Dave Taylor's African Safari: Trophic Level II - The Herbivores - Insects and Invertebrates

Dave Taylor's African Safari: Trophic Level II - The Herbivores - Insects and Invertebrates - Page 2

Trophic Level II - The Herbivores
Insects and Invertebrates
Page 2

Summary

Impossible as it sounds, the biomass of insects in the Serengeti-Mara ecosystem is greater than the combined weight of all of the mammal species. Little long range study about the role of insects in this ecosystem has been undertaken, but two insects have been looked at in depth: the locust and the termite. Termites live below the ground and create tall, chimney-like structures across the plains and woodlands. They are even more numerous than the grasshoppers and other insects that feed on the grass above ground.

Locust; The Vanishing Scourge

Several species of grasshoppers are common on the plains of Africa. All are strict vegetarians and most live primarily on grass. Eggs are laid below the ground and can endure long periods of drought and hot temperatures without drying up. Grasshoppers are found wherever there are grasslands for them to feed on.

Locusts are a type of grasshopper. There are 5,000 species of grasshoppers on Earth and only nine of them are locusts. For most of their lives they behave as typical grasshoppers. However, under certain conditions, they change their ways and gather in huge numbers to migrate. These migrations are made up of massive flocks that can eat grasslands down to the bare soil wherever they land. In one swarm, in the United States, there were an estimated 124 billion insects. Swarms that were 250 kilometers (155 miles) long and 20 kilometers (12 miles) wide have been reported. They are said to have appeared as a dark cloud moving across the land, blocking out sunlight. A huge swarm can eat 60 000 kilograms (132,000 lb.) of plant matter per hour and can do this for five days in a row! One migration devastated over 5 000 square kilometers (1,930 square miles) of land, leaving it a virtual desert with no grain for man or beast.

Things of the Past

Such swarms are largely a thing of the past. The species of grasshopper that become locusts are well studied and have been controlled since 1944. It is rare to hear or read about swarms today, but one was reported in 1997. The severe weather and flooding of January 1998 also raised fears of a locust invasion in East Africa and swarms were reported in Ethiopia and Madagascar that year.

When locusts are in their "grasshopper" stage, they live alone and do not migrate. When they become locusts, they change the shape of their body and their coloration. They begin to collect together in a mass. Then they migrate.

Before this happens, however, there may have been several years without an outbreak. During those years, they live alone. During this solitary stage as grasshoppers, they find each other for purposes of mating by chirping. The male chirps to attract a female. Different species have different "songs". As long as the male plays his song, the female will try to find him. After mating, the female digs a hole in the ground and lays her eggs. She will deposit 30 to 100 eggs. The hole is then plugged up with a foam that she produces. The foam allows air to reach the eggs. She may repeat this process ten times.

Fifty to sixty days later, the eggs hatch. The nymphs (worm-like larval staged locusts) crawl off and feed on grass. They are about 8 millimeters (.3 inches) long and each goes off on its own.

Let the Swarm Begin

Several weeks, or even months later, after the locust has passed its last molt, it acquires wings and become sexually mature. At that time, most locust species change color too. Females are straw-yellow and males turn lemon yellow. It will still be between three weeks and several months before they will mate and lay eggs.

Sometimes conditions are such that a swarm is created. Food shortages caused by drought force the locusts to congregate in one area. This overcrowding triggers hormonal changes. The change from a solitary hopper to the larger swarming locust takes time. The solitary phase is followed by a generation where there are fewer molts (6 instead of 7). The differences between males and females are less obvious. The nymphs become darker in color and it is these, when they become adults, that lay the eggs which produce the migratory generation.

Under these conditions, numbers of females lay their eggs in a more restricted area than usual. In one study, 560 billion eggs and 1,450 billion freshly hatched nymphs were destroyed. Estimates of the total number of potential eggs laid put the total at over 2,700 billion.

When the eggs hatch, billions of nymphs crawl out and move in one direction. They eat all the grass in their path. They are even able to digest dried grass and get some moisture from it. They provide a feast for plovers, gulls, storks, snakes, mongooses, monkeys and jackals, but even large numbers of predators cannot control the mass of locusts crawling and hopping their way across the land.

Overcrowding Equals Stress

The more often they meet locusts of their own kind, the more likely it is that they will change their behavior into a migratory swarm. When they reach sexual maturity they begin their flights.

One species, the red migratory locust of Tanzania (Nomadacris septemfaciata) has been well studied. It never flies when the temperature is below 18 C (64° F) and only rarely when it is below 22 C (71° F). It prefers to fly when the temperature is between 29 C (84° F) and 35 C (95° F). If the wind speed exceeds 3 meters per second (10 feet per second) at ground level, it will not fly.

At first, they take to the air a few at a time, then more join in and then still more. Once the whole swarm is in the air, they fly off. The swarm flies downwind to frontal systems where rain is falling. This ensures that there will be grass to eat. They will continue to feed and migrate until they run out of food or meet weather that kills them.

Millions die from predation too. Locusts eat the grass blades but they do not eat the roots. A good rain will soon have the grass growing again.

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