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P. Genovesi
ing all the removal efforts. The ability of the species to establish and expand
from a nucleus of very few individuals is thus another aspect to carefully take
into account in eradications. For example, eradication of squirrel species is
complicated by the ability of many species to establish and expand from a
nucleus of only two or three pairs (Bertolino and Genovesi 2005).
Nevertheless, analysing the target species’ life history may not be enough,
as ecological interactions with other species can play a major role in the final
outcomes of the eradication. For example, the eradication of rabbits from two
islands of the Kerguelen archipelago led to an overall increase in plant species
richness but also in the decline of some native species (Chapuis et al. 2004).
Eradication of two introduced brushtail possums and brushtail rock wallabies
from Rangitoto and Motutapu islands also resulted in a proliferation of several
weed species (Mowbray 2002). In general, when eradications are carried
out in areas invaded by more than one alien species, as in the case of islands
with domestic cats, rats, and rabbits, eradication of superpredators (cats) may
result in an explosion of mesopredators (rats), with the risk of aggravating
impacts on native species. Conversely, eradication of rats can cause a shift of
the diet of cats towards native species. Multi-species interactions also have
effects on the feasibility of eradication: Courchamp and Sugihara (1999)
showed that the removal of alien prey is in some cases an essential condition
to successfully eradicate alien predators, because abundant prey populations
may substantially limit the efficacy of predator removal efforts. Therefore,
eradication planning requires also an understanding of the ecological interaction
of the target species with other alien and native species and, in some
cases, concurrent control of more than one alien species can be the best alternative.
22.2.2 Lag Phase
Biological invasions are often characterized by a lag phase followed by a rapid
expansion (Williamson 1996), and this common pattern calls for starting
eradications at the earliest possible stage, when the chances of successfully
removing the infestation are highest. Prompt removal has been the key to success
in the few cases of eradications of marine organisms attempted to date as
well as eradications of freshwater fishes (e.g. Jackson et al. 2004), of the Canadian
beaver from France (Rouland 1995) and of alien plants (Rejmánek and
Pitcairn 2002). On the contrary, when the start of control activities is delayed,
eradication often becomes impractical (e.g. the American grey squirrel;
Bertolino and Genovesi 2003).
The case of Caulerpa taxifolia is paradigmatic of the need to adopt a precautionary
approach when addressing alien species. This alien alga was accidentally
detected in 1984 in France when the invaded area covered only 1 m 2 ;
by 1989, this had increased to 3 ha, and to 31 ha by 1991. Response was delayed