Hot Deserts
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What are biomes?


Introduction to biomes

What affects the distribution of biomes?


There is a range of factors that affect the distribution of biomes. These include climate, altitude and soil.

Climate – rainfall, temperature and sunshine hours are the main factors influencing biome distribution, with colder climates being found further from the Equator (remember that latitude is the main factor affecting climate!).

The deciduous forest ecosystem
The deciduous forest ecosystem

Altitude – height above sea level has a big impact on the growth of vegetation. In higher altitudes, it is colder (remember that the air is thinner so it can’t retain heat, and it is also windy and exposed) – so fewer plants grow and fewer animal species can adapt to the harsh environment. Cold upland environments also have thin soils which are lacking in nutrients, due to the absence of organic matter from plants and animals to decompose back into the soil.
Soil – soil type affects biodiversity too – nutrient-rich soils can support much more vegetation, and therefore more animal species. Plant growth is also affected by soil acidity, drainage and thickness.

What are the main biomes?

Tropical rainforest biome

Tropical rainforests are found within the tropics, between 23.5° north and south of the equator. They have a consistent climate – it is hot and wet, with about 12 hours of daily sunshine all year round. Because of this they are rich in plants and animals – almost all plants are evergreen and they grow quickly and are adapted to absorb maximum sunlight. Decomposition occurs rapidly due to the heat and moisture, supplying a constant supply of minerals to the soil. The dense vegetation cover provides food and habitats for many animal species.

Savanna grassland biome

Savanna grasslands are found within the tropics, mainly between 5° and 15° north and south of the equator. There is lots of sunshine all year and relatively low rainfall (800-900mm) – so it’s hot with a wet and dry season, with temperatures ranging between 15°C and 35°C. Vegetation is mainly grass, scrub, small plants and a few specially adapted trees, such as the acacia and baobab which can recover quickly after a fire – fires are common in the dry season. During the dry season the grass dies back and returns nutrients back to the soils, but these are washed out (leached) during the wet season. Savanna grasslands have many insects and lots of large mammals, such as lions, elephants, giraffes and zebras.

Desert biome

Deserts are found within the tropics, between 15° and 30° north and south of the equator, in a belt of high pressure and low rainfall (less than 250mm per year – sometimes only raining every few years), making them very hot and dry. The daily temperature range is high – from around 45°C during the day to below freezing at night – the lack of cloud means plenty of sunshine during the day, but no insulation once the sun goes down. The lack of rainfall means plant growth is sparse, limited to cacti and thornbushes, and many plants have a very short cycle due to the short growing season, so will grow quickly at the first sign of rain. Due to the sparse vegetation there is little leaf litter, which decomposes slowly, so the soils are thin and poor. There are few animals in this biome, including insects, scorpions, lizards and snakes.

Temperate biome

Temperate forests are found between 40° and 60° north and south of the equator, giving them mild temperatures and high rainfall throughout the year (up to 1500mm). There are four distinct seasons with warm summers and cool winters, hours of sunshine vary throughout the year and days are shorter in winter. Forests are made up of broad-leaved trees (eg. oak, ash, birch) that are deciduous, so drop their leaves in the autumn. Leaf litter decomposes quickly so soils are thick and nutrient-rich. The mild climate and rich plant-life provide food and habitats for mammals such as foxes, squirrels, badgers, birds and insects.

Taiga (boreal) biome

Taiga forests are found in high latitudes, around 60°north of the equator and in mountainous areas. They have long cold winters (-20°C) and short mild winters (-10°C). They get plenty of sunshine hours, but very little daylight in winter months. There is less than 500mm of precipitation per year, and most falls as snow. Most trees are evergreen, such as coniferous species like pine and fir, and there are low growing plants, such as lichen and mosses. Trees have needles rather than leaves, and these decompose slowly due to the cold, making soils nutrient-poor and acidic, and the soil is frozen for much of the year. Animals include black bears, wolves and elk – there isn’t much food available in this biome so there are relatively few animal species.

Tundra biome

Tundra areas are found far north, above 60° north of the equator, in Northern Europe, Alaska and Russia, and are below freezing for much of the year (summer 5-10°C; winter -30°C). Precipitation is low (less than 250mm) and most falls as snow. There is near-continuous sunlight in the summer, with little sunlight in the winter, which means there are few trees. Vegetation is mainly low shrubs and mosses, and produces little leaf litter which decomposes very slowly in the cold, so soils are thin and nutrient-poor, and have a layer of permanently frozen land below the surface (permafrost). Not many animal species except for Arctic hares, Arctic foxes and birds – some animals will migrate south during winter.

Polar biome

Polar environments are found further north, more than 66° north of the equator, within the Arctic and Antarctic circles.  This biome is very cold all year round – during the summer the average temperature at the north pole is 0°C and at the south pole is -18°C, and during the winter the average temperature at the north pole is -40°C and at the south pole is -60°C. There is very little precipitation – a lack of evaporation due to the extreme cold, and soils are permanently covered in ice, so are permanently frozen. Few animals can survive these harsh conditions – polar bears have adapted to the Arctic and penguins have adapted to Antarctica.



What might the examiner ask?

Explain how the Hadley cell works and why air sinks at 30 degrees N/S, what this causes (high pressure) and how high solar insolation gives high temperatures, forming an area of dry, warm deserts.

Lesson Summary Quiz

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