The eradication of alien mammals from five offshore islands, Mauritius

Bell, B. D. (2002). The eradication of alien mammals from five offshore islands, Mauritius, Indian Ocean. In Veitch, C. R., & Clout, M. N. (2002). Turning the Tide: The Eradication of Invasive Species: Proceedings of the International Conference on Eradication of Island Invasives,[University of Auckland, 19 to 23 February 2001] (No. 27). IUCN 40-45.

Following the removal of rabbits from Round Island in 1979 and the publication of a management plan in 1989, the Mauritius Government contracted Wildlife Management International Limited in 1993 to fulfil one of the plan’s recommendations to survey the offshore islands of Mauritius and Rodrigues and to prepare an offshore islands management plan. This plan made a number of recommendations and priorities in relation to the removal of alien species. In 1995 work on the priorities began with the removal of Norway rats (Rattus norvegicus) and hares (Lepus nigricollis) from Gunner’s Quoin, ship rats (R. rattus) from Gabriel Island and mice (Mus musculus) from Ile Cocos and Ile aux Sables. In 1998 cats (Felis catus), ship rats and mice were removed from Flat Island and rabbits (Oryctolagus sp.), which had been illegally released following the earlier eradications, from Gunner’s Quoin. These programmes were hand-laid operations. In all cases the main bait was grain-based pellets containing 0.02gm/kg brodifacoum. The bait was set out on at least half of the maximum grid recommended for the rodent species targeted. The exception was cats, which were trapped in leg-hold traps. Plans are being considered for the re-introduction of reptiles and birds. Some planting of native trees has begun. This paper covers the eradication sector of the management plan

Relative effect of cats and other factors in removing rabbits from islands

Flux, J.E.C. 1993. Relative effect of cats, myxomatosis, traditional control, or competitors in removing rabbits from islands. New Zealand Journal of Zoology, 20: 13-18.

The outcome of control on 607 islands of known area round the world was analysed to identify the most effective way of removing rabbits. More islands have been cleared of rabbits by traditionalmethods of trapping, shooting, and poisoning than by introduced predators, competitors, or disease, but their relative efficiency could not be calculated because unsuccessful attempts have not normally been recorded. Where hares were introduced, rabbits subsequently became extinct on 27% of 105 islands; introduction of cats removed rabbits from 11% of 80 islands;myxomatosis from 10% of 119 islands; and cats and myxomatosis together from 3% of 40islands. All these control techniques were more effective on smaller islands than on large ones, but rabbits have “died out” naturally on 11% of the 607 islands, and this factor is not linearly related to island area.

How the Macquarie Island parakeet became extinct

Taylor, R. H. (1979). How the Macquarie Island parakeet became extinct. New Zealand Journal of Ecology, 2, 42-45.

For 70 years following the discovery of Macquarie Island in 1810 the endemic parakeet Cyanoramphus novaezelandiae erythrotis remained plentiful, despite the introduction of cats (Felis catus) and other predators. The crucial factor in the bird's rapid disappearance between 1881 and 1890 appears to have been the successful liberation of rabbits (Oryctolagus cuniculus) in 1879. This led to great increases of feral cats and introduced wekas (Gallirallus australis) and presumably to greatly intensified predation on parakeets.

Changes in numbers of parakeets, rats,mice, feral rabbits, feral dogs, feral cats and wekas on Macquarie Island 1810-1920. Diagrammatic reconstructionfrom early accounts (Cumpston, 1968). Not to scale.

More on Macquarie island cats

Controlling cats through the control of rabbits

Courchamp, F., Langlais, M., & Sugihara, G. (1999). Control of rabbits to protect island birds from cat predation. Biological Conservation, 89(2), 219-225.

