Environmental Engineering Reference
In-Depth Information
major component of the stoat (
Mustela erminea
) diet, eff orts to reduce both species
was attempted through the control of the rats only (Murphy and Bradfi eld 1992;
Murphy
et al
. 1998b). h ese actions not only failed to eliminate the stoat popula-
tions, but the reduced availability of alien prey resulted in a diet switch by the stoat
to incorporate more native birds and eggs.
The competitor release effect
Having earlier covered the potential affects of control attempts on non-target spe-
cies through trophic interactions, this section looks at the consequences relating
to competitive interactions. The control of an invader has the potential to release
any species interacting with that invader from its pressure, be it predation or com-
petition (exploitation or interference) (Courchamp and Caut 2005). Control of a
superior competitor may lift the pressure of competition from an inferior competi-
tor, subsequently leading to an increase in its population; such a process is termed
the
competitor release eff ect
(see Fig. 15.1c).
While not always tested, there are numerous instances in which the removal
of a target alien species has coincidentally facilitated the emergence in the com-
munity of another long-suppressed non-indigenous species (Mack and Lonsdale
2002). For example, following the biological control of the weed St. John's wort
(
Hypericum perforatum
) at several sites using chrysolina leaf beetles (
Chrysolina
quadrigemina
), other non-indigenous invading species became more abundant
(Huff aker and Kennett 1959; Tisdale 1976). Similar results have been observed
with respect to invasive aquatic macrophytes. For example in southeast Florida, the
use of herbicides and grass carp to control the widespread invader
Hydrilla verticil-
lata
has coincided with an increase in the equally-unwanted invader
Hygrophila
polysperma
and its replacement of hydrilla as the number one non-native aquatic
weed in some southeast Florida canals (Duke
et al
. 2000).
h ere are a number of ecosystems to which both rats and mice have been intro-
duced. In such instances, rats generally dominate and are generally viewed as strong
competitors of mice (Ruscoe 2001). Techniques to control rodents (trapping and
poisoning) often lack specifi city regarding rodents; therefore their use in ecosys-
tems containing multiple introduced rodent species are often viewed as benefi cial
due to non-target rodent species mortalities. However, in a number of ecosystems
containing both rats and mice, the successful eradication of rats has corresponded
with a dramatic increase in mouse numbers, often to levels exceeding those prior to
the eradication programme (Brown
et al
. 1996; Witmer
et al
. 1998). For example,
rats and rabbits were successfully eradicated from Saint Paul Island, Indian Ocean,
but the control programme did not focus on the small mouse population that was
known to occur there (Micol and Jouventin 2002). h e complete removal of the
rat population released the mice from their competitors, causing such a demo-
graphic explosion that mouse numbers far exceeded the habitat carrying capacity.
While it may be argued that mice are less harmful than rats, mouse outbreaks can
be very problematic; mice have been shown to be active predators of invertebrates,
