Marion Island: Birds, Cats, Mice and Men

Abstract

Marion and Prince Edward Islands, 2 300 km off the southern tip of Africa, are home to several million breeding seabirds. But five domestic cats were introduced as pets to the meteorological station on Marion Island in 1948. By the mid-1980s, the feral cat population, by then over 2 000 strong, were estimated to be killing over 455 000 ground-nesting petrels and prions per year. A seven-phase campaign, combining the use of trapping, the release of feline panleucopaenia virus, and intensive hunting, across more than twenty years, led to the total extermination of cats on the island, the largest project of its kind undertaken to that date.

It appears to me that nothing can be more improving to a young naturalist than a journey in distant countries … the effect ought to be to teach him good-humoured patience, freedom from selfishness, the habit of acting for himself, and of making the best of every occurrence.

Charles Darwin (1839): The Voyage of the Beagle

1 Introduction – Africa’s Sub-Antarctic Wilderness

Africa is no longer the Dark Continent – no longer unknown, unexplored and inaccessible. Few places in Africa lie more than a few miles from a road, or a few hour’s drive from a village or even an airport. Internet access is increasingly ubiquitous. But there remains an African territory that is still seriously remote, wild and untamed: these are the Prince Edward Islands.

The Prince Edward Islands lie some 2300 km southeast of the southern tip of Africa. Discovered and forgotten in the mid-seventeenth century, re-discovered in the eighteenth century, plundered of their seal populations in the nineteenth century, and annexed by South Africa in the middle of the twentieth century, the islands remain little known beyond their rocky shores in the midst of the cold, wet, and gale-swept ‘Roaring Forties’ of the Southern Ocean.

The two islands, Marion and Prince Edward, form part of the circumpolar islands of the sub-Antarctic (Fig. 4.1). Despite their inhospitable climate, treacherous bogs, and volcanic and glaciated landscapes, the islands are home to hundreds of thousands of breeding seabirds – penguins, albatrosses, petrels and prions – and tens of thousands of breeding fur seals and elephant seals (Ryan and Bester 2008). Marion, the larger island, of 29,000 ha, rises to 1231 m; Prince Edward, of 4500 ha, to 672 m. With an estimated age of 450,000 years, the islands emerge as volcanic peaks from the ocean depths. Landscapes and vegetation include barren or fern covered lava fields, windswept communities of cushion plants, waterlogged mires and bogs, cliff lined coasts and snow-covered mountain peaks (Figs. 4.2, 4.3, 4.4, and 4.5).

Fig. 4.1

Marion and Prince Edward Islands lie approximately 2300 km southeast of the southern tip of Africa, slightly north of the Antarctic Convergence

Fig. 4.2

Exploring Marion Island in 1965 – the frequently snow-covered central peaks

Fig. 4.3

The fern and shrub-covered basalt lavas of the west coast plain

Fig. 4.4

Black basalt lava flows provide shelter for feral cats

Fig. 4.5

Boulder beaches occupied by breeding colonies of penguins and elephant seals. (Photo: Christiaan Brink)

The islands provide a fascinating story of species-poor, simplified ecological systems at work, and the impact of invasive species on the ecosystem’s fragile structure and functioning. They also provide a stage for remarkable actors who have devoted their professional careers to understanding the dynamics of sea-land-plant-animal and human interactions in what, for most researchers, are extremely hostile environments. What makes the islands unique for researchers in the African context is their remoteness, and the challenges that students have to face when physical contact, supplies, medical support, or in earlier years, even communication was a once-in-a-year opportunity.

At the time of annexation in 1947/48, the islands were truly terrae incognitae. Only one botanist had previously collected plants on Marion, on a hurried one-day visit during the voyages of the British Challenger expedition in 1873 (Moseley 1874). Shortly after annexation, Robert (Bob) Rand spent six summer months (1951/52) on Marion, undertaking a valuable baseline survey of birds and seals (Rand 1954). Like all visitors who followed him, Rand experienced the difficulties of working on the island: “No boat was available on the island; all excursions were made on foot. Food and gear for periods of four to seven days away from the base were carried in a rucksack, including heavy equipment. The varying nature of the ground and the unpleasant climate made movement from place to place very fatiguing, and detracted from the enjoyment and success of biological work. Rain, in particular, was a trying factor …”.

