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Intentionally or not, humans help invasive species spread, devastating unsuspecting ecosystems. If the damage is bad enough, drastic measures are required to restore balance in these places.

This week, New Zealand announced an aggressive plan to rid the country of rats, possums and stoats (a kind of weasel) by 2050. According to prime minister John Key, ever year these invasive predators kill millions of native species, mainly birds, and cost the economy NZ$3.3 billion (US$2.3 billion) annually.

The government will invest NZ$28 million to create a new company, Predator Free New Zealand, that will lead the cull. Hunting, setting traps, and spreading poison drops from the air have been used to control invasive species in the country in the past, eliminating predators from some smaller islands.

Can it be done on a much larger scale? Others have tried, with mixed success.

Goats in the Galapagos

Image: AP Photo/Reed Saxon

The problem
In the 1990s one of most pristine places on the planet developed a serious goat problem. With more than 100,000 goats eating everything in sight, native species like the endangered Galapagos tortoise were struggling to survive.

How did they get there?
Most likely, pirates and whalers brought goats to the Galapagos islands a few centuries ago. The islands served as a perfect pit stop on their routes, a place to fill up their fresh water supply and trap tortoises for food. They probably dropped off goats to graze on the land, so there would be something else to eat on their way back, by which time the sailors were sick of tortoise meat.

How to get rid of them?
The Isabella project was launched in 1997. The plan was simple. Goats are herd animals, meaning they like to gather together. Thus, you can chase them with a helicopter, let them group up, and get snipers to mow them down. But this only got about 90% of the goats, since they quickly linked the sound of propellers with imminent death. Then came the Judas Goats. Sterilized female goats were fitted with radio locators and scented with long-lasting hormones that made it seem like they were permanently in heat. This false promise lured previously hard-to-catch goats to their deaths. Some 250,000 goats were culled during seven-year project, with pretty much only the Judas Goats now left.

Lionfish in the Atlantic

Eat them to beat them.
Image: AP Photo/Michael Dwyer

The problem
Without predators, the venomous, ravenous lionfish—normally found in the Indo-Pacific—eats everything it can around coral reefs.

How did they get there?
Theories vary, but most likely over the past few decades aquarium owners dumped them the ocean when they wanted a change.

How to get rid of them?
Eat them. The flashy appearance that warns predators of their venomous nature also makes lionfish a rather striking additions to dinner plates. Campaigners have been promoting the concept of invasivorism (eating invasive species) to address the spread of invasive species, helping to reduce, but probably not eliminate, rapidly spreading invaders like lionfish.

Cane toads in Australia

A hopping great problem.
Image: AP Photo/Brian Cassey

The problem 
This giant tropical toad, normally found in South-America, spread from coast to coast in Australia, poisoning anything trying to take a bite out of it. Today, much local fauna is overwhelmed by the more than 200 million cane toads that are hopping around.

How did they get there?

A small batch of toads were imported by the Department of Agriculture in 1935 to get rid of sugar-cane eating beetles. Insecticides at the time were too destructive on the environment, so cane toads were meant be an eco-friendly solution. The plan backfired.

How to get rid of them?
The battle is still raging, with the methods getting more and more desperate. Australians have tried to deny their access to water in semi-arid areas by putting up fences. Another method takes a special chemical from adult cane toads’ toxins to attract and trap curious tadpoles. For future reference, once caught, scientists say the most humane way to kill the toads is to pop them in the freezer.