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| - Invasive non-native species pose a very serious threat to native plants and animals throughout Great Britain, and have been identified as the second biggest threat to biodiversity worldwide. Many species have been introduced into England from other parts of the world. They include some of our most widespread and familiar plants and animals, such as sycamore, rhododendron, grey squirrel, pheasant and Canada goose. In urban situations, these may offer people valuable contact with nature, as with buddleia which is welcomed as a shrub with colourful flowers providing nectar attractive to native butterflies. These introduced species now form part of England’s biodiversity and have spread into our most isolated areas, so that there is unlikely to be a single SSSI without at least one.
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| abstract
| - Invasive non-native species pose a very serious threat to native plants and animals throughout Great Britain, and have been identified as the second biggest threat to biodiversity worldwide. Many species have been introduced into England from other parts of the world. They include some of our most widespread and familiar plants and animals, such as sycamore, rhododendron, grey squirrel, pheasant and Canada goose. In urban situations, these may offer people valuable contact with nature, as with buddleia which is welcomed as a shrub with colourful flowers providing nectar attractive to native butterflies. These introduced species now form part of England’s biodiversity and have spread into our most isolated areas, so that there is unlikely to be a single SSSI without at least one. Non-native species are introduced in many different ways, both accidentally and deliberately. Early examples include poppies brought in with arable plants when farming was introduced into England, and rabbits introduced for food. Currently, the deliberate release and accidental escape of aquatic garden plants and exotic animals are particularly significant. Most introduced species either fail to survive in the wild or persist as rarities. Others live in native habitats without causing any threat to our native wildlife. A small fraction (perhaps 0.1%), however, can spread dramatically, increasing in range and abundance to such an extent that they cause marked effects. In some cases they can exist at low levels for some time before they expand to cause problems. Research has so far failed to explain how a species can become invasive. All newly-introduced species have the potential to become problematic, especially where they can change their habitat preference and behaviour to suit their new home. Freshwaters have seen dramatic transformations with the spread of Japanese knotweed, giant hogweed and Himalayan balsam on their banks, and with aquatic plants such as water fern, parrot’s-feather, floating pennywort and Australian swamp stonecrop in the water. These habitats are particularly vulnerable to invasion by non-native species which disperse by floating downstream and spreading in floods, including plants that would not normally produce seed in our climate but can grow from fragments. Such colonisation is often very rapid and difficult to control. Introduced animals can also transform habitats. Excessive grazing by deer (including several introduced species) is causing problems in woodland.
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