Biomes

Biomes as global ecosystems

An ecosystem consists of a community of living organisms and their interactions with each other and their physical environment within a particular location. The components of an ecosystem are either biotic (living) or abiotic (non-living, like rocks and bodies of water). Ecosystems also include habitats, which are the living conditions for a species within an ecosystem.

You can think of a biome as a global ecosystem. A single biome is essentially a combination of all of the many ecosystems of the same or similar type. For example, all of the desert ecosystems in the world are part of the larger desert biome, from the Sahara Desert ecosystem in North Africa to the Sonoran Desert ecosystem in North America.

Types of biomes

Let's get one thing straight: biogeographers do not agree on the number of types of biomes!

Some biogeographers like to keep things simple and may divide the world into as few as five or so biomes. Some nuance may be lost. For example, ecosystems as different as the tropical Amazon Rainforest in Brazil and the temperate Black Forest in Germany may be rolled into a single "forest" biome.

Fig. 1 - The Black Forest in Germany

On the opposite end of the spectrum, some biogeographers may divide the world into 30 (or more) different biomes in an attempt to differentiate as many distinct ecosystems and climates as possible.

Seven of the most commonly-used biome classifications are tropical rainforest, temperate deciduous forest, savanna grassland, temperate grassland, polar, desert, and coral reef.

Characteristics of biomes

There are lots of different characteristics for each biome. Let's explore them.

Tropical rainforest

The tropical rainforest biome is characterised by dense vegetation, high humidity, high heat, and high precipitation. Tropical rainforests also have very high biodiversity; many different types of plants and animals live in tropical rainforest ecosystems throughout the world. Most tropical rainforests are found around the equator.

Temperate deciduous forest

The temperate deciduous forest biome includes temperate forests that experience four seasons. Deciduous trees and shrubs drop their leaves in the autumn to conserve resources in the winter. Most temperate deciduous forest ecosystems are found in the United States and Europe.

Savanna grassland

Savanna grasslands are plains with a sparse population of spread-out trees. Savanna grasslands tend to be relatively dry and warm, which means most vegetation includes hardy grasses and shrubs. Most savanna grasslands experience two seasons, dry and rainy.

Temperate grassland

Temperate grasslands are dominated by shrubs and grasses; you will find very few trees here. The weather may be dry or humid, but temperate grasslands are not as consistently warm as savanna grasslands.

Polar

The polar biome includes most of the Arctic (North Pole) and Antarctic (South Pole). It's cold. Very cold. Most of the polar biome experiences below-freezing temperatures for most of the year and the ground is usually covered in permanently frozen ice.

Polar ecosystems are a type of cold environment. Another major type of cold environment is the tundra, which is often classified as a separate biome from the polar biome.

Desert

Deserts are the driest places on earth; on average, they receive less precipitation than anywhere else. As a consequence, they have relatively little biodiversity.

There are essentially two major types of deserts: hot deserts and cold/temperate deserts. While both of them are very dry, hot deserts experience sweltering daytime heat while cold desert temperatures tend to be milder. Often, hot deserts and cold deserts are treated as distinct biomes.

Coral reef

The coral reef biome is an aquatic biome found throughout our world's oceans. It includes all of the various coral reef ecosystems in the world. Corals that build reefs can create habitats for dozens of species of fish and crustaceans.

Climate classifications and ecozones

We mentioned earlier that not all biogeographers agree on the number of biomes. What can make things perhaps more confusing is that some biogeographers don't even use biomes at all!

Ecozones (also called biogeographic realms) divide the world into large, broad categories based on physical location. You can think of ecozones as an alternative to biomes; instead of combining geographically-distant ecosystems from all over the world, ecozones are mostly geographically contiguous regions. For example, Australia is not partially desert biome, partially forest biome – rather, the entire continent belongs to the 'Australasian Realm.' Ecozones are a way of looking at how different communities of animals are distributed in particular places.

On the other hand, climate classifications often overlap with biome classifications. Climatologists and biogeographers often use the same terms to describe climate types and biome types.

So, what is the difference between a biome and a climate? A climate is mostly concerned with an area's weather patterns and their effects, while a biome includes climate and wildlife.

Distribution of biomes

Take a look at the map below to see the global distribution of the biomes listed above.

As we've mentioned, it is common to see the desert biome split into "hot desert" and "cold desert," or to see the polar biome split into "polar" and "tundra."

The effect of latitude and climate

Why are the world's biomes where they are?

The answer largely boils down to both climate and latitude, which, in many ways, affect each other.

The equator receives more direct/concentrated sunlight than anywhere else on earth. Though other areas of the Earth receive a similar amount of sunlight, the angle of the sun in relation to the shape of the planet causes the sunlight to be less concentrated. In other words, it is spread out over a wider area. Additionally, the atmosphere is thinner around the equator, meaning sunlight has less to travel before reaching the Earth.

More simply? The further you get from the equator in latitude, the colder it gets. The warmest biomes, like deserts and tropical rainforests, tend to be closest to the equator, while the coldest biomes, like the polar biome, tend to be furthest from the equator.

But it's not just a simple matter of latitude, or else everything along the same latitude would always belong to the same biome! Climate, which you'll remember is closely linked to a biome's classification, is largely affected by physical geography. Mountains, wind patterns, and bodies of water affect the development of a local climate, which, in turn, affects the development of local ecosystems and global biomes.

Examples of biomes

Let's take a closer look at the temperate grassland biome to review the concepts we've covered so far.

Temperate grassland ecosystems are usually found above the Tropic of Cancer or below the Tropic of Capricorn. As a result, they tend to have milder temperatures than ecosystems in the tropics. Temperate grasslands also tend to be a little dry relative to other ecosystems (which is why they usually cannot support widespread tree growth) but not nearly as dry as deserts.

Fig. 3 - The temperate grassland biome in green

Different types of temperate grasslands include the steppe and prairie, and notable temperate grassland ecosystems include the Great Plains of North America and Mongolian-Manchurian grassland in Asia. All of the different temperate grasslands in the world belong to the temperate grassland biome.

The temperate grassland biome is characterized by grasses, shrubs, and large herds of migrating herbivores, as well as small rodents and ground-dwelling birds and the predators who try to feed upon all of them. To adapt to a 'sea of grass,' large herbivores constantly stay on the move, looking for new pastures to feed on, while smaller animals make their homes on or under the ground – with a lack of tree coverage, there aren't too many other options. In Mongolia, large herbivores include Przewalski's horse and the Mongolian gazelle, while North America is home to the bison and the pronghorn. These animals may not be related, but the nature of the biome has caused them to develop similar survival strategies.