Threaten biodiversity

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 fire

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.
 

 

 

 

 

 

 

Block waterways

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 Domestic Product.