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Alien species are defined in this report as ones not occurring naturally in Britain , i.e. species that have been introduced accidentally or deliberately by humans. (It should be noted that there may be some debate about identifying alien species for Scotland as opposed to the rest of Britain ). Accidental entry includes escape, outcasts from gardens, and carriage on vehicles, animals or persons moving between countries, or in products such as grain, hay and timber. The opportunities for alien species to become established are probably still increasing, with greater trade, more people travelling, new crops and management practices, and also climate changes. In particular, the predicted warming during the next century is likely to open Scotland to invasion from an increased number of species, as warmer regions tend to have greater species diversity than cooler regions. (The case of tree mallow, which is native to the south-west and west coast of the UK but now seriously spreading on bird islands in the Firth of Forth region and endangering the puffin population, is an example for the effects of slowly rising temperatures (CEH, 2005).

An SNH audit carried out in 2001 found a total of 988 non-native species in Scotland . The pathway by which the greatest number of species (mostly plants) is introduced to the wild in Scotland is from gardens. Alien species occur in greater numbers in the South and Central Belt regions of Scotland than elsewhere (partly due to the greater human population in these regions). The habitats into which most vascular plants have become established are man-made ones (arable land, waste ground), grasslands and woodlands; very few occur in upland habitats. (Welch et al., 2001)

In Scotland , rhododendron reduces the biodiversity of Atlantic oakwoods while American mink is held partially responsible for the decline of water vole populations. Hybridisation has occurred between non-native sika and native red deer as well as between native and non-native plants. Japanese knotweed undermines flood defences, and the impact of bark stripping by grey squirrels reduces forestry production.

Alien species can also affect human health, e.g. phytophotodermatitis through contact with giant hogweed or leptospirosis spread by the brown rat. There have been relatively few successful control eradication programmes against non-native species. Control measures are generally not implemented until a species becomes a problem, by which stage they are very expensive (Birnie et al., 2004).

A total of 13 mammals are considered to be alien in Scotland . Four species are of domestic/ feral origin: the domestic cat, the feral ferret, the feral goat and the American mink. Another four species have been long established in Scotland – the rabbit, the house mouse, the common/brown rat, and the brown hare. Three species have been introduced deliberately for "aesthetic" or sporting purposes: the grey squirrel, which is now implicated in the decline of the native red squirrel; Sika deer and fallow deer. Also, the hedgehog (although not an alien to Scotland) has been introduced for aesthetic reasons to the Outer Hebrides where it did not occur naturally, and has seriously damaged the ground-nesting native avifauna on some islands (Welch et al., 2001). Alien birds include pheasant and red-legged partridge.

Biological invasions, accidental or deliberate, by non-native species are a significant component of human-caused environmental change, often resulting in a significant loss of bio-diversity, economic value and function of the invaded ecosystem. Climate change is likely to facilitate future invasions of Scotland , as warmer regions have greater variety in species than cooler regions. Whereas some alien species have long been established in Scotland (e.g. rabbit, house mouse), others have only become a noticeable presence and potential problem more recently (American mink, Japanese knotweed, tree mallow).