Understanding invasive alien species in Cape Town

For the purposes of this report invasive species are defined as those species that are “non-native (alien) to the ecosystem under consideration, whose introduction causes or has the potential to cause harm to the economy, environment or human health”. Invasive species may be plants, animals or other organisms.   

Invasion by alien species is one result of globalisation, a process that allows for the easy movement of people and goods around the world. Species often arrive at different destinations accidentally, although many exotic species are intentionally introduced into gardens or native ecosystems for various purposes.  If the introduced species is able to adapt to the climatic and environmental conditions in their new destination they have the potential to naturalise and become invasive, especially if they are free from pests and parasites in their introduced environment. However, it is important to note that not all introduced species become invasive; only a small percentage become a problem in their new destination. 

Plants and animals that are introduced become a problem if they dramatically increase in numbers, outcompete indigenous species or alter the environmental conditions to the detriment of the local biota. Invasive species generally reproduce rapidly, spreading and invading local ecosystems. This may hold serious implications for water yields, rangeland productivity, biodiversity and associated tourism, human health, and fire safety. 

Cape Town experiences a number of problems caused by the presence of invasive species.

These include:

  • Loss of habitat and indigenous species due to being crowded out by invasive species. 
  • Fire risk, as invasive plants often burn much hotter and for longer periods of time than indigenous plants. 
  • Water loss in an already water-scarce city, as invasive trees and shrubs use a significantly larger amount of water than indigenous plants. 
  • Alteration in nutrient content of soils by invasive species with negative effects on the indigenous ecosystem.