Both introduced predators (e.g. domestic cats) and introduced small grazers (e.g. rabbits) are harmful to many island vertebrate species. The effects of cats on indigenous species are direct (predation), whereas the most obvious effects of rabbits are often indirect and in the longer term. Thus, in situations where both cats and rabbits are present, priority is frequently given to control of cats. However, the presence of rabbits can allow an increased predator population which can lead to extinction of the indigenous and less well adapted prey species, and increase the difficulty of predator control. Through a mathematical model, we show that control of introduced prey facilitates the control of the introduced predator population. Moreover, predator control may fail to protect the indigenous prey if control of the introduced prey is not undertaken simultaneously. Therefore, control of both introduced species is the best strategy.

Controversy on cat's eradication on Macquarie Island: predator or pathogen's release?

Bergstrom D, Lucieer A, Kiefer K, Wasley J, Belbin L, Pedersen T & Chown S (2009). Indirect effects of invasive species removal devastate World Heritage Island. Journal of Applied Ecology, 46: 73-81

A feral cat roams among baby penguins on Macquarie Island.
Photo: Geoff Copson/ Tasmanian NPWS

1. Owing to the detrimental impacts of invasive alien species, their control is often a priority for conservation management. Whereas the potential for unforeseen consequences of management is recognized, their associated complexity and costs are less widely appreciated.

2. We demonstrate that theoretically plausible trophic cascades associated with invasive species removal not only take place in reality, but can also result in rapid and drastic landscape-wide changes to ecosystems.

3. Using a combination of population data from of an invasive herbivore, plot-scale vegetation analyses, and satellite imagery, we show how a management intervention to eradicate a mesopredator has inadvertently and rapidly precipitated landscape-wide change on sub-Antarctic Macquarie Island. This happened despite the eradication being positioned within an integrated pest management framework. Following eradication of cats Felis catus in 2001, rabbit Oryctolagus cuniculus numbers increased substantially although a control action was in place (Myxoma virus), resulting in island-wide ecosystem effects.

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4. Synthesis and applications. Our results highlight an important lesson for conservation agencies working to eradicate invasive species globally; that is, risk assessment of management interventions must explicitly consider and plan for their indirect effects, or face substantial subsequent costs. On Macquarie Island, the cost of further conservation action will exceed AU$24 million.

Dowding, J. E., Murphy, E. C., Springer, K., Peacock, A. J., & Krebs, C. J. (2009). Cats, rabbits, Myxoma virus, and vegetation on Macquarie Island: a comment on Bergstrom et al.(2009). Journal of Applied Ecology, 46(5), 1129-1132.

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.

Bergstrom, D. M., Lucieer, A., Kiefer, K., Wasley, J., Belbin, L., Pedersen, T. K., & Chown, S. L. (2009). Management implications of the Macquarie Island trophic cascade revisited: a reply to Dowding et al.(2009). Journal of Applied Ecology, 46(5), 1133-1136.

1. 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, 2003–2006) 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.

More on Macquarie island cats

Rabbit control to control cats

Glen, A., J. Cruz & R. Pech. 2013. Can rabbit control reduce feral cat numbers at a regional scale? Kararehe Kino, Vertebrate pest research, 21: 20-21

One of the most important factors affecting the abundance of predators is the availability of their prey. In New Zealand, introduced rabbits support populations of introduced predators, including feral cats. The abundance of rabbits may therefore affect the level of predation by cats on native birds, lizards and invertebrates.

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By adopting a multispecies approach, in which rabbit and cat populations are targeted simultaneously, that both species can be supressed over large areas for long periods. This should have considerable benefits for pasture and for native vegetation and fauna.

Overview of the impacts of feral cats on Australian native fauna

Chris R.Dickman. 1996. Overview of the Impacts of Feral Cats on Australian Native Fauna. Institute of Wildlife Research and School of Biological Sciences. University of Sydney. Australian Nature Conservation Agency. Pp. 97.

