It was 1955 and it was only a single rat.
But then in 1964, it was lots of rats. Muttonbirders returning to Big South Cape Island found their huts infested. The rodents had got into the bedding, they’d chewed the wallpaper off the walls, evidence of their presence was everywhere.
By winter’s end, the island was devoid of life: plants were stripped bare, two bird species were gone, the forest floor was naked of seedlings. Rats had arrived and left a destructive trail in their wake.
Big South Cape Island/Taukihepa, the biggest of the Big South Cape group, sits just 1.5 kilometres off the southwestern coast of Rakiura/Stewart Island. It’s one of the Tītī islands where Rakiura Māori have rights to gather muttonbirds in perpetuity.
The scale and speed at which the rats invaded was detailed by Elizabeth and Brian Bell (with Don Merton posthumously credited) in the rodent eradication special issue of the New Zealand Journal of Ecology.
That very first rat that was trapped in 1955 might’ve been a one-off. Visits in 1956 and 1961 did not detect any more of the rodents. But when the muttonbird families arrived back on the island for their collecting season in February 1964, their huts at Murderer’s Cove had all the hallmarks of a rat invasion: wallpaper was stripped, food and supplies had been eaten or damaged and rat droppings and urine covered the beds and floors.
An island stripped bare
In March, poison was placed around the huts but it was too late. By September, the impacts were evident. Robins and fernbirds were gone from the island while saddlebacks and bellbirds were restricted to the southern end of the island.
And it wasn’t just the birds. The vegetation was showing signs of damage, with much being completely stripped, especially puriri and five-finger, and a complete absence of seeds and seedlings throughout the forest.
Wild South visited Big South Cape to produced the 1988 episode Island eaten by rats. One of the muttonbirders interviewed recalled the aftermath of the invasion:
One of the great things I think down there was the naturalness of the place. No vermin of any kind…it was really a little paradise on its own. And that’s the biggest effect it had on it was the number of birds we had there and after the rats came the place was empty and we’ve still got that feeling of loss now. What we had and what it is today…people just took it for granted, we didn’t realise what we had.
The effect on the birdlife was “shocking”, he said. Wrens “just disappeared overnight”; South Island saddlebacks used to nest inside like starlings.
The whole place was infested from one end of the island to the other.
Why Big South Cape was important
Led by Brian Bell, New Zealand scientists wasted no time leaping into action when the extent of the Big South Cape invasion became apparent.
It was the last haven for South Island saddlebacks, Philesturnus carunculatus, the beautifully-wattled bird with a special juvenile form – distinct from its North Island counterpart (Philesturnus rufusater). Thirty-six birds were caught and taken to safe refuge on nearby Big and Kaimohu islands, marking the first time translocation successfully rescued an endangered species anywhere in the world. The birds are now spread over many islands and are no longer considered endangered.
But witnessing a rat invasion in progress kicked off a new phase of conservation in New Zealand: island eradications.
It seems so simple now – rats and other introduced mammals caused waves of extinctions across New Zealand and other islands when they arrived, whether by predating birds or eggs, competing for food destroying habitat. But that wasn’t common knowledge when the rats arrived on Big South Cape.
Ecological understanding at the time was driven by continental science – the biggest ecological changes were driven by habitat loss. It wasn’t apparent that predation and competition by rodents could drive species to extinction, because much of the damage had already occurred before ecologists were watching.
Just a few years earlier, in 1959, Norway rats were eradicated from Ruapuke/Maria Island, marking the first rodent eradication in New Zealand. We were right on the cusp of figuring out how to fight back against the biggest drivers of ecological devastation across our archipelago, using tools we still use today.
Where did the rats come from?
But the story doesn’t end there. New research led by Judith Robins at the University of Auckland, and also published in the New Zealand Journal of Ecology special edition, aimed to answer the question: where did the rats come from?
You might not consider it so, but 1.5 kilometres is a manageable distance for a Norway rat to swim, though generally thought to be too far for a ship rat’s swimming abilities (that possibility still can’t be quite ruled out).
It’s far more likely that the rats arrived by boat.
Fortunately, historic samples of rats caught just after the Big South Cape invasion sit in storage at Te Papa, and Robins and her colleagues were able to look at a region of those rats’ mitochondrial DNA in comparison with other rats from the lower South Island and Stewart Island.
This particular segment of mitochrondrial DNA (the D-loop) bundles into clear haplotypes in the region. There’s a haplotype that’s common on Stewart Island, another common in the lower South Island.
If the Big South Cape rats had swum to the island, you’d expect them to be from Stewart Island stock. But they weren’t. Interestingly, they also weren’t from the group found around Bluff (a common port for muttonbirders to depart the mainland), nor from Riverton.
Robins et al. concluded the Big South Cape rats were most likely to have come from Fiordland or Port Chalmers/Dunedin, though the relevant haplotype is wide-spread so, in reality, any visiting boat could have arrived bearing rodent stowaways.
Big South Cape now
Eradication for Big South Cape didn’t occur until 2006.
In 1988, the tanker Command spilt 3,000 gallons of oil on the Californian coast, killing thousands of seabirds including an estimated 32,000 tītī. One banded bird was found on Whenua Hou/Codfish Island which allowed Rakiura Māori, supported by the University of Otago, to apply for mitigation funding made available through the Command Oil Spill Trustee Council.
Now, a campaign aims to keep the Tītī islands rat-free, but Big South Cape remains a poignant chapter in New Zealand’s conservation history. As Elizabeth and Brian Bell wrote:
Recognised as one of the worst ecological disasters in New Zealand history, we consider that the Big South Cape rat irruption changed the way many people thought about rats and their impact on native species.
What happened on Big South Cape could well be what happened throughout New Zealand when humans arrived, bringing their rodent cargo along with them. It’s just that on this occasion there were witnesses ready to intervene.
Featured image: Flickr CC, a saddleback on Tiritiri Matangi Island, Duncan.