Alien Introductions

The damage inflicted by the introduction of "alien" species is a well-recognised phenomenon but effects vary from ecosystem to ecosystem.

Continental forests appear reasonably resistant unless heavy cutting or partial clearing has disturbed them. Native meadows and prairies are also particularly susceptible to intruders if they have already been disturbed. For example many of the now common grasses in the U.S. and Canada are in fact transported Eurasian weeds; this is due to the damage caused to the original species by overgrazing through European colonisation. The species were able to invade and become established because the original perennial grasses couldn't recover fast enough to support the intense grazing.

Freshwater lakes and streams have little immunity to invading species. For example the introduction of the Nile perch, (whose diet is smaller fish), in Lake Victoria in 1960 to benefit commercial fishing led to the extinction of about thirty species of fish that were native to the lake.

The largest changes often occur when the intruder brings quite different traits from those of native species. In Hawaii an exotic tree, the Myrica faga, was introduced, that has the ability, like pea plants, to increase nitrogen content within the soil. This ability fundamentally changed the soils and its properties affecting soil organisms and altered the raw materials on which many other plant species depended.

Finally species that have evolved in isolated environments are often highly specialised and therefore extremely vulnerable to change. The saddest example of this is perhaps the fate of the flightless Stephen Island wren, the species was driven to extinction by a single cat owned by the lighthouse keeper on the isle off the coast of New Zealand.