Savannas and Hot Desert Biomes

Savannas are tropical grasslands with scattered trees and a pronounced wet and dry season, while hot deserts are very dry regions that receive less than about 250 mm of rain per year and have extreme temperature ranges between day and night. In brief: savannas, such as the Serengeti in East Africa, have 500–1,500 mm of rain, mostly in a wet season, and depend on fire and grazing; hot deserts, such as the Sahara, have sparse vegetation made of plants and animals adapted to save water. Both biomes lie mainly between the equator and about 35° latitude.

Together, these biomes cover a large part of the tropics and subtropics and are home to well-known wildlife as well as many farmers and herders. This guide explains the climate of each biome, how living things adapt, how they differ from each other, compares them in a table, works through example calculations, outlines the history of the study of drylands and corrects common mistakes.

What is the climate of the savanna?

Savannas lie between the rainforests near the equator and the deserts further north and south. They are found in Africa (the largest area), South America (Llanos and Cerrado), northern Australia and parts of India. Temperatures stay warm all year, about 20 to 30 °C. Rainfall is seasonal: the sun is overhead, the intertropical convergence zone brings rain in the wet season, and in the dry season the zone moves away and rainfall may stop for several months.

How do savanna plants and animals live?

Grasses dominate, and trees such as acacia and baobab are widely spaced. Plants cope with drought and fire in several ways. Grasses grow fast in the rains and then dry out, but their growing points are at ground level, so they regrow after burning. Acacia trees have deep roots and flat, umbrella-shaped crowns. Baobab trees store water in thick trunks and drop their leaves in the dry season. Fires, whether natural or set by people, prevent forests from taking over and renew the grass.

The African savanna supports large herds of herbivores such as wildebeest, zebra, gazelle and elephant, and predators such as lions and cheetahs. Many animals migrate to follow the rains, and the Serengeti–Mara wildebeest migration involves over a million animals. Termites and dung beetles recycle nutrients. Soils are often only moderately fertile and are vulnerable to erosion where overgrazing removes the grass cover.

What is the climate of hot deserts?

Hot deserts form mainly at about 15° to 35° latitude, where air that rose at the equator sinks. Sinking air warms and dries, so clouds rarely form. Examples are the Sahara, the Arabian, Thar, Sonoran and Australian deserts. Rain is not only low but unreliable, and may fall in single storms. Cloudless skies allow daytime temperatures to rise above 40 °C and then cool rapidly at night, so the daily range can be 20 to 30 °C or more. The Sahara covers about 9 million km², roughly the size of the United States.

How do desert plants and animals survive?

Plants are adapted either to store water or avoid drought. Cacti have fleshy stems that hold water, spines instead of leaves to reduce water loss and shallow, wide-spreading roots that quickly absorb rare rain. Other plants have very deep roots, small waxy leaves or seeds that lie dormant for years and sprout after rain (ephemerals).

Animals are often active at night. The camel can tolerate dehydration and stores fat in its hump (not water). The fennec fox has large ears that release heat. Many desert reptiles and rodents avoid the heat by sheltering in burrows, and some rodents get almost all their water from seeds. Light colors reflect the Sun, and efficient kidneys limit water loss.

Savanna and hot desert compared

Worked example: aridity and temperature range

1. Daily temperature range: a desert has a maximum of 44 °C by day and a minimum of 12 °C at night. The range is 44 − 12 = 32 °C.
2. Aridity index: the aridity index is precipitation (P) divided by potential evapotranspiration (PET), the water that could evaporate if it were available. If P = 100 mm and PET = 2,500 mm, the index is 100 ÷ 2,500 = 0.04. By the UNEP classification, values below 0.05 are hyper-arid, 0.05 to 0.20 arid and 0.20 to 0.50 semi-arid.
3. A savanna site: P = 900 mm and PET = 1,800 mm gives 900 ÷ 1,800 = 0.5, which is at the boundary between semi-arid and dry sub-humid.
4. Wet season share: if 80% of a savanna’s 900 mm falls in 5 months, that is 0.8 × 900 = 720 mm, or 144 mm per month, compared with 180 ÷ 7 = about 26 mm per month in the remaining 7 months.
5. Migration distance: if herds travel 800 km in a loop over 4 months (about 120 days), they average 800 ÷ 120 = about 6.7 km per day.

How have these biomes been studied?

Early explorers described the African savanna in the 19th century, and the Serengeti was protected as a game reserve in the 1920s and became a national park in 1951. Ecologists later showed that fire and grazing are natural parts of the system. For deserts, the drought and famine in the Sahel during the 1970s and 1980s drew attention to the vulnerability of dry regions. Studies of the Sahara’s past show it was much wetter, with lakes and grassland, roughly 5,000 to 11,000 years ago, a change linked to variations in Earth’s orbit. Satellite data now monitor vegetation and the movement of desert edges.

Which mistakes are common?

Mistake 1: Thinking that all deserts are sand dunes

Most desert surfaces are rock, gravel or stony plains. Sand seas (ergs) cover only a minority of the Sahara.

Mistake 2: Saying that camels store water in their humps

The hump stores fat. Camels survive by tolerating water loss and by drinking large amounts quickly.

Mistake 3: Believing that savanna fires are always harmful

Regular, natural fires keep the grassland open and renew the vegetation. Problems arise when fires are too frequent or too intense.

Mistake 4: Considering deserts lifeless

Deserts contain many specialized species and can burst into flower after rare rains.