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The Terrestrial Biomes
The terrestrial world can be divided into areas called biomes. A biome is a large area of land classified by its distinct plants and animals. The characteristics of each biome are dependent on its temperature and the amount of precipitation the area receives. The plants and animals found in each biome are adapted to the particular environment of the biome.
A biome is made up of many ecosystems. An ecosystem is the interaction of living and nonliving things in an environment. However, a biome is the specific geographic area in which ecosystems can be found.
For the purpose of this backgrounder we will identify major terrestrial biomes of the world based on the Whittaker biome classification scheme. It is interesting to note that not everyone agrees on the number and types of biomes.
Distribution of the Earth’s Major Biomes
The map below shows where each of the eight major terrestrial biomes are located in the world. Canada contains four biomes: temperate deciduous forest, grassland, boreal forest/taiga, and tundra. A biome has the same characteristics in any part of the world when it can be found. Therefore, the boreal forests of Canada look like the boreal forests of Russia. The characteristics of each biome are dependent on its climate, particularly temperature and the amount of precipitation the area receives.
When looking at a world map, the majority of deserts are found along two lines of latitude. These two lines are called the Tropic of Cancer (30 degrees North) and the Tropic of Capricorn (30 degrees South).
Around these latitudes, dry air coming from the equatorial regions (around 0 degrees) dries out the land. Some deserts are found in landlocked regions. These are places not bordered by an ocean. One example of this is the Great Basin Desert in North America. This is the largest desert in The United States. Coastal deserts form near the coasts of continents. As cool air moves from west to east across oceans, it can create cold foggy weather instead of rainfall along the western coasts of continents. Two deserts that fit this pattern are the Namib Desert in southwestern Africa and the Atacama Desert in South America.
Deserts differ based on where they are and the type of climate found there. Deserts are regions of land that receive less than 25 cm of rain each year. We usually think of deserts as being very hot, but some deserts can be very cold. Cold deserts can be found in the Antarctic as well as Greenland. What defines a desert is rainfall, not temperature. Some deserts receive more rain than others, but even when a desert does receive rain, the water evaporates quickly. With few plants, little water and extreme swings in daily temperatures, the soils in deserts tend to be rocky or sandy and have very little organic matter (from dead plants). These soils are known as aridisols or desert soils. Many deserts also experience a lot of wind.
Plants & Animals
A desert is often thought of as a windy expanse of sand. But there is more going on in deserts than meets the eye. The plants and animals in deserts have special adaptations which help them to live in this sometimes harsh biome.
Plants found in deserts have developed ways to reduce the natural evaporation from their leaves. They also have ways to protect themselves from desert herbivores (plant-eating animals). Plants reduce evaporation by having small leaves and waxy cuticles. The cuticle is a protective covering on leaves. Because the sunlight is so intense, plants have small or no leaves. Some desert plants only grow leaves in response to rainfall and then have no leaves during most of the year when it is dry. Other plants, in the cacti family, do not have any leaves. Instead they have hairs or spines covering them. These hairs/spines have a double benefit. Not only do they help to reduce evaporation, they also discourage animals from eating them. Like camels, cacti can store water in their tissues to use later.
Animals in hot deserts are good at avoiding the heat. When it is very hot, many animals will only come out at night, when the Sun has gone down. During the day, most animals will seek shelter in shady areas. An example of this is the desert scorpion, which hunts at night and spends the day hidden. Several different desert animals, such as rodents, burrow underground to keep cool during the day, similar to the rodents in the savanna biome. In the Sonoran Desert in the United States, ground squirrels spend large amounts of time in burrows built underground. The burrows they dig provide shelter from the Sun and from predators. When temperatures are moderate, the ground squirrels will venture above ground to look for food.
Human Impacts & Conservation
Human activities, such as allowing livestock to graze on grasslands, are turning other biomes into deserts. This is called desertification. Population growth and greater demand for land is making this a difficult problem to solve. Climate change is also making these hot dry places even hotter and drier. Off-road vehicles such as dune buggies, oil and gas production and urbanization, such as building towns and cities, are all causing damage to desert plants which take a long time to grow. For example, the saguaro cactus takes 200 years to grow to full size!
Some proposed solutions include planting bushes and grasses that keep the sand from blowing around and digging ditches that can store rain as well as wind-blown seeds. People are encouraged to use off-road vehicles only on designated trails and people living in desert resort cities, such as Palm Springs, California (in the Sonoran Desert), are encouraged to replace their water-loving grass lawns with native desert plants which do not require watering. This is known as xeriscaping.