and shrubs formed the largest dietary component (44% browse). At this season major
components for the other species were: euros, 83% grass; goats, 65% browse; rabbits,
25% browse. The rock-wallabies overall diet overlap was 75% with goats, 53% with
rabbits, and 39% with euros. In good conditions dietary overlap was still substantial
but lower than when drought prevailed. At that time the overlap was 47% and 45%
with goats and euros, respectively. Thus, potential competition was greatest with goats
and rabbits and least with the indigenous euro.
In North America the introduced starlings (Sturnus vulgaris) and house sparrows
(Passer domesticus) have competed with the native bluebird (Sialia sialis) for nesting
sites, with the result that bluebird numbers have declined considerably (Zeleny 1976).
Not all introductions result in competition. Chukar partridge (Alectoris chukar) have
been introduced to North America as a game bird. Their habitat includes semi-arid
mountainous terrain with a mixture of grasses, forbs, and shrubs. In particular, they
like the exotic cheatgrass (Bromus tectorum). Chukar introductions succeeded only
where cheatgrass occurred. These habitat requirements are unlike those of native game
birds such as sage grouse (Centrocercus urophasianus), and thus little competition
has taken place (Gullion 1965). Robley et al. (2001) showed that the endangered
burrowing bettong (Bettongia lesurur) in Australia was able to cope with drought
stress much better than rabbits because they could eat a variety of herbs and shrubs
that rabbits could not eat. If anything these bettongs could outcompete rabbits.
Thus, the decline of these marsupials was due to apparent competition from foxes
(Short et al. 2000).Interaction between species can be competitive or beneficial. Competition occurs when
two species use a resource that is in short supply, but a perceived shortage in itself
should not be used as unsupported evidence of competition. Instead, the relation-
ships must be determined by manipulative experiments reducing the density of one
to determine whether this leads to an increase of the other. Care should be taken to
eliminate other factors such as predation that may cause the response. Facilitation
is the process by which one species benefits from the activities of the other. It often
takes the form of one species modifying a less suitable food supply to make it more
suitable for another species, and where one species modifies a habitat making it more
favorable for another.
These two effects – competition and facilitation – can often be manipulated by
management to increase the density of a favored species.162 Chapter 9