Invasive alien plants in southern
Africa
Part 1
Terminology and Overview of Species
Southern Africa has a long history of problem plants which have been variously called ‘weeds’, ‘pest plants’, ‘plant invaders’, ‘invasive plants’, ‘bush encroachers’, ‘naturalised exotics or aliens’, ‘environmental weeds’, ‘transformers’, and others. The terms have been used to describe both indigenous plants (native, belonging to southern Africa) and non-indigenous plants (exotic, alien, introduced to southern Africa).
History
Non-indigenous or alien plants were first introduced to southern Africa
between one and two thousand years ago. These were plants mainly of central
and northern African origin and were associated with human activities. Most
plants from other continents were introduced to southern Africa by European
colonists from 1652 onwards. The first control campaign against an alien plant
in South Africa was initiated in 1860 against spiny cocklebur, Xanthium
spinosum.
Terminology
Much confusion exists in the English-language literature on plant invasions
concerning the terms ‘naturalised’ and ‘invasive’ and their associated concepts.
Richardson et al. (2000) have provided definitions that they hope will
permit widespread application and acceptance and these are given here in part.
Introduction implies that a plant or its propagule has overcome, through human agency, a major geographical barrier (intercontinental or infra-continental; usually >100 km).
Alien plants are plant taxa in a given area, its presence there due to intentional or accidental introduction as a result of human activity.
Casual alien plants do not form self-replacing populations and rely on repeated introductions for their persisitence.
Naturalised plants are alien plants that reproduce consistently and sustain populations over many life cycles without direct intervention by humans.
Weeds are plants (not necessarily alien) that grow where they are not wanted and that usually have detectable economic or environmental effects. Many indigenous pioneering species are weeds of disturbed sites such as roadsides, overgrazed land, and waste places. Alien weeds occur in the same disturbed sites and are also common in cultivated lands, planted pastures, and lawns.
Environmental weeds are alien plants that invade natural vegetation, usually adversely affecting native biodiversity and ecosystem functioning.
Invasive plants, plant invaders or invader plants usually refer to naturalised alien plants that have the ability to produce offspring, often in very large numbers, at considerable distances from parent plants. The most aggressive invaders can spread far from parent plants and cover large areas. The most damaging invaders may be termed transformers as they can change the character, condition, form, or nature of ecosystems over substantial areas. In southern Africa the term ‘invasive’ has sometimes been applied to indigenous species where distribution or density has increased in response to human-induced changes in the environment. Indigenous woody species that invade overgrazed land are referred to as bush encroachers.
Invasive
Plant Lists
The most comprehensive listing of weeds and invasive plants in southern
Africa was published by Wells et al. (1986) in their Catalogue of
problem plants of southern Africa. The catalogue lists 1,653 taxa (species
and varieties or subspecies) of indigenous and alien plants. The catalogue
includes a list of 984 ‘naturalized exotics’, which contains 789 plant taxa
that are generally agreed to be both alien and naturalised, a further 91 taxa
where naturalisation is uncertain, and 104 taxa of uncertain origin. The great
majority of these plants are herbaceous and invade sites of severe human disturbance.
They are classified as ruderal weeds (=of waste places) and agrestal weeds
(=of cultivated lands). Approximately 10% are classified as flora weeds—alien
invasive species competing with the indigenous flora.
Arnold & De Wet (1993), Plants of southern Africa: names and distribution, lists 858 taxa of naturalised alien species that are catalogued in the Pretoria National Herbarium.
The next most comprehensive catalogue of naturalised alien species is probably that contained in the Southern African Plant Invaders Atlas (SAPIA) Database, which is managed by the author. The SAPIA database currently contains 500 species, of which 200 are not listed by Wells et al. (1986). Not all of these species have become truly naturalised; some are occasional garden escapes that require considerable disturbance to become established. However, they are all regarded as worthy of listing, as they could become the invaders of the future. All the species in the SAPIA database have been recorded in natural or disturbed sites, excluding cultivated lands.
Invasive
Species
At least 1,000 alien plant species are known to be naturalised in South
Africa. About 200 (20%) of these species are regarded as important environmental
weeds invading natural or near-natural habitats. They represent 53 families
and 113 genera. The following nine plant families contribute 61% of the species:
Fabaceae
(legumes)
Asteraceae
(daisies)
Myrtaceae
(eucalyptus/myrtle family)
Solanaceae
(potato family)
Cactaceae
(cacti)
Pinaceae
(pines)
Poaceae
(grasses)
Rosaceae
(rose family)
Convolvulaceae
(bindweed family)
Acacia, with thirteen species, is the most important genus, followed by Eucalyptus and Pinus, with seven and eight species respectively.
Sixty-five per cent of the major environmental weeds are trees or shrubs, and 90% of species are perennials. Most species originate from South and Central America (38%), Europe and Asia (32%), and Australia (15%). Each major biome or vegetation type has a characteristic set of major invaders, some of which are virtually exclusive to that biome, for example, Hakea sericea (silky hakea) and Pinus pinaster (cluster pine) in fynbos, Chromolaena odorata (triffid weed) and Lantana camara (lantana) in savanna, and Pyracantha angustifolia (orange firethorn) in grassland. Some species are found in several biomes, for example, the cactus Opuntia ficus-indica (sweet prickly pear). Several very widespread invaders owe their extensive distribution to their spread along river systems; examples include Acacia mearnsii (black wattle) and Sesbania punicea (red sesbania).
Extent
of Invasion
The extent of plant invasions is difficult to assess. There have been
various surveys done, but comparing statistics is complicated by the variety
of measures used. It appears that aquatic habitats are the most severely invaded,
followed by watercourses, and next the winter-rainfall region, essentially
the fynbos biome. The summer-rainfall, subtropical coastal belt of KwaZulu-Natal
would be next, followed by the escarpment in Mpumalanga and the Northern Province.
Some estimates suggest that about 25% of the fynbos biome has been invaded.
Worst
Offenders
Some of the most widespread invaders include Acacia mearnsii, the
black wattle, which is naturalised in 428 quarter-degree squares (15 minute
squares) in South Africa. This represents 21% of the total quarter-degree
squares in this region. Prosopis spp., known as prosopis or mesquite,
have invaded 388 quarter-degree squares or 19% of the total area. Acacia
cyclops, red-eye or rooikrans, has invaded 167 quarter-degree squares
or 8% of the total area. It stretches along almost the entire Cape coastline,
a distance exceeding 2,000 km.
—by Lesley Henderson
SABONET News 6.2: 84
ARNOLD, T.H. & DE WET, B.C., (eds). 1993. Plants of southern Africa: names and distribution, Memoirs of the Botanical Survey of South Africa No. 62
RICHARDSON, D.M., PYŠEK, P., REJMANEK, M., BARBOUR, M.G., PANETTA, F.D. & WEST, C.J. 2000. Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6, 93–107.
WELLS, M.J., BALSINHAS, A.A., JOFFE, H., ENGELBRECHT, V.M., HARDING, G. & STIRTON, C.H. 1986. Catalogue of problem plants of southern Africa. Memoirs of the Botanical Survey of South Africa No. 53.
