Aliens compete with indigenous species for
space, food and water. Because they grow and reproduce very successfully,
they can eventually replace indigenous species. Invasive predators eat
many indigenous animals. Some alien species breed with closely related
indigenous species, producing hybrid offspring. This is a cross between
two species, which is no longer an indigenous species, e.g. Mallard and
Yellow-Billed Duck hybrid.
| Increase the severity of
Woody alien shrubs and trees grow very
densely. They therefore usually produce more fuel than the local
vegetation. Fires in alien vegetation often burn more fiercely and cause
more damage to people and nature than fires in indigenous vegetation.
Severe fires can kill the seeds of indigenous plants in the soil. This
reduces germination and recovery of vegetation after the fire.
Reduce water runoff
Woody alien shrubs and trees growing in
catchment areas often take more water from the soil than the indigenous
plants (e.g. fynbos and grassland). This reduces the amount of water
flowing into rivers and dams, which affects both people and nature.
Invasive water weeds grow
rapidly, especially when there are high concentrations of nutrients in the
water. They choke rivers and wetlands, making it difficult for people and
animals to get to or move through the water.
A costly nuisance
Some invasive alien species are a nuisance
because they are dirty and noisy (e.g. European starlings) or damage food
(e.g. rats) or buildings (e.g. borer beetles). Getting rid of invasive
aliens, for whatever reason, is usually very costly. In South Africa,
invasive alien plants cause economic losses of about 4% of the Gross