Terrestrial Biomes

Terrestrial biomes are large geographic regions on land defined by their average temperature, average rainfall, and dominant plant types. The geographic locations of terrestrial biomes are loosely correlated with latitude because of global patterns of air circulation. There are also patterns in ocean circulation based on latitude as a result of differential exposure to the Sun. These ocean currents influence the climate of coastal biomes.

Biomes surrounding the equator tend to be warm with regular, year-round precipitation. This is because the equator constantly receives steady, direct sunlight. The warm air rises at the equator and loses water as precipitation. Dry biomes exist around 30 degrees north and south latitude because the air has already lost its moisture. The dry air falls at these latitudes and becomes warm as it descends. At 60 degrees north and south latitude, air picks up moisture and rises again, increasing the precipitation in the area. However, the temperatures are cooler because there are fewer direct rays from the Sun at these latitudes. At Earth's poles, the dry air falls again, leaving the polar regions dry and cold. Variation in average temperature and rainfall leads to differences in the primary vegetation found in various terrestrial biomes. For example, a desert cactus adapted to an arid climate is unlikely to be found in a tropical rainforest, and maple trees that require a lot of water are unlikely to be found on the Arctic tundra. Similarly, heat-adapted and cold-adapted plants are found in biomes that match their physiology. As British naturalist Charles Darwin, who came up with the theory of evolution, observed during his travels, organisms tend to be found in environments to which they are well suited.

Tropical rainforests experience high rainfall (200–400 cm/year) and high average temperatures (25–29°C). Ecosystems of this biome occur near the equator. Animal biodiversity (number and variety of species) is high in these regions. For example, in a square kilometer of tropical rainforest in Central America, toucans, poison dart frogs, anteaters, boas, and bullet ants may be found. Plants experience competition for sunlight because tall trees form a canopy, or leafy covering, that shades vegetation at lower levels. Tropical rainforests are threatened by overharvesting and habitat destruction from human development. Despite the high biodiversity of rainforests, the soil is relatively nutrient-poor. As a result, after these lush, highly productive rainforests are destroyed to make way for agriculture, the soil is unproductive when it comes to growing food. Tropical rainforest biomes include the Amazon rainforest in South America and the Congo Basin in Africa.

The distinguishing factor of deserts is that they experience low rainfall (less than 30 cm/year). Temperature is not a deciding factor because there are both hot and cold deserts. Average temperatures can be either high (exceeding 50°C, such as summer in the Sahara Desert) or low (below -80°C, as in Antarctica). With a lack of water to moderate temperatures, deserts can show large temperature fluctuations. Plants tend to be small and spread out with space between individual plants. Desert plants are adapted to dry conditions with long tap roots, water storage in stems and leaves, and spines to protect against herbivory. They often have specialized strategies to prevent water loss during photosynthesis, such as CAM photosynthesis. Desert animals are also adapted to low-water consumption. Many animals are nocturnal to avoid intense heat and sun exposure, such as badgers, coyotes, and scorpions.

Savannas are dry regions (rainfall of 30–50 cm/year) that are generally near the tropics. The savanna is warm year-round (25–29°C). Rainfall is greater than in deserts but still in the low range compared to other types of terrestrial biomes. Plants in the savanna are mostly grasses and shrubs, with some trees. Large mammals such as lions, zebras, giraffes, and elephants roam African savannas, while the scarlet ibis and capybara live on the Venezuelan savanna. Fossil evidence suggests the earliest humans originated in this biome. The oldest human fossils have been found in the African savanna. However, modern humans are overhunting the current large mammal inhabitants, to the point of endangering species. For example, the black rhino and the Indian elephant are endangered.

Large grasslands, also known as prairies, are generally located about halfway between the equator and the poles. Rainfall is seasonal (30–100 cm/year), with dry winters and wet summers. Similarly, winters are cold (–10°C) and summers are hot (30°C). The grasses in these regions build fertile soils, which can be converted into successful agricultural land. Historically, large herds of bison lived in the North American prairie. In South America, grasslands can be found in the Andes Mountains. The Puna mouse is a common inhabitant of this location. Habitat loss caused by agriculture has endangered this biome as farmers push wildlife out of their natural habitat in order to grow crops or graze livestock. In addition, the overuse of these lands for grazing has left huge portions dry and no longer productive, because the livestock eat all the native plants, causing the soil to become loose and nutrient-poor.

Like temperate grasslands, deciduous forests are also located about halfway between the equator and the poles, mostly in the northern hemisphere. They are characterized by moderate temperatures, with warm summers (25°C) and cold winters (less than 0°C). Rainfall is moderate to high (70–200 cm/year) and occurs year-round. In the winter, precipitation falls as snow. The primary vegetation is hardwood, deciduous trees, such as oaks, beeches, and maples. Many of the animals hibernate or migrate to escape the cold winters. For example, brown bears hibernate in winter, while monarch butterflies migrate to warmer climates in latitudes closer to the equator. Deer, bears, squirrels, and other forest animals are common in these regions. Deciduous forests can be found in Canada, the United States, and Central Europe.

The largest terrestrial biome on Earth is the taiga, or northern coniferous forest, where conifers (cone-bearing trees that do not lose their leaves) grow. This biome can be found in Alaska and northern Scandinavia. Taiga winters are long and cold (–30°C) and the summers are warm (20°C). Rainfall is generally moderate (30–70 cm/year); however, some taiga regions receive high rain or snowfall. Cone-bearing trees such as pines and and spruces are the primary vegetation, and plant biodiversity (number and variety of species) is somewhat low. Large animals, such as moose, elk, wolves, and brown bears inhabit the taiga. Overharvesting of trees has endangered many old-growth forests of this biome because conifers can take as much as 70 years to grow from a seedling into a mature tree.

The tundra biome can be found in the northernmost and southernmost parts of the world, for example the Arctic Circle in the far north. Parts of Antarctica in the south also qualify as tundra, but this biome is mostly considered a desert due to its lack of rainfall. Tundra is also found on mountaintops, such as the top of Mount Kilimanjaro. This biome is characterized by cold winters (–30°C) and cold summers (10°C). There is very little precipitation (20–60 cm/year), and much of what falls freezes on the ground. It is also a windy region. Grasses such as polar grass and some herbaceous plants such as sedges can grow, but a permanently frozen layer of soil called permafrost prevents deep roots. Large grazing animals such as moose and elk are common, and some migrate to warmer regions for the winter.