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Many invasive cactus spread aggressively, to the detriment of other plants and the whole ecosystem.
Photo: C Mannheimer
Dr Iain Paterson applying biocontrol agents to a cactus stand near Windhoek.
Photo: R. Thomson
Biocontrol agents make infected cacti unhealthy and help to control their spread.
Photo: C Mannheimer

Alien plants

The control and eradication of invasive alien plants is a major priority of the Botanical Society of Namibia due to the devastating impacts they have on Namibia's native flora and fauna, as well as on livelihoods. Read about what BotSoc is doing to help tackle this serious problem.

 


 

 

What is an invasive alien?

An alien plant (or animal) is one that does not occur naturally in an area (not indigenous, exotic) and has been introduced by man, either deliberately or by accident. It can be harmful or benign. An invasive plant is one that spreads over large distances away from the original parent plants or population. These often spread aggressively, to the detriment of other plants and the general habitat or ecosystem. Not all alien plants are invasive, and not all invasive plants are alien. Indigenous plants can also be invasive, and these form a different problem that will not be addressed here.

Over 300 alien plants have been introduced to Namibia, many of them beneficial, such as food plants like mahangu and mielies, fruit trees like mangos and oranges, as well as ornamental garden plants like roses and pansies. Many of the introduced species only grow where they are planted and do not cause any problems. They do not propagate and spread on their own.

Known distribution of habitat transformers
Known distribution of habitat transformers: at least 27 species can form such thick stands that no indigenous plants can grow there and animals cannot pass by. Many are associated with water. The worst are rubber vine, cactus and Prosopis (now Neltuma). Map: Atlas of Namibia
Harrisia martinii
Moon cactus Harrisia martinii can form impenetrable mats that cover many hectares, out-competing and smothering virtually all other plants in the vicinity. Photo: C Mannheimer

In Namibia, about 70 alien plant species can or have spread beyond areas of cultivation, but do not form self-replacing populations. These are called "casual aliens" - mahangu and mielies are examples of casuals. At least 217 alien species have become naturalised, which means that they can propagate themselves without assistance from people. Most of these are harmless, but some have spread far from their points of introduction and may have a serious impact upon the habitats in which they occur, smothering and killing the indigenous vegetation. The latter are the invasive species, of which there are at least 60.

Why do aliens become invasive?

Most problematic aliens originate in countries with a similar climate to ours, such as South and Central America, or Australia, thriving in our conditions, without their natural predators and competitors. They do not arrive with the pests and diseases that control their natural populations in their home countries, and thus proliferate, to the detriment of our Namibian flora. Most are fast-growing, pioneer species, able to rapidly colonise disturbed areas such as road verges, urban areas, livestock kraals, cleared fields, while others are prevalent along water courses. They spread rapidly, either by producing many seeds that are distributed by wind, water and animals, or by reproducing vegetatively. Every cladode on a cactus plant can grow into a new plant.

Why are invasive aliens a problem?

  • Many outcompete, displace and smother indigenous plants, reducing biodiversity.
  • They form dense stands so that animals cannot move and land loses productive value.
  • Many are poisonous and kill or harm animals.
  • Animals become impaled on thorns, or infected by them, and die a slow and painful death.
  • Dense stands of aliens consume large quantities of water along watercourses, pans and dams, blocking access to the water for animals.
  • Aliens invade agricultural land, using soil nutrients and taking time, labour and money to remove.
  • They invade river systems, using water and nutrients and interrupting the flow of water.
Poisoners
Poisoners – nearly 20 species are poisonous to humans, livestock or other animals. As the map shows, they are widespread across the country, and should be eradicated. Many of them are associated with riverine habitats. Map: Atlas of Namibia.
Aquatic invasives
Three aquatic species (dark blue) live in water and are found in Namibia's perennial rivers, of which Kariba weed is the most problematic. About 20 species are associated with ephemeral rivers, streams and water points, of which the following three consume excessive amounts of soil water: prosopis (also called mesquite - pale blue), Eucalyptus gums and pepper trees (hatching). Aliens like rubber vine and lantana block animals' access to water. Prosopis has choked some southward-flowing rivers (red). Map: Atlas of Namibia.
Invaders of disturbed land
More than 30 species have invaded disturbed land such as roadsides, kraals, waterpoints, overgrazed or cleared areas, etc. Of these about 15 are crop pests. Map: Atlas of Namibia.

Are all aliens equally problematic throughout Namibia?

Just as the climate varies across Namibia, so too do the species that are invasive. Some prefer the more arid areas, e.g. Prosopis (now known as Neltuma). Others require higher rainfall e.g. Lantana camara. Some species known to be highly invasive in other parts of the world have been introduced to Namibia. As yet we are not sure of how invasive they are here, but we need to keep an eye on them. As a concerned citizen, you can help us by submitting your records to the Atlas of Alien Plants.

Invasive species in Namibia

Cacti

There are no indigenous cacti in Africa. Many species have been introduced as ornamentals and behave by staying where they were planted. Unfortunately, too many have spread around the country and have become a very serious problem.

