Dave Taylor's African Safari: Trophic Level I: The Producers - Grass

Trophic Level I: The Producers
Grass
Page 6

Summary

Generally speaking, the world’s grasslands exist between forests and deserts. They thrive in areas such as the Serengeti-Mara ecosystem due to a combination of biotic and abiotic factors. These factors are: seasonal rains with periods of drought, fire, and herds of grazing animals. Seasonal rains prevent trees from getting enough water to thrive and shade out the grasses. Fire kills young trees before they grow big enough to survive fires. Grazing herds "mow" the grass and stimulate it to grow faster.

Two Plants Dominate

Two types of plants dominate the Serengeti-Mara ecosystem: trees and grasses. There are other plants here including sedges, bushes, legumes. However, we will consider only the two main types that are most obvious to visitors to the region.

During the rainy season, a single square kilometer of grassland produces 525 tonnes of grass each month (525 tonne per square km per month) or (1500 tons per square mile per month). Another way to say this is that every square meter of grassland produces .5 kilograms (each square yard produces one pound) of edible grass per month. This works out to 13 125 000 tonnes (14,467,000 tons) of grass produced throughout the entire ecosystem. It is this incredible biomass of food for the herbivores that allows the great herds to exist here.

This figure is 200 times greater than the amount of edible food produced by the richest forests. A rainforest has a much greater biomass than a grassland does but it does not produce the same amount of accessible, edible food (leaves).

To Each Its Own (Grass)

Different grasses grow in different locations within the ecosystem. It would be incorrect to assume that there is only one species of grass growing here. The most wide-spread of the grassland community types is the one dominated by red oat grass (Themeda trianda). This community is found wherever the mean annual rainfall is about 700 mm (27 inches).

The short grass plains to the south are dominated by a species of grass known as dropseed (Sporobolus marginatus). This is the area where wildebeest calve. Most of the species of grass growing here are less than 15 cm (6 inches) high. Volcanic soils here prevent tree roots from reaching the water table. The grass grows well during the rainy season but dries out quickly during the dry season. During the wet season this grass is the most heavily used in the ecosystem. The shortness of the grass provides little cover for predators.

To the south and west of the short grasslands is a zone of tall grasses that average between half a meter and a meter in height. Here, the predators find the cover to their liking. However, during the rainy season it is so heavily grazed that in parts it appears to be a short grass plain.

To the north and west, towards the Mara and Lake Victoria, are regions of tall (over a meter or yard high) grasslands. These areas receive little attention from the grazers as the grass species here are coarser. Their height also allows lions more cover to hide in waiting to ambush their prey. The migrating herds do not linger here.

Many of the woodland areas are also covered primarily by grasses. Tree growth in much of the region is too sparse to shade out the grasses (see Trees).

Grassland Survival Factors

The existence of these grasslands is not due solely to abiotic factors such as altitude and rainfall. The great herds also influence them. Some grasses are not eaten by the wildebeest, while others are heavily grazed.

Grasslands need three factors to ensure their continuation. They are droughts, grazing and fires. Droughts prevent, or at least slow down, tree growth. Grasses have evolved to survive seasonal droughts but they must have rain sometime during the year.

Grazing by herbivores stimulates grass growth. Their hooves trample and kill young trees. If, as happened in the early part of the 20th Century, the number of grazers decreases, the grasslands of the Serengeti-Mara will once again convert to woodlands. The presence of elephants in the ecosystem has also helped the grasslands increase in area, especially in the Masai Mara. Large numbers of elephants kill trees.

As we’ve seen, fast moving fires do not kill the grass’ roots but remove dead material and replenish the soil (see Abiotic Factors: Fire).

Grazing Succession

Wildlife managers often cite grazing succession as the reason why the Serengeti is home to such incredible numbers of wildlife. One fifth of all of Africa’s large mammals can be found there. Three-quarters of the continent’s wildebeest live in the Serengeti-Mara ecosystem. A third of Africa’s zebras call it home.

"Grazing succession" refers to the practice used by farmers to rotate animals through a field. This same concept has been applied by wildlife managers and ecologists. Zebras are bulk feeders that prefer mature, older grasses. Wildebeest will eat any green grass. Together these two dominant species "mow" down the grass and create areas of short grass where Thomson’s gazelle feed. (On a farm, these species are replaced by horses, cattle and sheep.)

Wildebeest, topi and kongoni are closely related antelopes. How do they fit into this picture? Wildebeest eat new, green grass. Topi prefer intermediate grass and kongoni eat older, dry grass. Again, the resource is shared.

Competition

As you read, the text describes the foods preferred by each species. Most, but not all, species avoid direct competition with other species. Elephants compete with most grazers and browsers, at least to a degree. Lions and spotted hyenas compete for the same prey species.

Competition for resources, whether plant or animal, is a feature of any ecosystem. Grass, for example, competes with trees for sunlight and water. Lions compete with spotted hyenas for wildebeest. Living organisms also compete with their own kind for the resources needed to sustain life. This CDROM highlights several examples throughout the text. Some of the resources competed for include food, water, living space, and the right to mate and reproduce. Other forms of competition involve species competing with other species for their very lives; hunter against hunter.

Out of this need to compete comes the need to adapt. Every organism has a form and every form, a function. Adaptation allows an organism to compete as long as it is a positive adaptation. Negative adaptations usually condemn the life form to certain death.

Positive adaptations are found throughout the text. Migration, strength, horn and tooth development, mating rituals, plumage and hide color are all adaptations.

Adaptations can be physical or behavioral.

Select a Page:
1 2 3 4 5 6 7 8 9 10 11