Dave Taylor's African Safari: Abiotic Factors of the Serengeti-Mara Ecosystem - Fire

Abiotic Factors of the Serengeti-Mara Ecosystem
Fire
Page 12

Summary

Fire is a major abiotic factor in many ecosystems. In many parts of the northern hemisphere it burns off old growth forests and allows new ones to grow. In the Serengeti-Mara Ecosystem, fire is an important tool in maintaining the grasslands. If regular burning did not occur, much of the region would become forest again. Fire kills the young saplings and prevents them from maturing into adult trees that would shade out and kill the grass.

A History Lesson

Fire is one of the main reasons the wildebeest herd has been able to grow so large here. We tend to think of the region as always looking like what we see today. In fact, it is only since the 1960s that the Serengeti-Mara came to be the home of about 1.5 million wildebeest.

The ecosystem has undergone many vegetation changes in the last 150 years since its "discovery" by European explorers. From the 1880s to the turn of the century, the area was described as looking much as it does today; open grassland and wooded areas. The introduction of European cattle onto the continent in the 1880s changed the ecology of the area. The cattle brought with them a disease called rinderpest. Rinderpest is fatal to cattle and other ruminants. The disease spread rapidly through the herds of wildebeest, buffalo, giraffe and other antelopes causing their populations to plummet.

Rinderpest Arrives

Not only did rinderpest kill off the wild game, it also killed the African cows kept by nomadic and agricultural tribes in East Africa. By 1892, almost 95% of the African cattle were dead. Starvation and famine soon followed. Small pox epidemics, brought into Africa by Europeans, also took their toll on the local populations.

Meanwhile, with the disappearance of their natural prey, lions began to use an alternate source of food: humans. Man-eating lions provided another reason for the tribes that lived here to vacate the area. (Predation on humans by lions continued well into the twentieth century but ceases to be a problem in this area now.)

With the disappearance of the local tribes, the trees began to flourish. The two main factors that had controlled the forests changed. The herds of grazing animals that once trampled and fed on the young saplings were gone. Fires that burned off old grass and killed small trees were usually set by people to maintain the grassland for their cattle. Fewer people meant fewer fires and more trees.

It should also be mentioned that elephant numbers in East Africa were low during this period due to commercial ivory hunting. Elephants can also damage forests and in some recent studies have been shown to significantly limit the survival rate of saplings. However, accounts by hunters in the 1880s show that there were few elephants in the Serengeti at that time. The Serengeti-Mara ecosystem prior to the arrival of rinderpest did not support large numbers of them.

The Rise of Forests: The 30s and 40s

By the 1930s and 40s the area had become densely wooded. Now a new pest was increasing in numbers. Tsetse flies like wooded areas and their numbers skyrocketed. The tsetse fly’s bite often carries a disease that is fatal to both humans and cattle. Only one in twenty tsetse fly carries "sleeping sickness" (trypanosomes) and only 10% of the infected animals die. Even this low percentage was enough to keep cattle and humans out of much of the region. Indeed, the colonial government thought the land so poorly suited to human use that they created Serengeti National Park in 1929.

(Author’s note: Most travelers to East Africa take malaria pills during their trip. One out of three mosquitoes carry the parasites that cause malaria and, while not fatal, it can cause recurring bouts of malaria fever.)

By 1950, the rinderpest epidemic had run its course. During this time, the herds began to grow in number and slowly the woodlands were re-converted to grasslands. Tsetse fly numbers also declined as this change took place.

Helping the process was the revival of the Maasai (see The Missing Factor: The Maasai) population and their cattle herds. Grassfires were once again set during the dry season and slowly the forest retreated. The grassfires, especially in the 1950s, were devastating to the standing forest. The smaller herds of herbivores were unable to consume the dead grass, leaving more material to burn longer and hotter. Large areas were cleared of trees.

Ironically, it was the trees themselves that helped the fires to become so devastating. The added height of the trees on the land forced winds to cool off faster than when grass had covered the land. That extra height helped increase rainfall. So did the trees’ ability to reach water reservoirs deep below the ground. Water absorbed by the trees’ roots was released to the air where it too, fell as rain.

The early 1960s saw record rainfall in the region. As a result, the grasses that still grew in the region flourished in the woodlands. The smaller number of herbivores in the region at the time were unable to crop the grass. When a major drought hit the area a few years later, the combined effects devastated the forest. The Maasai set their fires and the abundant dead grass burned longer and hotter than usual. Trees were severely burnt and died off. Large areas of woodlands were converted back to grasslands.

Return to Grasslands: The 60s

In the Mara, elephant numbers had also increased, although elephants were not common in the Serengeti at this time. The return of large numbers of elephants to the Mara was due to the increasing human population of the region, which had the effect of pushing the elephant herds into protected areas.

The stage was set for the wildebeest herd to grow to the size we see today.

Why Doesn’t Fire Kill Grass?

Grasslands and fire have a rather complex relationship. Fire helps maintain grasslands by burning the young trees. Fire does not necessarily kill mature trees unless the fire is unusually hot and long lasting.

Most grass fires sweep rapidly across the plains and do not burn anything but the outer layer of bark. The trees that dot the savanna have built up a resistance to such fires and continue to survive. Their seedlings are not able to survive these fires and, as a result, there are many places in the savanna where only mature trees are present.

Why doesn’t the fire kill the grass itself? For the most part, the grass that is burned is old, dry grass with little nourishment left in it. It burns very fast; too fast for the heat to penetrate to the roots of the grass. When the next rains come, the roots quickly send up new shoots which are rich in nutrients.

Does the Fire Hurt Animals?

Fires on the African plains are little, if any, threat to larger animals. They simply move out of the way. Insects and smaller animals, such as rodents, small reptiles and amphibians, are often victims. They must flee the fire or die.

Many birds have learned to take advantage of grass fires. White storks, for example, will walk along the edge of the flames picking off the fleeing small animals. Lilac-breasted rollers and marabou storks will also do the same thing.

Larger herbivores, such as zebras, topi and wildebeest, actually benefit from the aftermath of the fire. They find the charred plain a safer haven from predators. The cleared land makes it more difficult for a lion to get close enough to attack or to hide in ambush.

(Author’s note: I’ve seen a lioness lie motionless in such a burned out area waiting for nearly two hours for the wildebeest to come near her. To our eyes, the lioness was visible but, because she lay so still , the wildebeest could not seem to see her. The wildebeest did stay far enough away from her to avoid being captured, although it was a close call when the cat finally made her move.)

Maasai-Set Fires

Fire does deprive the herds of some nutrition and it does force them to move on until new grass begins to grow. This is one of the reasons why the Maasai, in some areas, start the fires. The arrival of the wildebeest herds also brings the arrival of flies that feed off the herd. These flies will also attack the Maasai cattle. The Maasai do not hunt or kill wildebeest but they do not welcome their arrival either. Fires force the herds to move on.

The main reason why the Maasai start the fires during the dry season is to burn off dead grass. The ashes nourish the new grass which, in turn, is more nourishing for their cattle. The new grass also feeds the wildebeest and other herbivores so that they benefit as well.

Ecologists now recognize fire as an important abiotic factor in the ecology of grasslands. It is so important that, in parks where the Maasai no longer start the fires, park rangers have been assigned the task of doing controlled burns to simulate the "old ways". Serengeti National Park does this. It happens less often in the Maasai Mara because the herders still start fires there.

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