As plant ecologist on the first ‘Biological-Geological Expedition to the Prince Edward Islands 1965-66’ (Van Zinderen Bakker Sr. et al. 1971), I spent 15 months on the islands surveying the flora and vegetation (Huntley 1971, 2016). I could not anticipate the changes that were to follow. In the 1960s, the islands had a mean annual temperature of 5.5 °C, mean annual rainfall of 2726 mm, falling on 311 days per year, and with gale-force winds recorded on 107 days per year.

The climate has ameliorated over the past five decades, with annual precipitation down to.

1778 mm in the 2010s, and temperature means increasing by 1.2 °C, between 1949 and 2016 (Le Le Roux 2008, Le Roux and McGeoch 2008, Hedding and Greve 2018) with a consequent shrinking of much of the icefields of the high plateau (Meiklejohn 2011) and progressive drying of mires and bogs. Climate change has also resulted in multiple ecological changes such as increased house mouse abundance and in turn their impacts on the islands’ ecological structure and functioning (Chown and Smith 1993; Smith et al. 2002; Le Roux and McGeoch 2008; McClelland et al. 2018). The history, geology, biology, ecology and conservation of the islands is comprehensively covered in the synthesis volume edited by Chown and Froneman (2008), which draws on the considerable body of publications – over 1000 papers in peer-reviewed journals – of research conducted on the islands since the 1960s.

2 An Emerging Invasion Crisis

In common with most island ecosystems (Elton 1958; Mooney and Cleland 2001) but especially those of the remote oceanic islands of the sub-Antarctic, Marion and Prince Edward are highly vulnerable to invasive alien species – the greatest threat to the biodiversity of the sub-Antarctic islands (Frenot et al. 2005; Greve et al. 2017). The first human visitors to the islands were sealers, first recorded in 1802 (Cooper 2008). These early teams of intrepid men unintentionally introduced house mouse Mus musculus and several alien plants to the islands, probably from 1818 (Watkins and Cooper 1986). In 1948 five domestic cats Felis catus were introduced to Marion Island to try to control the mouse population at the small meteorological station that had been established on the recently annexed island. By 1952, offspring of the original cats had, according to Rand (1954): “gone feral and were preying on the smaller petrels or mice that are widespread over the coastal plain”. This is when the trouble really began.

By the time of my survey of the island, in 1965/66, the depredations of the cats on the smaller seabirds and the nightly foraging activities of mice were regularly observed. In hindsight, the 1965/66 Expedition should have raised the alarm regarding the crisis that would develop from cats and mice on the islands, especially given the warning call on the impacts of invasive biota raised a few years earlier by the seminal work of Charles Elton (1958). In their comprehensive review of the feral cat eradication project on Marion Island, Bester et al. (2002) noted that in October 1965, the expedition leader, Van Zinderen Bakker, informed the South African Scientific Committee on Antarctic Research (SASCAR) that: “The feral cat population was not large enough to threaten bird populations, and that the cats were indeed contributing to the control of mice in the meteorological station.” I suspect that Van Zinderen Bakker, a humanist and sensitive animal-lover, felt some compassion for the friendly cats that then lived around the meteorological station.

A decade later, research confirmed that the cats were exerting a negative impact on the bird populations (Anderson and Condy 1974). More dramatically, what focused attention on the ecological crisis on Marion Island was a report by Van Aarde (1975), that an estimated 455,000 birds were being killed annually by the population of feral cats. SASCAR was convinced that a cat eradication programme should be initiated.