  • Native to arid parts of the Americas, about 22 species are invasive
  • Deliberately planted by well-meaning people who want "water-wise" plants
  • Many spread laterally to form dense, impenetrable mats with vicious thorns
  • Animals cannot pass by, become impaled on the thorns
  • Thorns also stick in the throats of livestock, preventing them from feeding
  • Juicy fruits eaten by many; seeds spread far from the parent plants by birds and baboons
  • Stems (cladodes) or "pads" root where dropped
  • In South Africa, vast tracts of farmland have become useless – at huge economic cost through land loss and control
  • Many species are spreading rapidly in Namibia, thus posing a serious threat to indigenous vegetation
  • For details on individual invasive species see the Photo Guide to alien plants in Namibia.
Distribution of cacti
Map: Atlas of Namibia

Madagascar rubber vine

Cryptostegia grandiflora
  • Recently introduced as an ornamental but is also highly poisonous
  • Climbs over any support, thus smothering trees
  • Small seeds with long, silky hairs are blown far from the parent plant
  • In Australia, it has smothered and killed vast areas of forest and pasture and choked water ways
  • Present in all towns from Windhoek northwards, and has been planted on many farms and lodges, from where it is now spreading into the wild
  • As it prefers areas with over 400 mm rainfall, it has the potential to damage Namibia's northern woodlands
  • Has been known to poison elephants
  • For more information, read about it in the Photo Guide to alien plants in Namibia.
Rubber vine
Madagascar rubber vine Cryptostegia grandiflora
Photo: C Mannheimer

Prosopis

Three species in Namibia, of which Prosopis glandulosa is the most common. The genus name has recently been changed to Neltuma, but the plant is just as invasive as ever.
  • Introduced for shade and fodder from arid Americas
  • Although drought-tolerant and nutritious, they use a lot of ground water
  • Invaded pans and ephemeral rivers, such as the Fish and Nossob rivers
  • Outcompete indigenous riverine trees by depriving them of ground water
  • In places they have completely choked the riverbed
  • Provide valuable browse and shade at farm waterpoints BUT
  • Pods are eaten by livestock, spreading the seeds which wash into the rivers
  • City of Windhoek has a by-law prohibiting the planting of any of these species
  • For more information, read about it in the Photo Guide to alien plants in Namibia.
Prosopis chilensis
Prosopis chilensis
Photo: C Mannheimer

Leucaena

Leucaena leucocephala, Wonderboom
  • Widespread in Namibia
  • Produces vast amounts of winged seeds that germinate easily
  • Forms dense thickets that exclude other plants
  • Whole plant is toxic to livestock if eaten in quantity
  • For more information, read about it in the Photo Guide to alien plants in Namibia.
Leucaena pods
Wonderboom Leucaena leucocephala
Photo: C Mannheimer

River invaders

Many nasty aliens thrive along watercourses, although they are by no means confined to these habitats. They should all be eradicated wherever they are. Those in private gardens do not stay there, their seeds spread to the nearest watercourses. Problem species include:
  • Downy thorn apple (Datura inoxia)
  • Mexican poppy (Argemone ochroleuca)
  • Castor oil bush (Ricinus communis)
  • Wild tobacco (Nicotiana glauca)
  • Pepper tree Schinus mole
  • Eucalyptus spp

Waterberg and Otavi hills

Species such as Lantana camara, while a popular garden plant in many places, invade wetter areas. They have become a serious problem in higher rainfall areas and near permanent water. There are three indigenous species of Lantana in Namibia which could be planted instead of the invasive alien, none of which is well-known and none is as hardy and aggressive as the alien.

Solanum seaforthianum has caused a number of problems. It was originally introduced as an ornamental plant, but has since spread rapidly and become a invasive species, outcompeting native plant species and reducing biodiversity. It produces toxic chemicals that are harmful to livestock and other animals, and it can also affect the quality and quantity of water in rivers and streams, impacting both wildlife and human populations.

Solanum seaforthianum
Potato creeper (Solanum seaforthianum) is smothering many plants at Waterberg and has also been recorded along the Kunene River. Photo: B Curtis.

 

Keep an eye on these

Alien Moringa oleifera (left); indigenous Moringa ovalifolia (right)
Alien Moringa oleifera (left); indigenous Moringa ovalifolia (right).
Photo: C Mannheimer

The Asian Moringa oleifera (horseradish or drumstick tree) is being planted all over Africa for its nutritional, medicinal and other properties. It is now also being planted in Namibia, where we have our own near-endemic species, Moringa ovalifolia (Phantom tree). The alien species is regarded as potentially invasive or moderately invasive in tropical regions of the world. It has been banned in Florida, USA and parts of Australia, where the climate is similar to ours. We do not know the invasive potential of the alien in Namibia, nor do we know if it will hybridise with our species, thus creating genetic pollution. The two are difficult to tell apart unless they are flowering, particularly seedlings. It is essential that the spread of the alien is very carefully noted and reported.

Another known invasive alien in many parts of the world is the Neem tree, Azadirachta indica, also from Asia. This was introduced to Namibia some years ago as it is fast growing and used for shade and medicine. It is a declared pest in Australia due to its invasive properties, forming dense stands and excluding indigenous vegetation, especially along river banks. It is invasive in parts of Kenya, Tanzania and Uganda and is a serious problem along parts of the Kenyan and Tanzanian coasts, where it is spreading rapidly.

Recently introduced, the Kiri tree from China, Paulownia tomentosa, has become invasive in many parts of the world, including USA. It is an aggressive tree that invades disturbed natural areas including forests, roadsides and stream banks. Fast growing, it produces seeds prolifically. Once escaped from cultivation, it has the potential to become a serious pest in Namibia.