We shake our head when we see histories of animals driven to extinction. Why would people so callously kill off an entire species? Then sometimes, we discover there's a invasive, damaging species damaging the ecosystem, and we realize something needs to be done. But can there be a safe way to render something extinct?

The Passenger Pigeon

The bison is the most famous symbol of America's vanished wilderness, but a more apt symbol might be the passenger pigeon. While herds of bison still exist, they are contained within national parks. The pigeons could never be contained. They roamed through America and Canada, from the eastern seaboard to the midwest, in flocks of tens of millions. Seeing a flock of birds coordinate their flight is impressive today. In the 1850s, passenger pigeons would form living waterfalls as their flocks plunged down the slopes of mountains and into valleys. They'd block out the sun as they passed over towns. (They'd also create their own "snow storms," leaving the landscape white, but that's not as romantic.)

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The pigeons were not particularly fast, or smart, or strong. They overcame their predators and their competitors by sheer numbers. Whenever an overly-large crop of nuts or grains emerged, the passenger pigeons descended on it, eating so much and so quickly that omnivores or herbivores realized that the food stock wasn't worth fighting a flock of millions. Following the logic of the cicada, the pigeons nested in such numbers that carnivores glutted themselves, leaving most of the hundred-million birds in a nesting area to breed and raise their chicks in peace.

The pigeon's evolutionary strategy has come to be called "predator satiation," and at first humans were simply one more predator to be satiated. As long as the pigeons didn't eat up all their grain, people were happy to see them. Given a large enough flock of passenger pigeons, people could literally swat their dinner out of the sky.

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Then came railroads and sportsmen, and the predators started following the pigeons around. Farmers also, understandably, were sick of the threats to their grain and would try to get rid of the pigeons any way they could. Trees containing hundreds of nests were blown up. Flocks were netted. Grain was poisoned. Millions of birds were blasted out of the sky on a single day. Nobody was worried, because such an abundant species could never be wiped out, until suddenly only two breeding flocks were left. Both failed. The last passenger pigeon died in 1914.

The Wrong Way To Exterminate a Species

Similar forces nearly drove the American bison to extinction. It became profitable to kill off the animals, and so massive amounts of people turned their hand to it. But more than that, it was popular. Killing off populations was a sport, and for pest species a good deed. Railroads made it possible for an seasonal job to turn into a full-time occupation. The keys to extinction, then, are popular support and accessibility.

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Today, governments the world over are trying to recreate those forces in the name of conservation. Invasive species have overrun many of the world's ecosystems, and only killing off the invaders can save native species. This is not a pretty job, which means it will generally be hard to secure the first necessary component needed to kill off a species ‚ÄĒ namely, popular public support.

Public financial incentives are a great way to kill off large, hard-to-handle, and slow-to mature species like bears, bison, or deer, but not invasive species. Invasive species are good breeders, adaptable, and, in general, small. In the wild, they will give people a run for their money, hiding in inaccessible places and avoiding contact. On the other hand, they will be generally very easy to breed and raise. If killing off the animals is a simple business exchange, people become farmers of invasive species instead of exterminators of them.

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This is so common a mistake that it has been named The Cobra Effect, based on the time the British began offering rewards for dead cobras in Delhi, and locals began breeding cobras. Later, the French offered rewards for rats in Vietnam, and people bred rats. In both cases, the population of the problem species increased. People need to be on board with the goals of the extermination movement, not just its reward structure.

This can be tough to arrange, especially since people grow fond of some invasive species. When the Hebrides attempted to exterminate hedgehogs, locals resisted. The government needed to implement a capture, transport, and release program to get public support.

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Some invasive species have their own claim on the conservation movement. Exterminating non-native species from the Channel Islands National Park, a string of islands off the coast of southern California which had been used for farming, conservationists encountered protest when they began killing off the relatively cute invaders, like wild pigs and goats. Although no argument could be made for the pigs, the goats on San Clemente island had become a heritage breed - a breed that deserved conservation. The United States Navy, which was overseeing the extermination, delivered a breeding population to conservationists. Today you'll often see San Clemente goats on specialty farms and at petting zoos. Although they aren't an endangered species, their population is overseen by organizations that preserve rare breeds of domestic animals.

The Right Way to Exterminate a Species

Hokey as it sounds, one of the most important tools to exterminate some species and conserve others is a thorough understanding of the situation. Early attempts at repelling invasive species and saving endangered native species failed because people were just taking wild stabs in the dark. Even today, it's hard to know exactly what is driving a decline in a population of animals.

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When the Channel Islands National Park was first established, conservationists were pleased with the rapid recovery of the native island foxes. After a few years of recovery, their numbers began plummeting until one of the islands had a population of only 15 foxes. Desperately worried, researchers housed the foxes in shelters and went looking for the culprit. They found that the main predator of the island foxes was the golden eagle, an eagle that should be staying on the mainland. Why had it come out over the ocean?