What followed was the most ambitious, largest, longest and most expensive cat eradication programme yet undertaken on an island ecosystem. It still remains the most successful operations of its kind (Preston et al. 2019). As a model of the integration of elegant basic research, targeted monitoring and pragmatic conservation action by literally hundreds of contributors, it stands out as a conservation project that brings credit to South Africa. Yet, as noted at the end of this chapter, work on invasive alien mammals on the Prince Edward Islands has not yet concluded.

3 A Science-Driven Approach to Eliminating an Aggressive Invasive Species

Following the Van Zinderen Bakker expedition of 1965/66, nearly a decade passed before research resumed on the islands. Three institutions took the lead – the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, the Mammal Research Institute at the University of Pretoria and the Botany Department at the University of the Free State. The programme was multi-disciplinary, focusing initially on adding fine detail to the broad outlines of the biota reported by the 1965/66 expedition (Van Zinderen Bakker Sr. et al. 1971). By the end of the 1970s, detailed accounts were completed on the vegetation, cats, mice, ground-nesting birds, marine mammals and nutrient cycling in terrestrial ecosystems (see papers in Chown and Froneman 2008). Enough scientific evidence regarding the impact of cats on the burrow-nesting seabirds had been assembled to mobilise the cat-eradication project. Like similar science-driven projects, short funding cycles resulted in a rather unpredictable and frequently interrupted programme of work. But the commitment and persistence of the research leadership, and generous support of the main governmental funding source ensured ultimate success.

As with most large cooperative projects, there were many key players and champions. The stalwarts of the Marion Island research programme over many years include Rudi van Aarde and Marthán Bester (mammalogists), Valdon Smith and Niek Gremmen (botanists), John Cooper and Peter Ryan (ornithologists) and Steven Chown (entomologist) – plus a talented cast of over one hundred co-workers. Unsurprisingly, in contrast to the vast detail on the natural sciences included in the 1000-plus papers published on the islands’ biophysical history, biological diversity and ecological structure and functioning, only a handful of papers cover the human sciences, and even fewer study the social history of the island’s transient community of researchers, meteorologists and support teams. Fortunately, this near vacuum has been ameliorated by a fascinating collection of memories of the mammalogists, ornithologists, botanists, entomologists and cat-hunters involved in four decades of the cat eradication programme and of seal studies on the islands (De Bruyn and Oosthuizen 2017). The anecdotes – full of nostalgia of the blood, sweat and tears of working under physically and mentally demanding conditions, and of failures and triumphs have a primary focus – the remarkable leadership and charisma of the person who drove the project to success, Marthán Bester (Fig. 4.6). Here I will draw heavily on Bester et al.’s (2002) summary of the cat eradication programme.

Fig. 4.6

Marthán Bester with a tame member of the feral cat population. (Photo: Grant Craig)

Bester and his co-authors describe the seven phases of the feral-cat eradication programme, sometimes overlapping in chronology, and which extended over 25 years from conception to successful conclusion. Their paper provides an excellent model for such projects, even though the authors cautioned that there were many factors in the exercise that were peculiar to Marion Island, and that the programme: “… may therefore not be repeatable in the same way on other islands.” The phases follow a logical succession of a science-based collection of evidence for the development of policy and practice – a pragmatic exercise of ‘learning by doing’.

The first phase studied the ecology of the feral-cat population. Cat distribution, density, habitat selection, feeding and reproductive behaviour were studied between 1974 and 1976. The thorough research of Rudi van Aarde, published in a series of papers (Van Aarde 1978, 1979, 1980, 1983, 1984; Van Aarde et al. 1996), provided a firm baseline on the biology and demography of the cat population. His estimates put the 1975 population at 2139 individuals, increasing by 26% per annum, and annually consuming an estimated 455,000 burrowing petrels. A decade later, Schramm (1986) estimated that Marion had suffered a 25-fold decrease in burrowing petrel population densities as compared to those of cat-free Prince Edward Island, 19 km to the northeast of Marion.

