Management implications of the Macquarie Island trophic cascade revisited: a reply to Dowding et al. (2009)

Abstract

Summary1. The management of non-indigenous species is not without its complications. In Bergstrom et al.'s (2009) study, we demonstrated that feral cats Felis catus on sub-Antarctic Macquarie Island were exerting top-down control on the feral rabbit Oryctolagus cuniculus population, and that the eradication of the cats led to a substantial increase in rabbit numbers and an associated trophic cascade. 2. Dowding et al. (2009) claim our modelling was flawed for various reasons, but primarily that a reduction in the application of the rabbit control agent, Myxoma virus, coinciding with cat removal, was a major driver of rabbit population release. 3. We explore this proposition (as well as others) by examining rates of Myxoma viral release between 1991 and 2006 (with an attenuation factor for the years, [2003][2004][2005][2006] in association with presence ⁄ absence of cats against two estimates of rabbit population size. Myxoma viral release was a significant factor in the lower estimates of rabbit population, but the effect was small, and was not significant for higher rabbit population estimates. By contrast, the presence or absence of cats remained highly significant for both estimates. 4. Synthesis and applications. We re-affirm our position that top-down control of rabbit numbers by cats, prior to their eradication, was occurring on Macquarie Island. Nonetheless, we agree with Dowding et al. (2009) that systems with multiple invasive species represent complex situations that require careful scrutiny. Such scrutiny should occur in advance of, during, and following management interventions.

Eradication of cats (Felis catus) from subantarctic Macquarie Island

“…While Bergstrom et al . (,b) provide strong arguments that release from cat predation was the primary driving factor behind the rapid increase in rabbit number, other studies (e.g. Dowding et al .…”

“…The interactions between these variables have not been fully quantified as cats were only considered as a binary variable in the models of Bergstrom et al . (,b), and data on the efficacy of myxomatosis are inadequate to meaningfully assess its influence.…”

What to do in the face of multiple threats? Incorporating dependencies within a return on investment framework for conservation

“…, 1989; Banks et al. , 1998), creating an even greater ecological problem that is costly to rectify (Bergstrom et al. , 2009a,b).…”

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."…”