Environmental Engineering Reference
In-Depth Information
the majority of past management actions for alien species has been the formation
of separate control or eradication programmes, and most often for the most vis-
ibly destructive species (Courchamp and Caut 2005). While such single-species
eradications may be successful in terms of their removal of the target species, this
in itself may lead to unexpected and detrimental impacts on the ecosystem or
species which the original intent was to conserve. h e most common secondary
outcome is the ecological release of a second (plant or prey) alien species which
was previously controlled by the removed species (herbivore or predator) (Zavaleta
2002). Such outcomes can be anticipated, or ideally avoided, by fi rst obtaining
knowledge about species interactions occurring within the ecosystem and the gen-
eral ecological rules that they follow (Courchamp
et al
. 1999b; Zavaleta 2002;
Courchamp
et al
. 2003a; Courchamp and Caut 2005).
Species abundance and composition within an ecosystem (either natural or
modifi ed) exist largely due to the interactions between species that regulate these
factors. In any ecosystem, populations of producers, consumers, and predators are
in part controlled by one another through food web and other biotic interactions,
including competition and provision of habitat (Hairston
et al
. 1969; Fretwell
1987; Polis and Strong 1996). Such complex interactions necessitate a deeper
understanding of the system in order to predict properly the result of manage-
ment actions such as the removal of one species from the ecosystem (Courchamp
et al
. 2003a). Despite every invaded ecosystem being in some way unique, they all
follow, at least qualitatively, the same set of basic ecosystem rules (Zavaleta 2002).
With these basic ecological rules in mind, managers and eradication experts can
make great gains towards anticipating, planning for, preventing, and mitigating
the unexpected (Zavaleta 2002).
Any ecological release of a species from some pressure, such as competition
or predation, brought about by the removal of a species from an ecosystem, has
the potential to change subsequent species interactions and species abundance.
h erefore, before implementing an eradication or control programme, it is import-
ant to consider how food-web interactions (both vertical and horizontal) may be
limiting populations of producers, consumers, or predators within the ecosystem
(Zavaleta
et al
. 2001). Two ways in which food-web interactions may be working
is through top-down regulation by higher-level consumers or predators, and by
bottom-up regulation of populations by food availability or resource limitation
(Zavaleta
et al
. 2001).
Bottom-up
regulation of predators by prey implies that removing an alien prey
should reduce both alien and native predators (Polis 1999; Zavaleta 2002). In
comparison, the removal of an exotic predator from a single-invaded ecosys-
tem, can release native prey from strong
top-down
regulation, thereby potentially
increasing prey abundance. Similarly, removal of alien herbivores (in the absence of
predators) exerting top-down pressure on native plants can lead to rapid recovery
of native plant populations (Zavaleta
et al
. 2001; Zavaleta 2002). However, in an
ecosystem where diff erent trophic levels have been invaded, the scenario becomes
more complicated, as will be shown later in the chapter.
