Beware the Aliens

Sanderling
Migration Miracles – The Sanderling
28 December, 2020
Off-road tracks in the Dorob National Park Namibia
Stay on Track and Enjoy Pristine Nature
20 January, 2021
Beware the Aliens 1

Author: Barbara Curtis

 

Beware the Aliens

Part 1: The Worst Offenders
 
This title may conjure up visions of little green men with antennae.

But what I am about to write about are large green plants with thorns! (Although not all have thorns, all are nasty).

Imagine that these little green men invade your home, eat all your food, drink all your water, and kill you and your relatives and children.

Would you like that? I am sure you would not.

And yet that is exactly what invasive alien plants are doing to our indigenous plants and ecosystems.
 
 

What is an Invasive Plant


What is an invasive alien plant, you may ask?

An alien plant (or animal) is one that does not occur naturally in an area (not indigenous) 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 can also be benign (harmless), or they can 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.

We have introduced many alien plants to Namibia, such as food plants like mahangu and mielies, fruit trees like mangos and avos, and ornamental garden plants like roses and pansies.

Many of the introduced species, such as the last four mentioned above, only grow where they are planted and do not cause any problems. They do not propagate and spread on their own.

In Namibia, about 70 alien species can or have spread beyond areas of cultivation, but do not form self-replacing populations. These are called “casual aliens” (Pyšek et al. 2004); 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 (Klaassen & Kwembeya, 2013).

 
 

Alien Plants in Namibia

At least 217 alien species have become naturalised, which means that they can propagate themselves without assistance from people (Klaassen & Kwembeya, 2013).

Most of these are harmless, but some of them have spread far from their points of introduction and may have a serious impact upon the habitats in which they occur.

The latter are the invasive species, of which there are at least 60.

They can be further divided into “weeds”, “transformers” and “harmless”. In Namibia, at least 54 alien species are of uncertain status.
 
 

These trees should not be planted, and where they occur, they should be eradicated.

 
 
 
 

Which are the worst invader species and why are they a problem?

Probably the most widespread invasive aliens are the three Prosopis species, glandulosa, velutina and chilensis, also known as Mesquite.

As the name of the last implies, these come originally from the Americas. They were deliberately introduced for shade and fodder since they come from arid regions and are fast growing, with highly nutritious pods and leaves (they are in the Fabaceae, a family rich in protein).

However, although they can withstand drought, they are very thirsty plants. They soon left the farm homesteads and kraals where they were planted and invaded ephemeral river systems, particularly those flowing southwards, such as the Fish, both Nossobs and the Aub.

They outcompete the indigenous riverine vegetation, and take up vast quantities of water from the aquifers.

In places they have completely choked the riverbed (Strohbach, pers. comm).

Unfortunately, they are very useful, often the only trees around, and thus farmers are loath to remove then.

But they cannot be contained. The pods are readily eaten by livestock, which then spread the seeds around.

These trees should not be planted, and where they occur, they should be eradicated.
 
 

However, although they can withstand drought, they are very thirsty plants. They soon left the farm homesteads and kraals where they were planted and invaded ephemeral river systems, particularly those flowing southwards, such as the Fish, both Nossobs and the Aub.

 
 
 
 

Invasive Cacti

Another serious problem group is the invasive cacti. They are also from the arid Americas.

These succulent plants, prickly pears and the like, have very attractive flowers and some have delicious fruit. They all have thick green stems, sometimes looking like large leaves (cladodes or pads). None have leaves. Photosynthesis is done by the stems and leaves have been replaced by spines.

Some species were deliberately introduced as fodder (although they also have nasty spines, a hybrid spineless variety has been cultivated), or as garden plants for their fruit and flowers.

Not all cacti are invasive, and some garden species remain where they are planted.

But around 14 species spread both by dispersal of seeds and by vegetative growth and have become serious pests.

Birds and baboons love the fruit and spread the seeds far and wide. Where the plant germinates it spreads rapidly in all directions, completely smothering nearby vegetation and making it impossible for animals to walk past.

Many animals have experienced a slow and painful death where they have become impaled on the spines.

The infestations render the land totally useless to farmers.

This group of plants is more problematic in the northern parts of the country than the south, although the Boxing glove cactus (Cylindropuntiafulgida var mammillaria) is a problem in the south east (Mannheimer, pers.comm).

Our actual records of this group are very sparse as nobody wants to collect cacti.
 
 

While there are cactus species that are not invasive, it is far better not to plant cacti at all.

There are many indigenous species that do not require much water. Namib Trees is a nursery in Windhoek that specialises in indigenous plants (not just trees).

 
 
 
 
People have planted cacti for various reasons, often as a barrier to unwanted people and animals.

But this just increases the overall problem in the country.

These plants cannot be controlled and will spread to other areas. Well-meaning people who are trying to plant water-wise gardens often look to cacti.

While there are cactus species that are not invasive, it is far better not to plant cacti at all.

