As they die off, they will be gradually replaced by more ecologically pleasing local varieties - which may or may not be as pleasing to the eye. Some of the worst offenders in this regard are pine and gum trees. Scientists say the aliens often thrive because they have no local predators, such as birds or insects which eat their seeds, to keep their numbers in check. In some dams, alien water hyacinths have spread across the surface, blocking out the sun and choking out local plant and fish populations. Under the terms of the draft bill, they have been slated for complete removal.
Conservationists also say the outsiders contribute to soil erosion.
Naturally occurring plants tend to bind the soil better and prevent erosion. The consequences are not just confined to the natural world. Environmentalists say devastating floods that killed hundreds of people in Mozambique last year were exacerbated by the loss of vital wetlands, which have been under pressure partly because of exotic plant species.