The golden eagle had moved in on the islands in part because the bald eagle, its numbers dropping due to the egg-thinning effects of pesticides, had moved out. Meanwhile, a group of pigs introduced by farmers had gone feral and their numbers had exploded. Piglets were a good meal for the golden eagle, but if a fox was in range they would happily eat it instead. The bald eagle was a fish-eater and left the foxes alone. To save the fox, the researchers needed to reintroduce the bald eagle and kill off the pigs.

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Once the problem is identified, the nasty side of conservation-by-extermination begins. Railroads don't help much today, when it comes to access to the species to be eliminated. What gives people access to these species are dogs and helicopters. Dogs have been indispensable to hunters for millennia. Even if they aren't used directly for hunting, they are able to give hunters a good idea of where populations of the invasive species are centered, and where traps would be most effective. Helicopters are fast and can move over any terrain. They can fly low over an area, hover, and allow shooters to pick off animals from above. They also let researchers drop poison bait with perfect precision, making sure that every square meter of a territory has its own measure of poison.

When hunting, whether from a helicopter or not, the trick is getting the animals to come out in the open. In order to rid the Galapagos of an invasive goat population which was killing off all the local flora, the researchers captured a female goat. Coating her in pheromones, they sent her out in open areas, then killed every male that came near her. The female they spared only so they could recapture her and repeat the procedure.

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For the most famous invader of all, bullets don't work. People have spent a long time trying to get rid of rats. Cats work, but it's rarely a good idea to introduce one invasive species in order to kill off another. The second-favorite method of rat control is poison. When conservationists started using poison, they ran up against two problems - the rats' intelligence and the rats' social structure. Rats are smart and although they can live alone, they always keep an eye on each other. When one rat dies writhing in agony and spewing blood after eating some poison bait, other rats know it. Researchers had to come up with a poison that would kill rats stealthily, so that neither the poisoned rat nor its companions caught on.

What they came up with was an anticoagulant. Used to save people prone to blood clots, anticoagulants keep bloody wounds from healing. Unless taken in precise doses when absolutely necessary, they are very dangerous. Fortunately, most of them can be counteracted by a vitamin K shot ‚ÄĒ so if any small child were to eat poison bait, they could be treated. Anticoagulants provided researchers with a perfect poison. Rats ate the bait, and felt fine. They felt so fine that they often hoarded more of the bait. Slowly, over time, they sickened and died without ever connecting their condition to the poison. Any rats nearby moved in on their leftover stash of bait and ate it themselves. Whole islands have been wiped clean of rats this way.

If there is a practical problem with anticoagulants, it's that rats might adapt to the doses. The first version of this poison, a drug called warfarin, was very successful until the rats managed to adapt to the dose given them. Researchers had to create a much stronger version of the drug, called brodifacoum, in order to get the job done.

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Some people have come to believe there's a moral problem with anticoagulant poison. It is an ugly, slow way to die. The rats bleed from many tiny wounds, internal and external. While only a few people object to this when it comes to rats, many more protest when the poison is used on hedgehogs, rabbits, and possums. While most people acknowledge the need to stamp out invasive species in fragile ecosystems, many see poison as cruel. In New Zealand, conservationists are looking into chemical birth control as a method of gradually ridding the island of invasive species like the possum. The problem is, this would mean injecting millions of possums with a chemical that may or may not work. Some researchers are considering introducing a special nematode worm, which renders possums sterile, but that would go back to the old game of introducing one foreign species to kill off another foreign species.

Many people also want modifications to traps. While some trappers prefer the traditional leg-catcher traps, the traps are painful and do not give the animals a quick death. Some trappers are looking into faster traps, using pressurized carbon dioxide, which would kill their occupants immediately.

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Again and again it comes down to getting the public on the side of conservationists, which can be tough. Everyone now knows that cats devastate an ecosystem, but people love cats. In fact, New Zealand, a country that leads the world in such exterminations, has many of its current invasive species due to the 1861 Animal Acclimatization Act. Designed to make new transplants more at home, the act allowed the introduction of comforting species from the person's homeland, which is why New Zealand had hedgehogs, cats, goats, and other problem species. It's likely that cats will be the last species to leave. They're beloved pets. Meanwhile, people are cautioned not to take native species into their home because "they're not pets." Perhaps one of the ways to get invasive species out of the way is to allow people to develop a fondness for domesticated native species.

Rat Campaign: Campaign Against Rats (FDA 188), Cobra Image: Patrick Jean, Passenger Pigeon Image: John James Audobon, Goat Image: Cliff, Fox Image: National Park Service, Rat Image: Chris Barber, Possum Image: JJ Harrison.

[Sources: Why the Passenger Pigeon Went Extinct, Buffalo Tales, Skirting Death, The Curious Case of the Kiwi Hedgehog, A Review of the Potential of Fertility Control to Manage Brushtail Possums in New Zealand, Rat Island, In New Zealand Stoats are On the Most-Wanted List, When Conservation Means Killing.]

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