There are many indigenous species that do not require much water. Namib Trees is a nursery in Windhoek that specialises in indigenous plants (not just trees).
 
 

The best method for eradicating cactus is bio-control, where a host-specific insect is released onto the plants.

 
 

Methods to Eradicate Cacti

The invasive species should never be planted and should be eradicated.

Manual removal of cactus is not easy due to the spines.

Where a piece of the plant (cladode) falls to the ground, it will root and produce new plants. Thus, care must be taken when disposing of these plants, or else the discarded plants will grow where they are placed.

The best method for eradicating cactus is bio-control, where a host-specific insect is released onto the plants. The most effective agents are plant-sucking bugs, generally of the group known as cochineal insects due to a red substance, carminic acid, which they secrete to prevent predation by other insects, although other insects can infect some species of cacti.

Each of about ten species of invasive cactus has a host-specific insect parasite which will generally attack that species only, thus there is no chance of non-invasive garden cacti being infected.

Namibia already has the control agent for one species of cactus, and is about to release the control agents for three more of the worst species.
 
 
 
 

Madagascar Rubber Vine

A “new” species of extreme concern is the Madagascar rubber vine, Cryptostegia grandiflora.

This species has been introduced into Namibia fairly recently as an ornamental due to its attractive pink flowers and the fact that it grows fast.

As a climber it grows into and over any form of support, including trees, which it can totally smother. In Australia, where it has been declared a noxious weed, it has smothered and killed vast areas of forest and pasture and choked water ways. In 1995 the loss of grazing due to this plant in Queensland was estimated to have cost the beef industry of Australia $ 18 million (CRC 2003).

In Namibia we have seen it in almost every town from Windhoek northwards, and it has been planted on many farms and lodges. Since it prefers areas with rainfall of 400-1400 mm, it has serious potential to destroy our already-compromised northern woodlands.

Not only does the species suffocate plants and destroy habitats, but it is also highly poisonous, and should definitely be eradicated.
 
 

Other Invasive Plant Species in Namibia

Other invasive species include various Agave spp., the two most invasive being A. americana and A, sisalana, Sisal.

These aloe-like succulents also come from the Americas and spread laterally by runners.

They are often planted as barriers due to their thick, spine-tipped leaves.

But they invade and block water courses and can affect soil fertility negatively where planted.

Other invaders of water courses and disturbed areas are the three species of Datura, Thorn-apple, Ricinus communis, Castor oil bush, and Nicotiana glauca, Wild tobacco, two species of Argemone, Prickly or Mexican poppy, and Leucaena leucocephala, Wonderboom.

All of these are poisonous and displace the indigenous vegetation.

Wonderboom is of high concern as it is spreading rapidly.

At the time the Tree Atlas of Namibia was published there was one record of this species. Now it is all over the place.
 
 
 
 

Please help me with this task by sending in records of places that you have seen aliens, and by spreading the word as widely as possible.

It would be best if you can upload the information directly onto the Alien database (www.the-eis. com).

 
 

Adressing the Alien Plant Problem in Namibia

In order to address the problem of invasions by alien plants, we need to know what their distributions are, and we need to make as many people as possible aware of the problem.

A recent initiative by the EIS (Environmental Information Service of Namibia) invites all citizen scientists to send in records of 36 invasive aliens via their Alien Atlas app.

The Atlas of Namibia, first compiled in 2002 by Mendelsohn et al., is being revised, with new up-to-date information on all aspects of Namibia. I have been asked to submit a section on invasive aliens. Because this is an atlas, naturally it would be good to have distribution maps. However, on looking at the currently recorded distributions, it is clear that the records vastly under-represent the actual extent of infestation.

In an attempt to address this situation, I am calling on all members of the public to assist with records. I am interested in all invasive aliens, and also those whose status is currently unclear.

However,my main interest is in the worse three groups first mentioned in this article, particularly the cacti and the Rubber Vine.

Please help me with this task by sending in records of places that you have seen aliens, and by spreading the word as widely as possible.

It would be best if you can upload the information directly onto the Alien database (www.the-eis. com).

Failing that, or if you are uncertain of the identity of the plants or do not have exact co-ordinates, you can email the information to me at curtis.namibia@gmail.com. Photos are always good to aid in identification. If you do not have co-ordinates, quarter degree square or name of town or farm will also help.

Please help us to rid the country of little green men! Every bit helps.

Acknowledgements: Thanks to Ben Strohbach (NUST) for information on Prosopis, Coleen Mannheimer for commenting on this article and for photos, Petra Mutota for alien poster.

References:

CRC 2003. Weed Management Guide.

Klaassen E & Kwembeya E 2013. A checklist of Namibian indigenous and naturalised plants. Occasional Contributions 5, NBRI.

Pyšek et al. 2004. Alien plants in checklists and floras: towards better communication between taxonomists and ecologists. Taxon 53(1):131-143.

 
Join NEWS