The removal of introduced organisms is controversial (Myers et al., 2000; Simberloff, 2002), especially when they are relatively charismatic vertebrates. Even attempts at eradicating such species far from the public gaze can generate negative reports from the press (Ebbert & Byrd, 2002) and hostility from the public (Towns, Atkinson & Daugherty, 2006; Webb & Raffaelli, 2008). Conflicts over such eradications are reduced with increased public awareness and education (Bremner & Park, 2007), but this requires a thorough understanding of the effects of invasive species on native ecosystems (Towns, in press).
Demonstrating the relationship between invasive species of mammals and the degradation of native ecosystems is not an easy task. Only rarely have there been detailed reports of simple single-species invasions, and the observed subsequent responses of native ecosystems. Likely pathways for interaction are difficult to test; planned invasions are irresponsible and can have unforeseen consequences (Ricciardi & Simberloff, 2009). If we are to learn about the effects of invasive species, often the best remaining option is to remove them and study how ecosystems respond (Veltman, 1996; Courchamp & Caut, 2005). This is not necessarily easy, even on small islands, which often have complexes of introduced mammals. The most instructive examples are thus those where only one species of invasive mammal is present.
Brodier et al. (2011) provide such an example. They describe the responses of plant cover and three species of petrels to the removal of rabbits, Oryctolagus cuniculus, from an island in the Kerguelen archipelago. They found that rabbit eradication benefited a species of burrowing seabird in deep soils, most likely the result of a release from competition for space. Interestingly, two other species of burrowing seabirds in shallower soils showed no response to the removal of rabbits.
Rabbits are notorious for their detrimental effects on island ecosystems, in particular for suppressing native plants (Courchamp, Chapius & Pascal, 2003). Perhaps the most famous example is on Laysan Island in the Hawaiian islands, where 26 species of native plants and three species of birds disappeared after rabbits were released (Atkinson, 1989). Similarly, rabbits combined with the earlier presence of goats had devastating effects on Round Island in Mauritius (Atkinson, 1988), ultimately leading to severe erosion, destruction of habitats used by burrowing seabirds and the extinction of an endemic species of snake. The rabbits have since been eradicated, but soil replacement could take centuries. Ecological effects such as these have been a powerful incentive for rabbit eradications. However, rabbits can have hidden effects, such as suppression of invasive plants. Eradication of rabbits may be followed by unpredicted outbreaks of weeds. On Motunau Island off New Zealand, rabbit eradication was followed by the spread of boxthorn, Lycium ferocissimum, a shrub that invaded native grasslands and impeded seabird access to nest areas (Beach, Wilson & Bannock, 1997).
Fortunately, the unpredicted consequences of the rabbit removal studied by Brodier et al. (2011) were largely beneficial. Predicted declines of skua after the loss of their rabbit prey did not happen, whereas blue petrels, Halobaena caerulea, increased in abundance. However, like previous examples, introduced plants have progressively invaded native plant cover (Chapuis, Frenot & Labouvier, 2004), which may cause problems similar to those found after rabbit eradications elsewhere. Brodier et al. (2011) emphasized that their aim was to understand the ecosystem responses of eradications of invasive animals. The need to understand eradications in a whole of ecosystem context has been emphasized for some time (e.g. Zavaleta, Hobbs & Mooney, 2001; Courchamp et al., 2003). We often see complex and unpredicted responses to eradications, especially of herbivores. With more studies like those by Brodier and co-workers, such ‘surprise effects’ should reduce in frequency. This should help to add scientific credibility to proposed eradications as their scale, complexity and cost increase.