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| Savanna grasslands and climatic specialism
Like the boreal forests, the savanna regions of the world have experienced huge changes in the last 10,000 years. The retreat of the ice led to an expansion of the main climatic belts in a poleward direction. Much of the Sahara Desert was green, temperate and rich in wildlife during the ice age when the temperate zone moved south towards the equator. The rainforests shrank to their core areas, and the savanna and desert regions were compressed into narrower strips than they currently occupy. The present day savanna areas therefore have a complex history - some having been desert quite recently, others having been on the edges of temperate zones. There is also uncertainty about the origin of savannas. They seem closely related to human activity - in particular the use of fires (since prehistoric time) for hunting and for clearing vegetation. It is clear, therefore, that we should expect variety in different savanna areas. This variety will reflect past climate, present climate and human activity. |
| An overview of savanna grassland characteristics
Savanna grasslands lie in a unique position between two major regions of the global climate machine. On one side lies the low pressure area of upward moving air along the equator - creating conditions of high precipitation.
On the other side lies the area of downward moving air of the subtropical high - creating conditions of aridity and drought. With the seasonal movement of the climatic belts in summer and winter, the savanna region alternately experiences a wet season and a dry season. The animation (below right) shows the movement of the ITCZ (Inter Tropical Convergence Zone) which defines the length of the dry season. |
On the map of global savanna locations the simple pattern which should exist (according to the model on the left) is complicated by a number of other factors in the real world such as:
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Adaptations In the savanna, plants and animals have adapted to extremes of moisture
availability. As the graphs show, there is a gradation in the savanna climate
depending on whether the location is closer to the equatorial zone or the
desert zone. However, both graphs demonstrate clear patterns of high temperature
and varying rainfall. Since the rainfall is delivered in a short time by
intense convectional storms these areas are very vulnerable to soil erosion;
partly because of the intensive rainfall and partly also because the vegetation
cover is low at the end of the dry season. If humans reduce the vegetation
cover further by overgrazing or tilling the land, severe erosion can result.
The key plant and animal adaptation is to dry season drought. Most plants
show xerophytic (drought-resistant) characteristics. These might include
deep tap roots, rapid life cycling, etc as shown in the diagram. Productivity
is highly seasonal, but for at least a portion of the year it will have
excellent conditions for growth so net annual productivity is fairly high.
The figure will vary, however, depending on the length of the wet season
compared to the dry season. Productivity will also vary with soil quality.
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This typical savanna scene illustrates some of the ways
in which climate influences ecological processes. |
| Species diversity is not as high as in the rainforest but can be surprisingly
rich. The lush growth of grass in the wet season creates ideal conditions
for a wide range of herbivores including insects, birds, gazelle and all
their predators. Species diversity is very seasonal, with many migratory
species following the rains. Competition is complex and also seasonal.
There are many finely tuned ecological niches during the wet season and
some interesting mutualism arises where species co-operate to benefit one
another. For example, oxpecker birds are tolerated by the buffalo. The
bird benefits by having access to food (ticks and flies) on the host. The
buffalo benefits from a free cleanup and an early warning system in the
form of keen sighted birds. In the dry season competition for water favours
the better adapted species. Food webs are - as with all things in this
climate - seasonally changing. They are complex and long in the wet season,
but much shorter and more fragile in the difficult dry season.
Economic potential
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| Questions to consider:
What are the implications of seasonally extreme rainfall totals for agriculture and farming? What are the implications of inappropriate farming for soil erosion? |
| Links within GIP:
Climate graphs Agriculture in West Africa |
External links: |
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