It is not just individual species that suffer in invasions; the effects of invaders can have repercussions for the wider environment as well.
Changing the landscape
Invasive species can have impacts that affect the whole community of animals, plants and micro-organisms. They can alter soil composition, water flow, nutrient availability, or make an area more susceptible to fires. For example, Chinese mitten crabs (Eriocheir sinensis, Milne-Edwards, 1854), whose population in the British Isles is rapidly increasing, tunnel into soft riverbanks such as that of the river Thames. The burrowing activity of these crabs can cause riverbanks to collapse, damaging earthworks and even the foundations of riverside property.
Knowing no boundaries
Attempts to conserve species and habitats by creating protected areas, such as nature reserves, can be effective at stopping human activities such as habitat destruction. Invaders, however, do not differentiate between protected and non-protected areas, and consequently have become an issue for most protected areas. Lundy Island, off the coast of Devon, is a Site of Special Scientific Interest. It is the only place in the world where the Lundy cabbage (Coincya wrightii,O.E. Schulz, 1936), and two unique insects that depend on it, can be found. The survival of all these species is currently threatened by invasive rhododendron (Rhododendron ponticum, L.), which has to be regularly managed to prevent it from taking over the habitat of this rare cabbage.
The crops and animals we rely on for food are often exotic species, but not all exotic species are welcomed in agriculture. Invaders are a big problem for agriculture and have large economic, as well as environmental, costs. Most aspects of agriculture are affected by invaders, from invasive weeds to the invasive varroa mite (Varroa jacobsoni, Oudemans) affecting honey bees (Apis mellifera). The cost of invasive plants in the British Isles has been estimated at £200-300 million, and the damage caused and cost of control of introduced vertebrates, (mammals birds, fish, reptiles and amphibians), is estimated at £239 million per year. In the USA, the total cost of invaders is estimated at $137 billion per year. These economic costs do not include the consequences of treating invaders with herbicides or poisons, which may also affect native species.
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