Meddling With Nature Can Create Ecological Disasters

by Tony Orman

Macquarie Island  halfway between Australia and Antarctica is a prime example of Man meddling with Nature and causing ecological disasters.  In 1985, Australian scientists set about a plan to eradicate wild cats that had been introduced in the early 19th century. The basis for extermination was the cats were preying on native burrowing birds.

But the grand plan turned into a nightmare. Twenty-four years later, a team of scientists concluded the cat removal unexpectedly wreaked havoc on the island ecosystem.

With the cats gone, the rabbits began to breed, exploded in numbers and decimated the native plants. The lessons were in history.

Seal hunters had introduced rabbits to Macquarie Islands in 1878. Rabbit numbers were excessive  estimated at 100,000, so a rabbit virus was introduced and the rabbit number dropped to 20,000.

But the cats which had preyed largely on rabbits, “switched prey” and began eating seabirds.

The earlier rabbit population explosion had destroyed the grassy areas on coastal hillsides and exotic grasses and herbs took over, forming a dense network that prevented native sea birds from accessing nesting sites.

An article in the New York Times in 2009 by Elizabeth Svoboda said “the Macquarie debacle is not an isolated incident; several other species removal programs have inflicted damage on surrounding ecosystems. In New Zealand, conservationists decided to wipe out three introduced species in one go –  rats, possums and stoats by poisoning the first “two.”

The rationale was that the poisoning operation would eliminate stoat populations by association because rats were a critical part of the stoats’ diet. But when the plan was begun in the early 1990s, the stoats did not disappear. With the absence of rats, the stoats preyed on native birds and bird eggs.”

Prey Switch

It was the classic “prey switch” just as happened on Macquarie Island.

Sudden removal of one species disrupts the inter-related predator prey network that makes up the food chain.

While “prey switching” is a theoretically accepted phenomenon, fully proven examples (such as Macquarie Island) are actually uncommon.  Predator-prey relationships can be notoriously difficult to measure.

However, the fervour and haste which the Department of Conservation and local councils applies with toxins is reckless and fraught with ecological repercussions and problems.

Large scale poisoning with eco-toxins such as 1080 and brodifacoum may heavily reduce predator numbers initially but with a few short years, the outcome is disastrous. The science is there to show the resurgence in predator numbers and subsequent wrecking of the food chain.  

Wendy Ruscoe in a study published in Landcare Research’s publication 2008 showed aerial dropping of 1080 will temporarily knock back a rat population but due to the rodent’s amazing reproductive capacity, the surviving rats – perhaps 10 to 20percent – recover rapidly and within 18 months, are two to three times greater than before poisoning began.

A 2007 study by Landcare scientists Graham Nugent and Peter Sweetapple showed rat numbers recovered within 18 months and at the two year mark, rat abundance could be four times greater than before poisoning.

Rats are amazing breeders. At just nine weeks of age, a young female can become pregnant and bear a litter.

 “A female rat typically births six litters a year consisting of up to 12 rat pups, although 5 to 10 pups are more common. Rats reach sexual maturity after nine weeks, meaning that a population can swell from two rats to around 1,250 in one year, withy the potential to grow exponentially.”

Stoat Prey on Rats

The disruption to the native ecosystem ripples further.  A major prey for stoats is rats.  When rat numbers are reduced by 80% – 90%, the stoat deprived of its major food source, invariably switches prey to birds. But later as rat numbers surge and boom and pass original numbers, stoats enjoy a virtual banquet of rats and their breeding rate increases in response. 

Numbers surge and then explode.

The well intentioned but ignorant predator extermination programme usually using 1080, has merely stimulated, within a few short years, major population explosions of rats and stoats. 

Attempting to poison-away rodent surges in beech-mast years is the ecological equivalent of farting against thunder. All this does (if anything) is delay the inevitable, as the fast-breeding ability of rodents will eventually allow population growth to match the food supply. 

Rather than benefiting the birds and overall ecological health, there is massive ecological disruption by the man-induced mega rat and stoat plagues

Ecological Damage

That is not counting the birds killed directly or by secondary poisoning scavenging on toxic carcasses. 

Nor is it accounting for the loss of insects and other invertebrate organisms killed by 1080. 

The poison 1080 was originally patented as an insecticide in 1927.

Research by DSIR scientists Mike Meads and Peter Notman,in the 1980’s,  warned of the vital loss of organisms and resulting ecological damage. 

Examples are many of human interference directly or indirectly into Nature’s food chains resulting in profound consequences. In a classic 1966 experiment, biologist Robert Paine removed the purple seastar, Pisaster ochraceus — a voracious mussel-feeder — from an area of coastline in Washington state. Their predator gone, mussels exploded in numbers, crowding out biodiverse kelp communities with monoculture.

Less than a decade after Pisaster, marine ecologists James Estes and John Palmisano reached the astonishing and widely reported conclusion that hunting of sea otters had caused the collapse of kelp forests around the Aleutian Islands. With otters reduced to low levels, the prey (sea urchins) stripped the kelp forests. 

When otters eventually returned, they regulated urchins, allowing “luxuriant” regrowth of biodiverse kelp communities.

Toheroa Decline

In New Zealand, the decline of the toheroa shellfish was attributed unofficially to heavy over-fishing of snapper which preyed on paddle crabs which in turn preyed on toheroa. With the heavy decline in snapper, paddle crabs proliferated and almost obliterated toheroas.

New Zealand has a long history of an obsession with attempted extermination of predators. In the 1950s acclimatisation societies managing trout fisheries blamed freshwater eels and shags for perceived declines in trout numbers. Bounties were paid out on eels. It had little effect. Ironically the best trout fishing rivers had healthy populations of both trout and eels. Eels simply removed the sick, the old or the unwary thus making for a quality trout population.

The concept of being ”predator free” or “zero predators” has no ecological justification, except in limited circumstances on smaller offshore islands and “mainland islands” . Even in islands where predators may have been eliminated e.g. Secretary Island in Fiordland, the success is short-lived and temporary as animals can and do swim from the mainland to recolonise.

Sadly the Department of Conservation appears to have little appreciation of the unintended consequences of eradication campaigns. Currently it is conducting massive 1080 poison drops particularly in the South Island and in areas of high tourism and recreational values such as the Milford and Heaphy Tracks.

Every Five Years

The time patterns are obvious. The department is finding excessively high rat numbers about four or five years after an earlier 1080 aerial drop.

Remember that 2007 Landcare Research research on rat numbers following a 1080 drop?

To recall that – aerial dropping of 1080 will temporarily knock back a rat population but due to the rodent’s amazing reproductive capacity, the surviving rats recover rapidly and within 18 months, are two to three times greater than before poisoning began.

So the ill conceived programme continues and every five years for so after 1080, rat numbers will boom and soon after stoat numbers will soar.

It seem incomprehensible that an agency such as the Department of Conservation and the Predator Free 2050 and ZIP concepts should go unquestioned in the light of the international understanding of the dangers of playing God with the ecosystem’s food chains.

Footnote: Tony Orman is a conservationist with involvement going back to the Save Manapouri campaign in the 1970s. He describes himself as a ‘rational greenie’ who doesn’t vote for the Green Party.