All living things are connected, ecology teaches us – yet it’s perhaps not immediately obvious how feral cats can change the hue, texture and composition of huge swathes of plants.

Yet that’s exactly what has happened on Marion Island, one of the two volcanic islands that make up South Africa’s only overseas territory, deep in the heart of the feared “Roaring Forties” of the vast Southern Ocean.

Journalist John Marsh, who accompanied the team that annexed Marion and Prince Edward Islands for South Africa in December 1948 and January 1949 respectively, was entranced by what he saw on arrival. In his subsequent book of this historic event, No Pathway Here, he wrote of his first glimpses of Marion:

“She rose, a jade jewel, out of the sea. Her lush green coat was fringed with the black lace of the cliffs and her heights draped with scintillating snow.”

That’s not the same scene experienced by current travellers to the island, such as those who sailed there this month for the opening of the new research base. Stellenbosch University researcher Professor Steven Chown, who has visited Marion on many occasions since 1983, says Marsh’s “jade jewel” has disappeared.

“You think to yourself ‘Hang on, it looks more like a dirty piece of amber’,” he remarks.

The major reason for the dramatic reduction of the island’s bright green hue, at least in some areas, was feral cats, whose numbers exploded exponentially after being introduced to this near pristine environment. Because they killed literally millions of seabirds over decades, they were directly responsible for reducing the flow of nutrients, like nitrogen, via the birds’ guano into the ecosystem.

This in turn resulted in sharply reduced cover by nutrient-loving species such as the sub-Antarctic tussock grass Poa cookii and other tussock grassland species which help give the sub-Antarctic islands their distinctive bright green colour.

Scientists say the contrast is now striking between Marion and its neighbour, Prince Edward Island, 19km to the north, which has never been invaded by mammals and which still hosts huge numbers of breeding petrels whose waste products promote its tussock grass species.

Marion Island’s cat saga is a salutary lesson as to why invasive alien species pose such immense ecological dangers, especially in closed island ecosystems.

A handful of cats were taken to Marion Island in 1949 with the most innocent of motives – to control the house mouse plague at the meteorological station. The mice had been introduced inadvertently by sealers, probably during the 1800s.

Two groups of two and three cats respectively were brought, and one of them, a tortoiseshell male, was sterile, judging from the fact that his pelt colour was never reproduced. So the devastating and completely unanticipated ecological disaster that followed was initiated by just four moggies.

By 1975 Marion’s cat population was estimated at about 2 140 and was increasing at a rate of 26 percent a year. They were estimated to be consuming some 450 000 birds of the 12 species of burrowing petrels each year, and one of these species – the common diving petrel – appeared to have been driven to local extinction.

Despite warnings as early as 1952, it was only in 1975 that a decision was taken to eradicate the cats. When the first serious control operation through the deliberate introduction of the highly infectious cat virus Feline panleucopaenia was instituted in March 1977, the population was already 3 400.

By the time the problem was finally resolved some 32 years later, with the death of the last remaining cat in July 1991, the environmental and animal welfare cost had been immense:

l Millions of indigenous seabirds killed, with some populations still not recovered;

l Possibly as many as 1 200-plus cats killed by the virus;

l At least 1 080 cats shot dead, with close on 200 of them having been caught beforehand in one of up to 1 387 traps;

l An unknown number of cats poisoned with the deadly bait 1080 painstakingly placed in the sterilised carcasses of 12 000 day-old chicks and hung from overhangs and in caves where they would not be accessible to seabird scavengers like sheathbills;

l A handful of cats killed during an unsuccessful hunting effort using dogs; and

l A deadly “by-catch” in the traps of just over 350 seabirds, including 149 Salvin’s Prions and 92 Subantarctic Skuas.

“That’s the price we paid, and we thought it was reasonable,” says Professor Marthan Bester of Pretoria University’s Mammal Research Institute who co-ordinated the eradication project.

The human effort was also immense: between 1977 and 1993 some 82 people were involved. They included eight two-man hunting teams who spent up to nine hours a night, and sometimes during the day, tramping the difficult island terrain, often in extreme weather, using 12-bore semi-automatic shotguns and spotlights powered by motorcycle batteries.

The feral cat eradication project on Marion Island has been hailed as, arguably, South Africa’s most successful large-scale conservation achievement, and it’s a project Bester is rightly proud of.

But the effects of the cats are still visible today: although the breeding success rate of some of the burrowing seabird species has recovered, their numbers are nothing like the pre-cat historical incidence.

Now, unfortunately, the inexorable march of human-induced global climate change seems set to continue some of the changes induced by the cats, also affecting the colour, texture and composition of the island’s verdant vegetation quilt. New visitors to Marion will always be entranced, smitten, even overwhelmed – but it may not be because of the “jade jewel” that emerges from the sea to greet them.

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