untitled

regenerating vegetation. This provides a habitat for African buffalo, which in turn

provide short grass patches that can be used by the smaller antelopes such as topi.

In this case elephants are creating a habitat for buffalo and topi that would not

otherwise be able to live there. Therefore, the presence of elephants increases the

number of herbivores that can live in the Lake Rukwa ecosystem. Vesey-Fitzgerald

called this sequence of habitat change in the grasslands a grazing succession.

Bell (1971) has described a similar grazing succession amongst the large mammals

of Serengeti. In certain areas of Serengeti there is a series of low ridges bounded by

shallow drainage lines. The ridges have sandy, thin soils and support short, palat-

able grasses. The drainage lines have fine silt or clay soils that retain water longer

than those on the ridges and so support dense but coarse grasses, which remain green

long into the dry season. Between these two extremes there are intermediate soil types

on the slopes. The whole soil sequence from top to bottom is called a catena. In the

wet season when all areas are green, all five non-migratory species (wildebeest, zebra,

buffalo, topi, and Thomson’s gazelle) feed on the ridge tops. Once the dry season

starts the different species move down the soil catena into the longer grass in

sequence, with the larger species going first (Fig. 9.17). Thus, zebra is one of the

first species to move because it can eat the tough tall grass stems. By removing the

stems, the zebra make the basal leaves in these tussock grasses more available to wilde-

beest and topi, and these in turn prepare the grass sward for the small Thomson’s

gazelle. Thus, there is a grazing succession.

Zebra, wildebeest, and Thomson’s gazelle also have much larger migratory popu-

lations in Serengeti separate from the smaller resident populations discussed above.

It is tempting to think that the movements of these migrants follow the same

pattern as those of the resident populations. Indeed, McNaughton (1976) has shown

that migrating Thomson’s gazelle prefer to feed in areas already grazed by wildebeest

because these areas produce young green regrowth not found in ungrazed areas. The

gazelle take advantage of this growth, which was stimulated by the grazing, and so

benefit from the wildebeest.

The relationship between the migrating zebra and wildebeest is more complex.

Although zebra usually move first from the short grass plains to the long grass

dry-season areas, the wildebeest population (1.3 million), which is much larger than

COMPETITION AND FACILITATION BETWEEN SPECIES 155

100

80

60

40

20

0

AMJ J AS

1967

% of population

Buffalo

Gazelle

Zebra

Topi

Wildebeest

Fig. 9.17The
proportion of the
population of different
ungulate species using
short grass areas on
ridge tops (upper
catena) in Serengeti.
The larger species leave
before the smaller at the
start of the dry season.
(After Bell 1970.)