Environmental Engineering Reference
In-Depth Information
15.2.3.1 Interactions resulting from conspicuous aliens
Hyperpredation
When a decline in a native prey species is observed in an ecosystem also containing
an alien predator and prey, generally the initial response is to attempt to remove
the most visibly devastating species—the predator. However, such actions have the
potential to lead to a further decline in the native prey species; the availability of
abundant exotic prey can infl ate alien predator populations, which then increase
the predators' consumption of native species, subsequently driving the indigen-
ous prey to very low numbers and potentially to extinction (Zavaleta
et al
. 2001;
Zavaleta 2002; Courchamp and Caut 2005). This process, termed
hyperpreda-
tion
illustrates how introduced prey can have an important indirect effect in such
ecosystems. A prey species introduced into an environment in which a predator has
also been introduced is likely to allow a high enough increase of this predator that
native prey, less adapted (in terms of behaviour and life history traits) to high levels
of predation, could suffer a population decline (Courchamp
et al
. 1999b, 2000,
2003a; Courchamp and Caut 2005).
h e interaction between introduced cats (
Felis catus
), rabbits (
Oryctolagus cunic-
ulus
), and native birds through the hyperpredation process on Macquarie Island (a
Tasmanian State Reserve) resulted in the decline of burrow-nesting petrels, as well
as the extinction of an endemic parakeet (
Cyanoramphus novaezelandiae erythro-
tis
)
and a banded rail (
Rallus philippensis
) (Taylor 1979; Brothers 1984). Despite
cats being introduced to the island 60 years before rabbits, the dramatic impact
of cat predation on the bird populations dated back only 10 years after the intro-
duction of rabbits (Taylor 1979). h e presence of the rabbit population not only
maintained, but signifi cantly increased, the cat population during winter (when
seabirds are absent from the island), therefore resulting in increased predation
pressure on the land bird species. Rabbits are more adapted to cat predation and
were thus able to support the increase. However such an increase was fatal to sev-
eral native bird populations (unadapted to mammalian predation pressures) which
were extirpated by an over-sized cat population that no longer depended on the
presence of birds to survive (Courchamp
et al
. 1999b, 2000).
Diet studies of the predator in question should be conducted in order to assess
not only the importance of the impact on the local population, but also potential
hyperpredation processes (Courchamp and Caut 2005). Not surprisingly, the pres-
ence of hyperpredation has consequences on the management actions required:
should a control programme be aimed at the predator only or at the introduced
prey and predator simultaneously? Removing only an introduced predator popu-
lation without controlling the introduced prey is not recommended for several rea-
sons. First, eradicating the predator may be di
cult to achieve since the introduced
prey constitute a constant source of food (Fig. 15.1a). Second, removing the pre-
dation pressure would increase the di
culties of later coping with the introduced
prey, which are often characterized by high reproductive rates. On the other hand,
,