Having established the magnitude of the problem, a diversity of solutions was considered by the advisory group established by SASCAR. Biological control was recommended, and here the extended experience of the Veterinary Research Institute at Onderstepoort, Pretoria, provided leadership. The second phase researched the feasibility of using feline panleucopaenia virus (a highly contagious parvovirus that causes cat distemper) as a biological control. Serological testing of a small sample group of Marion cats proved the efficacy of the approach (Howell 1976, 1984). However, despite the potency of the virus, it was still considered necessary to have an integrated control programme using a variety of techniques, of which the virus would be a primary component.

Basic research advanced into applied research in the third phase, from March 1977, when the cat population was estimated at 3405 individuals. This phase saw the release of 96 cats inoculated with the virus, at 93 sites around the island, using helicopter support. Evaluation followed introduction. Phase four, from November 1976 to May 1978 assessed the impact of the virus. It was found that there was an estimated 53% reduction in the cat population after 18 months (Erasmus 1979). It was suspected that the cats could develop resistance to the virus, indicating the need for a multifaceted approach. Phase four explored a variety of other controls. Cage traps and gin traps, plus different baits and lures and a range of poisons were tested. Hunting with shot guns by day and by night was evaluated, as was hunting with dogs – three Jack Russell terriers, a German shepherd and a Labrador.

All these experimental approaches proved too costly or inefficient or held potential environmental risks. Despair started to set in. Bester et al. (2002) recorded that by February 1978 some doubt was expressed that the cats could ever be eradicated. Undaunted, three years after the virus release, SASCAR wisely recommended that further control, monitoring and research were needed.

The fifth phase, from April 1981 to May 1983, intensified studies on the impacts of the virus on cat demographics and ecology, their effect on bird populations, and the utility of hunting as a secondary control measure. These detailed studies gave results that confirmed the promise of an integrated approach. The cat population had been decreasing by 26% a year, and by 1982 it was down to 615 individuals or 18% of the 1977 population. But the news was not uniformly positive. By 1983 the cats were found to be developing immunity to the disease (Van Rensburg et al. 1987) and parallel studies concluded that the bird populations were still threatened (Fugler et al. 1987; Newton and Fugler 1989).

With a much-reduced cat population, hunting became a feasible (but very costly) option (Van Rensburg and Bester 1988). Phase six, between August 1986 and May 1989, focused on hunting by night using 12-gauge shot guns and battery-operated spotlights (Fig. 4.7). Eight teams of two men killed 458, 206 and 145 cats in the three winter hunting seasons. But the decline in hunting success indicated that hunting alone would not be sufficient. An independent review of the programme was commissioned. The reviewers recommended continued hunting in winter, and that dogs and baits be tested once more, and that controlled gin trapping and poisoning be used. It became obvious that complex problems such as invasive species control need multiple approaches to their solution.

Fig. 4.7

Teams of hunters used spotlights to locate and shotguns to kill 809 cats during three hunting seasons. (Photo: Kevin Language)

Having already spent many millions of dollars on the programme, the funding source (Ministry of the Environment) needed a compelling motivation to invest more millions. Fortunately, the strong and courageous departmental and institutional leadership was convinced by the quality of the science, the seriousness of the developing conservation crisis, and by the commitment of the researchers, hunters and support teams. Funding was approved for the final, and most expensive, seventh phase which commenced in April 1989 and ended in March 1993.

Increased intensities of trapping, hunting and poisoning led to the last cat being trapped in July 1991. Other than a few skeletal remains, no further signs of live cats were found despite even higher intensities of hunting and trapping effort over the following 22 months. During the core control period, from 1986 to 1993, no fewer than 76 hunters were engaged, a total of 14,357 hours of hunting was invested, 3155 cat sightings were made and 768 cats shot. In total, 197 cats were trapped, using 3227 traps and 30,000 poisoned day-old chicken baits. The total count of cats killed (excluding those that died, undetected, from panleucopaenia or poisoning), was 1080 (Bester et al. 2002).