This report provides an overview of the impact of feral cats Felis catus on native fauna of the Pacific region, with particular reference to Australia and its island territories. In Australia, cats take a wide variety of native species of mammals, birds and reptiles, but show evident preference for young rabbits or small marsupials where these are available. Reptiles are taken primarily in and habitats, while birds often feature predominantly in the diet of cats on islands. Despite their catholic diet, population-level impacts of feral cats on native fauna have been poorly documented. There is considerable potential for competition to occur between cats and carnivorous species such as quolls and raptors, but no critical evidence has yet been adduced. There is also potential for amerisal impacts to occur, either via transmission of the pseudophyllidean tapeworm Spirometra erinacei or of the protozoan parasite Toxoplasma gondii, but evidence for deleterious effects in freeliving animals is not compelling. Direct predatory impacts have been inferred from anecdotal and historical evidence, more strongly from failed attempts to reintroduce native species to their former ranges, and most critically from the decimation of island faunas and responses of prey species following experimental removal of cats or reduction of cat numbers. Attributes of the biology of feral cats and their prey species derived from the literature review were used to develop a rank-scoring system to assess the susceptibility of native species to cat predation. Species listed federally as endangered or vulnerable were designated as being at zero, low or high risk of impact from cats according to their attribute scores, and their distributions mapped from primary sources and actual locality data. Based on the number of threatened species they contain, localities and regions within Australia were placed in order of priority for future research to clarify the precise impacts of feral cats. Although difficult and expensive to carry out, controlled and replicated field removal experiments are recommended to elucidate cat impacts in all mainland areas. Removal of cats should take place also on offshore islands and island territories, but only if pilot studies show that this will not release populations of alternative predator species such as introduced rats. If release appears likely, cats should be removed only as a component of an integrated control program that targets all relevant predators.

Rabbits killing birds revisited

Zhang, J., M Fan & Y. Kuang. 2006. Rabbits killing birds revisited. Biosciences, 203:100-123


We formulate and study a three-species population model consisting of an endemic prey (bird), an alien prey (rabbit) and an alien predator (cat). Our model overcomes several model construction problems in existing models. Moreover, our model generates richer, more reasonable and realistic dynamics. We explore the possible control strategies to save or restore the bird by controlling or eliminating the rabbit or the cat when the bird is endangered. We confirm the existence of the hyperpredation phenomenon, which is a big potential threat to most endemic prey. Specifically, we show that, in an endemic prey–alien prey–alien predator system, eradication of introduced predators such as the cat alone is not always the best solution to protect endemic insular prey since predator control may fail to protect the indigenous prey when the control of the introduced prey is not carried out simultaneously.

Rabbits killing birds

Courchamp, F., M. Langlais & G. Sugihara. 2000. Rabbits killing birds: modelling the hyperpredation process. Journal of Animal Ecology, 69 (1): 154-164

1. Introduced rabbits are known to have catastrophic effects on oceanic islands, either by direct destruction of the vegetative cover, or by the resulting disturbance of indigenous vertebrates.

2. Another dramatic effect, less well known, but potentially of major importance, is the hyperpredation process. This process, related to apparent competition, predicts that an introduced prey species, well adapted to high predation pressure, could induce the extinction of an indigenous prey, through the sudden increased population size of an introduced predator. In many island ecosystems, the simultaneous presence of introduced feral cats and rabbits is thus potentially a further threat for small vertebrates endemic of these islands.

3. Through a mathematical model, we tested this hypothesis, using a tri-trophic system comprising an indigenous prey (birds), an introduced prey (rabbits) and an introduced predator (cats), and we demonstrated the theoretical existence of the hyperpredation process.

4. In addition, the numerical analysis of the model allowed a quantification of this process. It shows that the conditions required for an indigenous species to cope with the hyperpredation process imply very high intrinsic growth rates and/or carrying capacity, as well as behavioural anti-predator response to the introduced predator. Since these conditions are unlikely to be met, this process is a further potential threat to most indigenous vertebrate prey.

5. Finally, our model shows that, although it can be induced by both types of adaptation together or alone, behavioural adaptations alone are more powerful in generating the hyperpredation process, than are life history traits adaptations.