You only have to look out of your window to see alien invaders, but before you start to search the skies for spaceships, look in your garden, your nearest park, or on the banks of your local river. It is here you are likely to find the invaders, species of exotic plants and animals that have arrived as stowaways or escaped from gardens and farms, and are now causing problems both for humans and for wildlife.
Hundreds of exotic plants and animals have been introduced to Britain as crops, livestock, pets or garden plants. It is estimated that 55,000 varieties of exotic garden plant alone are available in the British Isles. The majority of introduced species are harmless, but a few species have established themselves, and are spreading across the countryside. These invaders may outcompete native species for resources or even eat them. In some cases invaders seriously disrupt ecosystems and can cause drastic changes the landscape.
It is not just the British Isles being changed by exotic species, the problem of invaders is global, and it has been going on for centuries. When people travelled, they carried familiar plants and animals with them, and brought exotic animals and plants back home. As travel has become easier, the opportunity for species to spread has grown, and with it the opportunities for invasions.
Although only a few species cause problems, the effects these troublesome few have on native habitats can be devastating. Their effects can be even more serious in habitats already damaged by other human activities. The United Nations Environment Programme (UNEP) has identified invaders as the second largest threat to global biodiversity, second only to habitat destruction.
Himalayan balsam, it may look beautiful but it costs millions of pounds to control this invader in the British Isles.
Invasive species are also proving to be an expensive problem. The Environment Agency estimates that the cost of controlling Himalayan balsam (also known as Indian Balsam or Policeman's Helmet, Royle), just one invader in the British Isles, is between 150 and 300 million pounds. This is just a fraction of the global cost of invasive species, running to hundreds of billions of pounds per year. Stopping these invaders is now a major challenge for scientists across the world.
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The cost of invaders