By 2000, with no cats having been seen nor their signs detected in over nine years, it was concluded that cats had been successfully eradicated from Marion Island (Bester et al. 2000). This was unquestionably the largest successful cat eradication programme yet undertaken at the scale of Marion Island’s 29,000 ha of unforgiving volcanic mountains in the sub-Antarctic. Nearly three decades passed before another extensive cat eradication project was successfully concluded – on tropical Dirk Hartog Island, of 63,000 ha – just off the West Australian coast (Algar et al. 2019).

4 Lessons Learned

Bester et al. (2002) suggested that, with the benefit of hindsight, greater efficacy could have been achieved by a quicker, more intense initial reduction, followed by a large and persistent effort. However, the Marion programme required considerable, lengthy and repeated consultation due to the risks and sensitivities of such a novel project. The media regularly published articles and letters on the project, positive and negative, reasoned and emotional. Further, the inevitable lags associated with mobilising the very considerable finances required for such a large programme on a remote and inhospitable island resulted in its extended timespan. Bester et al. (2002) humbly concluded that: “Few would understand what it takes to be a successful hunter on Marion Island, and particularly what was required to prove the absence of cats over the last 22 months of the programme.”

In their assessment of the programme, Bester et al. (2002) noted three critical drivers of success, of which they considered the first the most important:

• The susceptibility of the Marion cat population to the feline panleucopaenia virus;

• The lack of tall stands of vegetation, which would have rendered the hunting campaign impossible due to decreased sighting rates and hunting success; and

• The recolonisation of preferred habitats, cleared of cats, from neighbouring sub-optimal habitats, which served continually to concentrate surviving cats in smaller areas.

Other factors contributing to the success of the programme noted by Bester et al. (2002) were:

• The initial study of cat biology and distribution provided detailed knowledge of the cat population;

• The absence of other terrestrial predators which would have interfered with the trapping and poisoning campaigns;

• The inclusion of experienced personnel from previous teams in each new team to continue the programme; and

• The resolve of the funding bodies to provide the necessary support.

What Bester et al. (2002) did not mention, but which were equally important factors included:

• The early recognition by young researchers of the gravity of the impact of the feral cats on the bird population and the urgency to mobilise population control actions;

• The importance of the scientific and technical capacity of South African institutions, in particular the Veterinary Faculty of the University of Pretoria at Onderstepoort, and the Mammal Research Institute, University of Pretoria;

• The high quality of the basic biological studies undertaken by young researchers spending long periods, usually without direct mentorship, on the island;

• The availability of young, resilient, adventurous and extremely dedicated cat hunters, willing to work under extremely difficult weather conditions in hazardous terrain, usually in the pitch darkness of freezingly cold winter nights;

• The strong leadership and intellectual maturity of the projects’ young field supervisors; and

• Finally, and most importantly, as repeatedly emphasised in the 464-page tribute volume to Marthán Bester (De Bruyn and Oosthuizen 2017), one fact is abundantly evident: the inspiring leadership of the man. Through the multiple challenges of logistics, personalities, inclement weather, hazardous terrain and financial uncertainties, Bester was able to provide calm, decisive and firm leadership of the 76 cat hunters, dozens of researchers and the inevitable demands of government administrators, advisory committee members and the interested public.

5 Postscript: When the Cat’s Away, the Mice Can Prey

Eradication of the cat population has not removed the problem of alien predators from Marion Island. As early as 1990s the impact of cat eradication on the house mouse population, and the knock-on effect of higher mouse populations on their invertebrate prey, was discussed by Van Aarde et al. (1996). The island’s mice, weighing in at a mere 20 grams, are now proving to be mortal predators on the bird populations, from the smallest burrowing petrels to large fledglings of wandering albatrosses, of up to 7 kilograms.

