest desert on Earth.
Temperate Forest Biome
The temperate deciduous or broad-leaf forest biome occupies a significant portion of the mid-latitudes, particularly in the Northern Hemisphere. With four distinct seasons (warm summers and cool winters) and a moderate amount of rainfall (typically 30 to 60 inches (75 to 150 centimeters), the climate supports large deciduous forests. Deciduous trees, such as oaks, maples, hickories, poplars and chesnuts, lose their leaves in the fall in anticipation of winter, and produce new leaves in the spring. Temperate forests once covered most of Europe, and still covers the majority of Eastern North American. Temperate forests can also be found throughout Asia, most notably in southern Russia and northern China.
Temperate Grassland and Savanna Biomes
Found on six different continents, grasslands cover a significant portion of the Earth’s surface. Grasslands occupy a climatic zone somewhere between forests and deserts; they are typically too dry to support forest vegetation, but too wet to be classified as deserts. Grass tends to grow well in these semi-dry environments, and therefore covers much of this climatic zone.
Grasslands typically receive between 25 and 75 centimeters (10 to 30 inches) of precipitation per year. Their temperature, however, can vary widely from season to season and from place to place. Temperate grasslands (sometimes referred to as the temperate steppe biome) are found in the mid-latitudes and, like temperate forests, usually experience warm summers and cool or cold winters. Examples of temperate grasslands include the Great Plains of North America, the Pampas in South America, and portions of the Eurasian Steppe.
The term “savanna” is typically used to refer to grasslands that also contain a limited number of trees with an open canopy. Savanna’s occupy a large portion of Africa and South America. One of the largest savannas in the world is the Serengedi in Africa, which occupies a large swath of land between the Sahara Desert to the north and the Congo Rainforest to the south. The expansive grassland supports 70 species of mammals and over 500 species of birds.
Taiga (or Boreal Forest) Biome
The Taiga biome, characterized by coniferous forests, is the largest terrestrial biome on Earth. The taiga covers much of Canada and Russia, the two largest countries in terms of area, and contains about 30 percent of all the forested land on Earth. The defining climatic characteristic of the taiga biome is cool annual temperatures, which are lower than any other biome except the tundra. In the taiga, winters are long (typically 6-7 months) and cold. Some of the lowest temperatures in the Northern Hemisphere have been recorded over northern Russia within a vast expanse known as Siberia. With annual mean temperatures around 0° C (32° F), the soil in the taiga is frozen throughout the year, a feature known as permafrost.
The tundra biome is found in the very northern latitudes where annual temperatures are too cold support tree growth. The tundra thus begins at the “tree line,” the location or elevation at which trees no longer grow. The vegetation in the tundra consists of shrubes, grasses, mosses and lichens. The majority of the tundra biome is located north of the taiga in the Northern Hemisphere along the arctic coast of Canada, Alaska and Russia. Like the taiga, the winters of the tundra are long and cold, with temperatures averaging around −28 °C (−18 °F) during the winter months. In the summer, temperatures average about 12 °C (54 °F), but night-time lows are commonly below freezing. With such low temperatures, the tundra’s biodiversity is relatively low.
The Mediterranean biome is characterized by warm, dry summers and cool, wet winters. The biome is named after the area surrounding the Mediterranean Sea, including Spain, southern France, Italy, Greece, and northern Africa. Several other locations around the world, however, have “Mediterranean-like” climates, including much of California, West and South Australia, and Southwestern South Africa. The dominant vegetation in the mediterranean biome is shrubland, which is called maquis in the Mediterranean Basin and chaparral in California.