What is a stoat? If you don’t know, not to worry. I had never heard of them either until I came here. A stoat (Mustela erminea) is a smallish, about 15 inches long including the tail, predator animal in the Mustelid family, a group that also includes ferrets and weasels. In fact, the stoat is sometimes referred to as the short-tailed weasel.
If you read a previous post, you remember that the possum is considered Public Enemy Number One for New Zealand vegetation. Well, the stoat is Public Enemy Number One for New Zealand birds including New Zealand’s iconic national bird, the kiwi. Reading about the devastation this introduced species, the stoat, has caused on the native bird population here particularly the kiwi can only be described as heart-wrenching.
The stoat is not a native species to New Zealand. In fact, there are no ground mammal species that are native to New Zealand. During its long 80 million year unfolding, New Zealand evolved without ground mammals with most of the ecological niches commonly filled by them being filled by birds, amphibians and reptiles.
The stoat was introduced into New Zealand in the 1880’s from Europe to control the rabbit population. But you just said there were no ground mammal species here. Then how did the rabbit get here? Well, the rabbit was also introduced here but got out of control.
If this is beginning to sound like the convoluted story in some children’s book, then you’re right. Rabbits were introduced into New Zealand for food and fur but got out of control eating all the plants so that the sheep and cows (also both introduced) had nothing to eat. Then stoats were brought in to control the rabbits but the stoats got out of control eating all the birds . . .
One ecologist described the introduction of the stoat to control the rabbit as trying to “correct a blunder by a crime.” In any case, stoats are now found throughout all of New Zealand. They have long bodies and are dark brown with whitish underparts and a bushy black tail. They have short round ears and black eyes. Despite living only about 1.5 years in the wild, they are extremely fertile. The females are pregnant by the time they leave the nest where they were born, and although they usually only have one litter in a lifetime, they can carry up to 12 babies at a time with 6-8 being more common.
Sadly, within only a few short years after the introduction of the stoat, along with his buddies, the ferret and the weasel, drastic declines in bird populations were noted. New Zealand birds are particularly vulnerable since many of them can’t fly and build their nests on the forest floor. The eggs and juvenile birds are particularly at risk. Some species of kiwi, for example, only lay one egg a year, such that a predator like the stoat with a far greater reproductive rate, can threaten their long-term survival. Stoats are implicated in the actual extinction of some of the indigenous bird species in New Zealand, and are the major cause of decline of many others.
Stoats hunt day and night. As an added note, they have heart rates of around 400 beats per minute and reparatory rates of 100 breaths per minute. These hyper-active machines kill more than they can eat. They have been known to swim up to 2 kilometers across open water.
As such it doesn’t take many stoats to cause substantial devastation to bird life.
The kiwi, the flightless bird endemic to New Zealand and its national symbol, was and is particularly affected. For example, on the North Island, it is estimated that stoats kill approximately 15,000 brown kiwi chicks each year. This accounts for 60% of those born each year. Other predators including ferrets, dogs and cats kill another 35% resulting in only 5% of kiwi chicks born each year surviving.
During certain years, seasonal increases (called mast years) in the amount of seeds beech trees in New Zealand produce causes huge increases in the numbers of rats and mice (both also introduced) which stoats feed on and this ramps up the reproduction rate of stoats. Once the rodent population is reduced, the stoats increasingly turn to birds with devastating consequences. For example, a mast year in 2006 allowed the stoats to wipe out more than half of the population of takahe, a native bird, in one area.
In areas where birds are particularly at risk, poisoning and stoat trapping (killing) is often implemented. Poison pellets, commonly a formulation called 1080 (sodium fluoroacetate), which is felt to be safe for the environment although some disagree, are often commonly dropped from the air. A stoat trap consists of a long wooden box with a small entrance at one end, usually an egg for bait and a springed trap like a mouse trap which kills the stoat when he contacts it.The effectiveness of these methods vary, and the Department of Conservation is constantly evaluating new methods of controlling the stoat pest.
Stoat trapping commonly occurs in forest reserve areas and national parks with the traps commonly numbered and placed several meters off the tracks (trails). When we hiked the Kepler Track, one of New Zealand’s Great Walks on the South Island, the ranger at one point advised us that the distance from where we were to the next hut was 135 stoat traps. Each trap was commonly 100-200 meters apart, and you could keep track of the distance as you hiked by counting off the stoat trap numbers pinned to trees.
For a while, stoat trap distances even became a convenient way for Rebecca and me to measure distance threatening to supersede the use of yards, meters, city blocks or even miles.
If the grocery store were several blocks away, Rebecca might say to me, “It’s two stoat traps away.”
“Thank you,” I’d reply.
Finally, for completion, here is a list of all the mammals that have been introduced into New Zealand: possum, rat, hedgehog, cat, rabbit, chamois, deer, sheep, cow, pig, ferret, goat, dog, mouse, tahr, wallaby, weasel and stoat.