Environmental Engineering Reference
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reptiles, and even the chicks of large birds such as the albatross (Newman 1994;
Smith et al . 2002; Wanless et al . 2007).
Caut et al . (2007) were able to demonstrate how some control strategies that
overlook the competitor release eff ect may fail to restore the ecosystem through an
unexpected increase of the inferior competitor, even if that species is being con-
trolled too. Using mathematical models to mimic the eff ects of controlling intro-
duced species in the presence of their competitors, Caut et al . (2007) found that it
was possible for a competitive release eff ect to occur even when both introduced
competitors were being controlled simultaneously (as is the case in most rat eradi-
cation programmes). h e competitive release of the inferior competitor (mouse)
is due to the indirect positive eff ect of control (the removal of their competitor,
the rat) exceeding its direct eff ect (their own removal). Furthermore, both control
levels and target specifi city were found to have a direct infl uence on the extent
of the competitor release process: the stronger and more specifi c the control, the
greater the eff ect (Caut et al . 2007).
Because the intensity of the competitor release is directly proportional to
the control eff ort, indiscriminate intensifi cation of the control will exacer-
bate this process, with high potential impact on native prey species (Caut
et al . 2007). Furthermore, while most control programmes aim for high target
specifi city (Simberloff and Stiling 1996b; Murphy et al . 1998a), this recent
study highlights the role of control specifi city in terms of the likelihood of a
competitor release eff ect. While conservation managers appear to be faced with
a dilemma regarding control intensity and specifi city, Caut et al . (2007) suggest
the following:
Obtain an understanding of the invaded ecosystem as a whole in order to
assess potential processes (including competitor release) that may occur dur-
ing or following control.
Use as many specifi c methods as there are species to be controlled. If resources
are limited, as is the case in most instances, controlling the inferior competitor
should be the fi rst priority so that the combination of control and competi-
tion (or predation) eliminates it; after which the remaining resources can then
be used to target the superior competitor without the danger of releasing the
inferior competitor (Caut et al . 2007).
It is important to note that these should only be taken as guidelines, and that
assessment of the data gathered during pre-eradication studies should provide a
much better basis on which to construct an optimal strategy.
15.3 Mitigating actions
Several tools have already been mentioned in this chapter, which will reduce the
likelihood of unexpected outcomes in eradication and control programmes.
 
 
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