Alien-invasive species are animals and plants that have been introduced to a country in which they were not naturally found in the past, and which have the potential to grow their population and range in that country.
Some were introduced intentionally, such as game birds released for shooting, others unintentionally, such as escaped aviary birds, and still others were introduced unknowingly, such as stow-aways on ships. These compete with local species for resources and can ultimately completely displace local species in the landscape, thus directly threatening local biodiversity.
We are quite fortunate in South Africa in that only 23 bird species have been listed as alien invasives in terms of the National Environmental Management: Biodiversity Act’s Alien and Invasive Species List released in October 2014. Of these, only five have well established, growing populations: the mallard duck (Anas platyrhynchos), feral pigeon (Columa livia), common mynah (Acridotheres tristis), common starling (Stumus vulgaris) and house sparrow (Passer domesticus).
Many other bird species have been introduced to South Africa; some died out, whereas others like the common chaffinch (Fringilla coelebs) remain restricted to localised habitats.
The mallard duck is the only bird species that hybridises readily with local species – usually with yellow-billed ducks (Anas undulata), but they are not fussy. Even Egyptian geese (Alopochen aegyptiaca) are not immune to its doubtful charms. Mallards are not common in South African national parks, but they and their hybrid offspring are spreading, and now occur in remote areas, including De Hoop Nature Reserve.
The other four widespread alien birds occur in many of our national parks. Fortunately, they do not favour natural habitats, therefore so are mostly confined to the built-up areas of camps and entry gates. The house sparrow is found in all of South Africa’s national parks, but it is not thought to pose a real displacement threat to any local birds. However, the common mynah has expanded its range dramatically over the past two decades and now occurs in the Marakele, Mapungubwe and Kruger national parks. They are also fast approaching the Mokala National Park in the south west, which is a concern since they have the potential to out-compete local birds and have been known to kill chameleons by pecking their eyes out.
Another alien bird species worth mentioning, if only for the success of efforts to halt its invasion, is the house crow (Corvus splendens). In the eThekwini Municipality alone there would have been more than 250 000 individuals by now if the decision had not been taken to eradicate them. Following several eradication efforts they have been eliminated from Durban and Richards Bay, and there are thought to be fewer than 1 000 left in Cape Town. Their impact on native birds and other wildlife (including in national parks) would have been devastating, had they been allowed to continue to spread and grow.
One of the ways in which we support alien birds is by feeding them at picnic sites and rest camps, so please don’t feed any birds, alien or native. There is also no room for complacency, with another 20 bird species on the prohibited list. The invasive alien-species regulations are important for maintaining our indigenous biodiversity.
For more information consult the gazetted Nemba invasive-species lists at www.gpwonline.co.za
Written by Rob Little for the SANParks Times: www.sanparkstimes.co.za)