INDIGENOUS
An indigenous plant or animal is one which occurs naturally in the place in which it is currently found, and has not been assisted in its travels by people. Being indigenous does not mean that a species has always occurred where it is now found - some species shift their distribution quite quickly, for example, when blown by strong winds of naturally transported by ocean currents.

The indigenous concept also applies to habitat, and does not refer just to a geographical area. For example, the birds indigenous to dry thornveld are very different from the birds indigenous to mistbelt forest, even though they may be only 10 km apart.

An endemic species is not only indigenous, but is restricted to a particular area. The blue crane, for example, is indigenous to southern Africa, and is found nowhere else. This makes it endemic. The wattled crane is also indigenous, but it is not endemic to southern Africa as it is also found further north in Africa.

ALIEN
A species which does not occur naturally in an area (i.e. is not indigenous), but which has been introduced there by people, is called an alien. Sometimes people use the word "exotic" instead of alien, but this is not the best word as it has many other meanings.

INVADERS
A few alien species reproduce and spread, unassisted by man, into areas where they are not wanted - these invasive aliens pose a serious threat to nature conservation. Expensive and drastic measures are required to control invaders.

Some indigenous species also invade new areas but these invasions are brought about by changes in the area invaded, e.g. Acacia karoo rapidly takes over overgrazed grassland. Control of such invasions involves correcting the management of the area, in addition to control of the invading species itself.

WHY DO SOME ALIENS BECOME INVADERS?
There are several reasons, but most important is that aliens find themselves in an environment different from that in which they evolved. Usually such a drastic change in environment is fatal to an alien. Occasionally, however, conditions are superior to those in the native land. In particular the alien animal or plant is normally free of the diseases, parasites and predators which keep its numbers in check in its natural environment. Under these conditions populations can explode with the invaders overwhelming the indigenous fauna and flora, usually by crowding it out.

In southern Africa very few mammals have become invaders. Alien fish, however, pose a serious threat to the indigenous fish of our inland waters. For example, smallmouth and largemouth bass prey heavily on indigenous fish. Fish parasites accidentally introduced with some aquarium species cause even greater problems. Parasites such as `white spot' are alien to southern Africa.

Plant invaders are plentiful, especially in the southwestern Cape, KwaZulu/Natal, and the eastern Transvaal. Cape fynbos, a unique floral kingdom, is severely threatened by Australian acacias, which were originally introduced for timber, bark products, or to stabilise sand dunes. In the eastern Cape, KwaZulu/Natal, and the Transvaal lowveld the worst invaders are lantana, chromolaena (triffid weed), seringa, and pereskia (Barbados gooseberry), black wattle, prickly pear, bugweed, and bramble. Many alien plants have been declared undesirable and may not be grown on any public or private property. These alien plants were originally introduced as commercial plants or as ornamental garden plants. Many rivers and dams are clogged with water hyacinth, Kariba weed or parrot's feather, also escaped ornamentals. Highveld areas suffer most from wattle and bramble, and the arid west is being invaded by mesquite. There are more than 700 alien plant species in southern Africa and 10% of these are invasive aliens.

None of these plants and animals cause problems in their native lands. Their invasive qualities were unsuspected, which goes to show that no alien plant or animal should ever be released into the veld without screening for its invasive potential. Almost any alien could become an invader.

THINGS TO DO
* List and map all the invaders in your area and find out how to control each invader.

* Draw up a report outlining an overall control plan (specialist advice will be needed for this) and present it to your local authorities.

* Check all gardens and nurseries to ensure that none are cultivating invaders - bring offenders to the attention of your local conservation authority.

* Do not move any aquatic plant or animal (including fish) from one water body to another - to do so is illegal. * Never release unwanted pets into the wild - rather give them to a pet shop.

* Abide by the regulations regarding the importation of plant and animal material when returning from abroad, and encourage friends to do so.

FURTHER READING

SOUTH AFRICA'S UNDERWATER INVADERS. I.J. de Moor and M.N. Bruton. African Wildlife 42(4), 1988.

INVASIVE ALIEN PLANTS AND NATURE CONSERVATION. I.A.W. Macdonald. in African Wildlife 42: 333-335, 1988.

ATLAS OF ALIEN AND TRANSLOCATED INDIGENOUS AQUATIC ANIMALS IN SOUTHERN AFRICA. Southern African National Scientific Programme Report No. 144. CSIR, Pretoria. 1988.

NATIONAL LIST OF INDIGENOUS TREES. F. von Breitenbach. Dendrological Foundation, Pretoria, 1990.

NATIONAL LIST OF INTRODUCED TREES. F. von Breitenbach. Dendrological Foundation, Pretoria, 1989.

CATCHMENT ACTION: 28 ALIEN PLANT INVADERS IN NATAL. P. Caldwell. Share-Net, 1992. PO Box 394, Howick, 3290.

PLANT INVADERS: BEAUTIFUL BUT DANGEROUS. C. H. Stirton (ed.) Department of Nature and Environmental Conservation of the C.P.A. Cape Town, 1987.

* A CATALOGUE OF PROBLEM PLANTS IN SOUTHERN AFRICA INCORPORATING THE NATIONAL WEED LIST OF SOUTHERN AFRICA. M. Wells et al. Memoirs of the Botanical Survey of South Africa, no. 53. Botanical Research Institute and Department of Agricultural and Water Supply.

* PLANT INVADERS OF THE TRANSVAAL. L. Henderson and K.J. Musil. Department of Agriculture and Water Supply, Bulletin 412. Government Printer, Pretoria, 1987.

* DECLARED WEEDS AND ALIEN INVADER PLANTS IN SOUTH AFRICA. M. Henderson et al. Dept. Agriculture & Water Supply, Pretoria, 1987.

* Available from the National Botanical Institute, address below.

USEFUL ORGANISATIONS

Dept. Water Affairs and Forestry, Forestry Branch. P. Bag X313, Pretoria, 0001. Tel. 012-299 9111. Publish various pamphlets on invasive plants.

Directorate of Agricultural Information. P. Bag X144, Pretoria, 0001. Tel. 012-206 2829/206 2181. Publish a selection of pamphlets on weeds.

The National Botanic Institute. P. Bag X101, Pretoria, 0001. Tel. 012-804 3200, ask for Information Officer. Branches nationwide.

Plant Protection Institute. P. Bag X134, Pretoria, 0001. Tel. 012-206 9112. Branches in Pietermaritzburg, Stellenbosch, Pretoria.