Alien Invaders Most Destructive Species of all Time

Hundreds of thousands of organisms have been transported around the world by humans, making us the most destructive species of all. The majority fail to escape into the wild, but some go on to establish populations. While a lot of these species don’t cause much trouble, the few that do are generally referred to as ‘invasive’, and can wipe out native creatures. Here are some of the most destructive ‘aliens’ that are wreaking havoc around the globe.

BROWN TREE SNAKE

Native to Australia and New Guinea, the brown tree snake arrived on the Pacific island of Guam as a stowaway in military cargo in the late 1940s. As the snake spread, it wiped out the island’s birds. Ten forest birds are now extinct and the remaining species are very rare. The snakes also frequently short-out power lines, causing power cuts. But the main economic cost of the snake falls on Guam’s two main sources of income: tourism and the military. The presence of a large venomous snake doesn’t exactly encourage visitors, and clearly birdwatchers needn’t bother visiting. The island is also the site of a US military base and the staff have their hands full preventing the snake from repeating its original stowaway trick. It could hitch a ride to other islands, such as Hawaii, where it is estimated it could cost US$1.7 billion a year in power cuts alone, plus all the other environmental damage. An attempt to control them with poisoned mice is underway, but it’s too early to know if it will be successful.

THE ROSY WOLFSNAIL

Put on islands where they shouldn’t be, predators often cause mayhem, especially if nothing there wants to eat them. Florida’s Euglandina, or the rosy wolfsnail, is such a beast. It was released on many Pacific islands to control the giant African snail, which was itself introduced, and became a problem. But the cure turned out to be much worse than the disease. The problem is that Euglandina isn’t very keen on eating the giant African snail, but it’s an extremely effective predator of smaller snails. To make matters worse, the Pacific islands support (or used to support) a staggering diversity of snails: 931 species in the Hawaiian archipelago alone. Everywhere Euglandina has been introduced, most of the native snails are now extinct. Experience shows that, given enough money and commitment, eradicating introduced predators from islands is possible. But Euglandina is just too abundant, on far too many islands, for there to be any realistic prospect of eradicating this devastating species

CANE TOAD

Having tackled the prickly pear cactus with an Argentinean moth, Australia was in the mood to try other control organisms in 1935. The South American cane toad looked like a good bet. It had (apparently) been successful at controlling cane beetles in Hawaii, a major pest of sugar cane. Unfortunately, cane toads had no effect on cane beetles in Australia, but did eat nearly everything else they came across. This is bad enough, but they soon caused other problems. The toads secrete toxins that are deadly to predators, and in Australia they have been responsible for declines in native reptiles, which are killed when they try to eat the pest. The prospects for ridding Australia of them
looks bleak, but in the longer term, natural selection should come to our aid. Native Australian predators, from birds to ants, are figuring out how to eat cane toads, while native reptiles are evolving to avoid eating them and also resistance to the toxin. One snake has even evolved a smaller head, making it less likely to attempt to munch on larger toads

WATER HYACINTH

Originally from South America, but now worldwide, water hyacinth was widely introduced as an ornamental plant and is indeed very attractive, with large, purplish-blue flowers. But it is notoriously difficult to prevent aquatic plants from escaping into the wild and spreading through river systems. Water hyacinth grows rapidly and forms dense mats. These clog irrigation channels and intakes for hydroelectric plants and power station cooling water. It also puts native submerged plant species in the shade and reduces dissolved oxygen levels, harming fish. Mechanical control is expensive and herbicides undesirable, so the plant has been a target for biological control. Two moths and
two weevils have been introduced to tackle it and have certainly had an effect, but haven’t won the war. Interestingly, water hyacinth’s dried, woven leaves, which are tough yet flexible, now form the basis of a thriving furniture industry. One factory in Thailand employs over 1,000 local villagers to harvest the plant from waterways.

DROMEDARY

The first four aliens here are all on the Global Invasive Species Database of the world’s 100 worst invasive species. Camels aren’t, but there are many Australians who think they should be. Large numbers of them were imported Down Under in the 19th Century, and then released when motorised transport arrived. There are now about 1 million, accused of causing soil erosion and damaging livestock watering stations. The problem is that without a predator, the population is out of control. Ironically, the dingo could do the job, if only Australians would stop persecuting the wild dog for its habit of eating sheep. The final irony is that dingoes do help to control both foxes and cats, which are on the list of the world’s 100 worst invasive species.