Cats, rabbits, Myxoma virus, and vegetation on Macquarie Island: a comment on Bergstrom et al. (2009)

Abstract

1.Eradication of a single pest species from a multiply invaded island system may have unpredicted and detrimental impacts. Bergstrom et al. (2009) describe damage to vegetation following an increase in the number of rabbits on Macquarie Island. They propose that the increase in rabbit numbers was caused solely by eradication of cats. 2. However, their modelling is flawed and their conclusion that cats were controlling rabbit numbers is unsupported. We suggest the increase was because of some combination of four factors: reduced releases of Myxoma virus, abundant food after 20 years of vegetation recovery, release from cat predation and climate variability. 3. Recent high numbers of rabbits on Macquarie Island are not unprecedented; vegetation has been damaged in the past but has recovered. Rabbit numbers appear to be in decline again in the absence of both cats and Myxoma releases, suggesting that other factors can contribute to regulation of rabbit numbers in this system. 4. We do not agree with the implication that pest management could have been better integrated. Eradication techniques for rodents and rabbits on an island the size of Macquarie were unavailable when cat eradication was deemed necessary. The benefits to seabirds of cat eradication have been rapid. Our analysis further highlights the complexity of multiply invaded island ecosystems.

Indirect Effects of Introduced Predators on Seabird Islands

“…Unfortunately, this led to cats diet-switching from rabbits to seabirds, so managers began controlling cats, and eventually eradicated them (Copson and Whinam 2001 ). Since cats were eradicated in 2001, rabbits have once again increased in numbers (Bergstrom et al 2009 ), although it is unclear whether this release is attributable to the cessation of cat predation or the decrease in effi cacy of viral control (Dowding et al 2009 ). Vegetation cover has also decreased with rebounding rabbit density, although this effect is confounded by climate warming over the past 50 years.…”

“…On Tatoosh Island, Washington, peregrine falcons ( Falco peregrinus ) directly reduce the abundance of Cassin's auklets ( Ptychoramphus aleuticus ) and rhinoceros auklets ( Cerorhinca monocerata ), but through their predation on crows ( Corvus caurinus ) they release common murres and pelagic cormorants ( Phalacrocorax pelagicus ) from crow predation on eggs (Paine et al 1990 ). Plausible hypotheses for indirect interactions (Bergstrom et al 2009 ) can be confounded by other processes, both biotic and abiotic, operating from within and outside of these systems (Dowding et al 2009 ). Shift s between indirect interactions among many species will depend heavily on the mode of system regulation or other externally driven changes to systems.…”

Novel Tropical Forests: Nature's Response to Global Change

“…Each scenario has different costs and requires different levels of effort to address them. For example, restoring natural vegetation on small islands through the eradication of introduced animals can be costly with unpredictable effects and controversy [65], [66][67][68], while natural processes at no cost to the economy powered the reforestation process that led to increasing the forest area in Puerto Rico from 6 percent to 57 percent. We face a conundrum as articulated by Botkin [69]: "One can either preserve a 'natural' condition, or one can preserve natural processes, but not both."…”

Eradications of invasive mammals from islands: why, where, how and what next?

“…The removal of herbivorous mammals can also lead to dramatic changes in vegetation cover and structure, including the proliferation of exotic plants (Zavaleta et al 2001). Although funding and feasibility constraints may result in a staged rather than a holistic approach (Bergstrom et al 2009;Dowding et al 2009), ideally all introduced vertebrates should be eliminated at once.…”