Management of interacting invasives: ecosystem approaches - Invasive Species Management - page 232

15
Management of interacting
invasives: ecosystem approaches
Leigh S. Bull and Franck Courchamp
It is usually easy enough to shoot the goats, wild cattle or sheep from small islands, but
unfortunately this very often creates only fresh conservation problems
R.H. Taylor, 1968
15.1 Introduction
Too often the success of an invasive species management programme is measured
solely by the decrease or eradication of that species. However, this way of think-
ing distracts from the ultimate goal of these programmes, which is not just the
removal of the alien species but rather the restoration of the ecosystem's biodiver-
sity. Several decades ago, Taylor (1968) alluded to the fact that the former does
not necessary lead to the latter; the incorrect management of an invaded ecosys-
tem can in fact result in potential problems following the removal of a species.
Ecosystems, be they invaded or pristine, consist of a community of organisms and
their physical environment that interact as an ecological unit (Lincoln et al . 1998).
Because of these interactions, any alteration to the species composition can have
fl ow-on effects throughout the ecosystem (Chapin et al . 2000). It is this poten-
tial for fl ow-on effects, as Taylor (1968) suggested, that researchers, managers,
and conservationists must consider before attempting any control or eradication
programme. While a number of invasive species eradications have had the desired
positive effects on native biodiversity, there are instances in which such actions
have had either no effect, an unexpected, or even opposite impact on an ecosystem
(Mack and Lonsdale 2002; Zavaleta 2002; Courchamp and Caut 2005). Such
outcomes have largely been as a result of not considering the importance of the
interactions between species (both native and introduced) within the ecosystem.
Unfortunately, multiply-invaded ecosystems are now the rule rather than the
exception. h ese ecosystems are generally more di cult to manage than those
that have been invaded by a single species, because as the numbers of interacting
invaders increase in an ecosystem, and as aliens in late stages of invasion elimin-
ate native species, they are more likely to replace the functional roles of the native
species (Zavaleta et al . 2001). In the cases of these multiply-invaded ecosystems,