because it produces the same changes in prey populations as would be predicted from
interspecific competition (Holt 1977, 1984). Examples of apparent competition are
given in Section 9.10.2 and Chapter 21, where predators are causing the demise
of secondary prey, the rare roan antelope in Kruger National Park, South Africa
(Harrington et al. 1999; McLoughlin and Owen-Smith 2003), and the wildebeest
in Manyara National Park, Tanzania, as a result of a high abundance of buffalo, the
primary prey.
If two prey species live in the same habitat, as in the wildebeest and buffalo
example in Manyara, then at high intensities of predation coexistence is unlikely. On
the other hand, coexistence is promoted if the two species select different habitats,
that is, niche partitioning occurs.
Another version of apparent competition can occur through shared parasites. One
species can be a superior competitor if it supports a parasite which it transmits to a
more vulnerable species. For example, when gray squirrels (Sciurus carolinensis) were
introduced to Britain, they brought a parapox virus that reduced the competitive
ability of the indigenous red squirrel (S.vulgaris) (Hudson and Greenman 1998).
The latter has largely been displaced, occurring now in only a few small locations of
its former range. Gray squirrels are displacing red squirrels through competition in
Italy, and could be spreading through Europe (Wauters and Gurnell 1999).Since the observed responses of prey populations to changes in predator numbers
are similar to those from interspecific competition, we cannot infer such competi-
tion simply from observations or even experiments that show either changes in species
population size or niche shifts. We need to know (i) whether resources are limiting;
and (ii) the predation rates and predator numbers.Facilitation is the process whereby one species benefits from the activities of another.
In some cases the relationship is obligatoryas in the classic example of the nereid
worm (Nereis fucata), which lives only in the shell of hermit crabs (Eupagurus bern-
hardus). The crabs are messy feeders and scraps of food float away from the carcass
that is being fed upon; these scraps are filtered out of the water by the worm. While
the worm benefits, the crab appears not to suffer any disadvantage (Brightwell 1951).
In other cases the relationship may be facultative, by which we mean that the depen-
dent species does not have to associate with the other in order to survive, but does
so if the opportunity arises. Thus, cattle egrets (Ardea ibis) often follow grazing
cattle in order to catch insects disturbed by these large herbivores. Although the birds
increase their prey capture rate by feeding with cattle, as they probably do by fol-
lowing water buffalo (Bubalus bubalus) in Asia and elephants and other large ungulates
in Africa, they are quite capable of surviving without large mammals (McKilligan 1984).
The European starling (Sturnus vulgaris) also follows cattle on occasions. In contrast,
its relative in Africa, the wattled starling (Creatophora cinerea), seems always to
follow large mammals and in Serengeti they migrate with the wildebeest like camp
followers.
Vesey-Fitzgerald (1960) suggested that there was grazing facilitation amongst
African large mammals. Lake Rukwa in Tanzania is shallow and has extensive
reedbeds around the edges. The grasses, sedges, and rushes can grow to several meters
in height, and in this state only elephants can feed upon the vegetation. As the
elephants feed and trample the tall grass they create openings where there is lush154 Chapter 9
9.8.2Implications
9.9 Facilitation
9.9.1Examples of
facilitation