Perhaps the biggest problem with invasive species is that by the time they are recognised as invasive, the species tend to be so well established that it can be very difficult to eliminate them.
Eradicating an entire population is not easy. Invaders are usually spread across huge areas, and are intermingled with native species. Plants and fungi may also have seed or spores left in the soil, or fragments that may be able to grow into new individuals. It is not surprising then, that very few invader eradication programmes have been successful. Without suitable scientific information, eradication programmes are unlikely to work, as it is vital to understand the ecology and behaviour of the organism you are trying to eradicate.
Coypu – a rare success story
Coypu (Myocaster coypus, Molina, 1782), beaver-like rodents from South America, were introduced to the British Isles in the 1920s and farmed for their fur. By the 1960s, a large wild population had become established from escapees. Mainly found in East Anglia, this increasing population caused considerable damage to wetlands as they dug into banks and ditches. Around 100,000 individuals were trapped between 1961 and 1962 but this barely dented the growing population.
After several unsuccessful efforts at eliminating the coypu, a strategy group was established in 1977. Scientists created a simulation model based on the coypu’s biology and environmental conditions of the invaded area. This enabled them to predict how much effort they needed to put into trapping coypu to reduce the population, and how many years of trapping would be needed in order to eradicate this species. The project was a success, and by 1989 only 1 coypu was caught. The programme took 10 years to complete and cost an estimated £2.5 million.
Prevention is better than cure
Given the cost, effort, and difficulty of eliminating invader species it is clear that stopping species that could become invaders from becoming established in a new territory is an important priority. The need to stop invaders has become the focus of legislation throughout the world. Some countries, such as New Zealand, have strict laws on which species can be imported as a way of protecting their native species from new invaders.
After the introduction of numerous invaders from ballast water, including zebra mussels (Dreissena polymorpha, Pallas, 1771) and Chinese mitten crabs (Eriocheir sinensis, Milne-Edwards, 1854), a series of guidelines have been produced to prevent transport of organisms in ballast water. Ships now choose carefully where and when they pick up water to reduce the number of species they may collect. They also exchange the ballast water picked up from coastal areas in mid-ocean; coastal species cannot survive well in the open ocean and vice versa. Ports have designated areas where ballast water can be deposited and may have treatment facilities such as ultraviolet screening to kill any exotic species present. Ports also conduct regular surveys so they can attempt to eliminate any exotic species that do manage to establish.
Next: Living with invaders