Many species, which in their native ecosystems are in reasonable balance with others, exert a much greater, and often adverse influence, if introduced accidentally or deliberately elsewhere. In the USA alone, it is estimated that there are between 2000-4500 exotic insects (i.e. non-native), more than 200 alien plant pathogens and around 4000 exotic escaped plants (i.e. growing outside cultivation). Of these a third of the insects, more than half the plants and almost all the pathogens are known to have a harmful effect (Pasek et al, 2000; Campbell, 2002).
Exotic or alien plants (neophytes) can be harmful in a variety of ways. In the UK, efforts are being made to eradicate the rapid-growing turkey oak Quercus cerris from stands of native oaks, as it hybridizes with them producing trees with poor quality timber due to 'shakes' (internal cracks along the rays or growth rings). The large woody shrub Rhododendron ponticum, native to Turkey and Spain, has become a notorious intruder, especially in the British Isles where it is extensively naturalized and spreads by seeding and suckering. Introduced as cover for game birds such as the pheasant, it spreads to form dense thickets under which little else can live. Japanese knotweed Fallopia japonica and Himalayan balsam Impatiens glandulifera are amongst the larger herbs creating similar problems. A number of garden escapes - Spanish bluebell Hyacinthoides hispanica and variegated yellow archangel Lamiastrum galeobdolon ssp. argenteum - are causing concern in the UK as they hybridize with the native species.
Virtually every country in the world has similar problems with invasive species. Any number of web pages can be found that deal with weeds causing problems in forests and many other habitats. This is especially true in North America and Australasia where European weeds were brought in by colonists both deliberately and accidentally.
The dangers of globalization and diminution or potential loss of endemic species are well illustrated by the Island of Madeira where a decision to replant many forest areas previously cleared by fire was taken as early as 1515, some exotic species being brought in at a very early date. Douglas fir, Monterey cypress, Japanese cedar, sweet chestnut, sycamore and the common fig are amongst the very many introduced plants listed by Sziemer (2000) as having become naturalized or showing a tendency to do so. Eucalypts are the most prominent of the trees introduced for silvicultural purposes, often occurring on steeply sloping hillsides beside the Laurel Forest, which they overshadow and tend to invade. More remarkably, native trees are in some places also being overgrown and killed by the climber Passiflora x exoniensis, one of the passion flowers.
Globalization and the displacement of native forest is by no means a oneway affair. Wax myrtle Myrica faya, from Madeira and the Azores, is much more vigorous in Hawaii, where this introduced plant is spread by a bird introduced from Asia and supplants the native forest.
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