The task of proving that there are no cats left on the island was a considerable challenge. It took several years of concerted effort by teams of cat hunters spending tedious and often dangerous nights searching the coastal cliffs, soggy mires, rocky plateaus, broken lava flows and desolate mountains of the island. Having concluded that the cats had been successfully eradicated, assessing the recovery of the burrow-nesting seabird populations was the next priority. In 2013, 22 years after the last cat was killed on Marion Island, Ben Dilley re-surveyed the same study sites where petrel burrows had been counted by Michael Schramm in 1979. Schramm revisited Marion with Dilley to ensure replicability of the surveys. In the northeast sector of Marion Island, 741 quadrats of 10 × 10 m were searched for burrows, following Schramm’s design. Based on the known dynamics of burrow-nesting seabird populations on neighbouring Prince Edward Island, free of introduced predators, Dilley et al. (2016a) expected a three- to five-fold increase in Marion petrel burrow densities following cat eradication. However, within the 1041 ha study site, the number of burrows of eight petrel species showed only a 56% increase between 1979 and 2013 – from a 1979 estimate of 156,000 to a 2013 estimate of 243,000 burrows. Dilley et al. (2016a, 2018) concluded that the slow recovery rate of burrows was due to predation of petrel eggs and chicks by the increasing mice population.

It was not long before mice started attacking larger birds. Dilley et al. (2016b) recorded a seemingly sudden eruption of mice attacks on grey-headed, sooty and wandering albatrosses (Fig. 4.8). Following a first record of attacks on wandering albatross chicks in 2003, one-third of sooty albatross fledglings were found ‘scalped’ at a remote colony on the island’s southwest coast in 2009 (Jones and Ryan 2010). An unprecedented increase in the frequency of mouse attacks on grey-headed, sooty and wandering albatross chicks was recorded in 2015 (Dilley et al. 2016b). Filming at night with motion-activated infra-red cameras provided confirmation that the mice attacked the heads of chicks, debilitating the birds, which in some cases were then attacked by giant petrels. The records showed that 11% of 2201 grey-headed and 9% of 1045 sooty albatross chicks were attacked by mice in the autumn of 2015. Most of the chicks died. Even the large chicks of wandering albatross have not escaped the predations of mice. Between 2003 and 2018, 32 wandering albatross chicks have been noted with mice-inflicted wounds, of which 72% died (Dilley et al. 2016b). In 2016, mice attacks on large chicks of three albatross species continued at similar levels to those seen in 2015. In 2017 and 2018 fewer albatross chicks were attacked, but in 2019 attacks were again frequent and widespread, but the reasons for these annual fluctuations are not clear – what has become clear is the sudden increase of this mouse behavior was not a one-off event in 2015.

Fig. 4.8

Two grey-headed albatross chicks with typical ‘scalping’ wounds due to predation by mice, Marion Island, 2015. (Photo: Ben Dilley)

Even more alarming were the records, in 2017, of mouse wounds on breeding adult giant petrel on Marion Island and adult Tristan albatross and Atlantic yellow-nosed albatrosses on Gough Island (Jones et al. 2019). The adult birds were not killed by the mouse attacks, but the presence of many chick carcasses in the vicinity pointed to the probable cause. As Jones et al. (2019) observe, mortalities of breeding adults would have far greater impacts on the breeding populations than loss of eggs or chicks. Dilley et al. (2016b) concluded that although mice were not an important food source for cats, feral cats might have influenced mouse demography before the cats were eradicated. As Chown and Smith (1993) and Le Roux and McGeoch (2008) have suggested, a warming climate, together with cat removal, might have resulted in increased mouse densities. These reached up to 237 mice per ha (McClelland 2013). With the approach of winter, and with decreasing invertebrate food populations recorded over the past 40 years, mice might be switching their prey. Rayner et al. (2007) attributed this prey switch process to the ‘mesopredator release’ effect, whereby mesopredators (mice) increase after the eradication of top predators (cats). Marion Island’s bird populations are again threatened by an invasive mammal (McClelland et al. 2018).

