Once [G 40 / rinderpest] disappeared from the wildebeest population in 1962, their numbers rapidly expanded over the next 15 years before leveling off at their present numbers. This increase has had direct and indirect effects on vegetation and, as a consequence, on other [G 22 / herbivores].
Wildebeest graze on the plains during the wet season when grasses are growing. Their constant grazing prevents the flowering or maturing of leafy green vegetation with soft stems, such as herbs. The continuous cutting and saliva stimulate new shoot production through the rainy season. This type of vegetation grows from the roots which are protected from grazing; whereas, grasses that grow from the stems are damaged by grazing and their growth is inhibited on the plains.
As a result, the Grant's gazelle, which eats herbs almost exclusively, has increased in population on the plains. Similarly, the extensive grazing of the wildebeest has created more food for the smaller [P 020 / Thomson's gazelle] that follow the wildebeest on part of their migration. It is not certain what effect this increase in food supply has had on the population because Thomson's gazelles are difficult to count from the air.
In the woodlands, wildebeest' grazing takes place after the growing season while the plants are dormant so their grazing has little direct impact on species composition. However, their vast numbers trample the tall, dry grass reducing the fire hazard. This allows regenerating trees, especially those below 3 feet, to survive and grow to a size that can withstand any future fires. This has led to the increase of Acacia trees in the woodlands and consequently, an increase in the [P 015 / giraffe] population which prefer to browse on small Acacia trees rather than tall trees.
The tall dry grass trampled by the wildebeest, however, reduces available food for [I 002 / buffalo]. After rinderpest died out of the buffalo population in the early 1960s, the number of buffalo rapidly expanded. But by the early 1970s, the buffalo population, one tenth the size of the wildebeest's, leveled off in response to the competition with the more numerous wildebeest in the area.
The vast numbers of wildebeest also remove increased vegetation that results from higher rainfall. Other antelope species such as the impala, [I 010 / kongoni], and [I 011 / topi] populations have not benefited from periods when more vegetation was available; therefore, their populations have not increased in the woodlands.
