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that of zebra (200,000), often do not follow the zebra but take their own route and

eat the long dry grass. Therefore, most migrant wildebeest obtain no benefit from

the zebra. In contrast, zebra may be benefiting from the wildebeest for a completely

different reason. In the wet season, when there is abundant food, many zebra graze

very close to the wildebeest, and by doing so they can avoid predation because most

predators (lions and spotted hyenas (Crocuta crocuta)) prefer to eat wildebeest. Only

if there are no wildebeest within range will predators turn their attention to zebra.

Therefore, it pays zebra to make sure there are wildebeest nearby. In the dry season,

however, zebra compete with wildebeest for food. Zebra, therefore, show habitat

partitioning and avoid the wildebeest. However, by doing so they probably make

themselves more vulnerable to predators again (Sinclair 1985). Thus, zebra have to

balance the disadvantages of predation if they avoid wildebeest with competition if

they stay with the wildebeest. We can see a seasonal change from facilitation in the

wet season when there is abundant food, to competition in the dry season when food

is regulating the wildebeest.

An example of facilitation has been recorded on the island of Rhum, Scotland. There

cattle were removed in 1957 and reintroduced to a part of the island in 1970 where

they grazed areas used by red deer. Pasture used by cattle in winter results in a greater

biomass of green grass in spring compared with ungrazed areas. Gordon (1988) found

that deer preferentially grazed areas in spring that had previously been used by

cattle (Fig. 9.18), and subsequently there were more calves per female deer.

On the North American prairies both black-tailed prairie dogs (Cynomys ludovi-

cianus) and jackrabbits (Lepus californicus) benefit from grazing by cattle. If grazing

is prevented, then the long grass causes prairie dogs to abandon their burrows. At a

site in South Dakota where cattle were fenced out, there were half as many burrows

as on adjacent areas where grazing was continued. Snell and Hlavachick (1980) showed

that a large prairie dog site of 44 ha could be reduced to a mere 5 ha by the

elimination of cattle grazing in summer to allow the grass to grow. Presumably under

natural conditions when American bison grazed the prairies there was facilitation by

bison allowing prairie dogs to live in the long grass prairies. Facilitation could be

mutual because both pronghorn and bison respond to the vegetation changes caused

by prairie dogs, both species using prairie dog sites (Coppock et al. 1983; Wydeven

and Dahlgren 1985; Huntly and Inouye 1988; Miller et al. 1994).

This example illustrates two management points which follow from the understand-

ing of the interaction (facilitation) between large mammal grazers and prairie dogs:

156 Chapter 9

15

10

5

0

15 May – 11 June 12 June – 9 July 10 July – 7 August

Number of deer fecal

pellets / 10 m

2

Grazed

Ungrazed

Fig. 9.18Facilitation of
deer grazing by cattle is
demonstrated by deer
fecal-pellet groups on
cattle-grazed plots
during each of the 3
months of study. Deer
preferred to graze plots
used by cattle the
previous winter. (After
Gordon 1988.)