The concern regarding mice is not new. As early as 1995, a workshop of Marion Island biologists considered that the eradication of mice from Marion Island would be beneficial for the restoration of the island’s ecosystem functioning (Chown and Cooper 1995). The workshop made specific recommendations on research activities needed to precede an eradication programme, including mouse population dynamics and feeding behaviour, and the differentiation between the effects of climate changes and mouse predation on the island biota. A decade and a half later, in a detailed unpublished review, Angel and Cooper (2011) concluded that most of the research proposed by the 1995 workshop had been undertaken, “justifying and providing the information necessary for a feasibility study for the eradication of the species on the island.” There have been a series of studies on Marion’s house mouse population, at irregular intervals from the late 1970s through to the present (Gleeson 1981; Matthewson et al. 1994; Ferreira et al. 2006; McClelland 2013; McClelland et al. 2018). Much has been learned about the interactions of mice with their physical and biological environments. The annual peak populations of mice have been increasing in response to ameliorating climatic conditions, in turn suppressing native invertebrate biomass (McClelland et al. 2018).

In order to accelerate action, John Cooper, one of the pioneer researchers on the islands’ bird populations, encouraged funding from BirdLife South Africa, in partnership with the South African National Antarctic Programme (SANAP) to commission a feasibility study: “to assess whether eradication of the mice is feasible, and to review the constraints and risks to be resolved or mitigated before making such an attempt.” The field study was undertaken in April 2015 and the results presented to SANAP later that year. The report (Parkes 2016) concluded that the eradication of mice from Marion Island is definitely possible, mice eradication having been successfully achieved on 62 islands, including sub-Antarctic Macquarie Island, which at 12875 ha is half the area of Marion. The tried and tested approach recommended would use up to four large helicopters to disperse cereal-based pellet baits containing the second-generation anticoagulant toxin brodifacoum. The exercise would need to be completed within a few days and repeated within 10 days. As Parkes (2016) noted: “Eradication is the permanent removal of all individuals from a defined area, Marion Island in this case. It is an all or nothing management goal – one cannot almost eradicate a pest”.

What is clear, from the available scientific evidence reviewed by Preston et al. (2019) is that: “Left uncontrolled, it is feared that 18 of the 28 species [of seabirds] breeding on Marion Island may be vulnerable to local extirpation should the mice not be eradicated.” The significance of this statement is that it is the consensus of its 31 co-authors, all with extended experience in conservation science, action and administration. The lead author, Guy Preston, as Deputy-Director General of the Department of Environment Affairs, Forestry and Fisheries, in the Ministry of Environment, spear-headed South Africa’s massive Working for Water project, which has invested more than US$1000 million in nation-wide invasive species control projects over the past 25 years, the largest conservation programme in Africa. The proposal carries the weight of sound science, decades of experience and strong government endorsement.

The concern for Marion’s bird populations is paralleled by a similar crisis on Gough Island, in the South Atlantic, where an international partnership was established to implement a mouse eradication project in 2020, led by the United Kingdom’s Royal Society for the Protection of Birds. Gough Island, of just 6500 ha, is one of the world’s most important seabird breeding islands, home to an estimated 12 million seabirds of 22 species. Predation by mice, with very heavy mortality rates, has been reported for several of its albatross, petrel and prion species (Dilley et al. 2015). The Gough Island mice eradication project has provided a testing ground for South African teams in preparation for a similar project on Marion Island in the southern autumn of 2025. The South African government has already budgeted the equivalent of US$2.2 million for the project, while BirdLife South Africa is mobilising donors and crowd-funding to provide further support for the project. However, the estimated total budget for the project is over US$20 million, a challenge that will require extraordinary fund-raising strategies.

It took over two decades of heroic efforts by more than 100 scientists and cat hunters to eradicate the cats of Marion Island. It would seem unrealistic to expect that the much more numerous, much better concealed, and much more resilient mice population could be eradicated within a few weeks of helicopter-borne aerial bombing with a shower of toxic bait. But as Nelson Mandela famously said: “It always seems impossible until it is done.”