Introduced animals arrive in their new environments without their natural predators, competitors and pathogens that would otherwise control their population numbers. This allows them to establish self-sustaining populations and spread significantly to the point where they are considered to be invasive. Animal invaders include vertebrates and invertebrates, parasites and pathogens and are found in a wide range of environments such as terrestrial, freshwater, marine and in the soil, they can be hosted in humans, plants, or animals.
Invasive animal introductions can be grouped in two categories viz deliberate and accidental. Most of the invasive vertebrates in South Africa have been deliberately introduced, while most of the invertebrates were accidentally introduced through e.g. potted plants, freight, food and timber goods. Examples of accidentally introduced vertebrates are rats and Indian house-crows all of which were introduced via ship. Animals such as European fallow deer, grey squirrels and common starlings were deliberately introduced by Cecil John Rhodes into Cape Town during the colonial era to enhance the sense of “home away from home”.
Many of the species introduced during this era became invasive with detrimental consequences for indigenous plant and animal communities. More recent introductions occurred through the aquaculture trade as a deliberate or accidental means of introduction of several freshwater species. Some of these species were deliberately of accidentally released or have escaped, or spread from aquaculture facilities. Examples include aquarium fishes and their associated parasites and a few free-living freshwater invertebrates, mostly snails. One of the world’s most harmful alien insects, the Argentine ant was accidentally introduced into South Africa around 1898. Argentine ants are found in disturbed areas such as drainage lines, they also invade fynbos and Afrotemperate forests in the Western Cape.