The battle against invasive alien species in Namibia

Invasive alien plants and animals are one of the five major threats to global biodiversity. These are defined as non-indigenous species that adapt well to conditions where they are introduced and then spread or flourish rapidly, often in the absence of their natural enemies. If nothing is done to reduce the spread or eradicate these species, they can take over large pieces of agricultural and protected land, cause disease or injuries to both livestock and wildlife, negatively affect indigenous plants and animals, and ultimately damage native ecosystems.

In Namibia, 38 trees or large shrubs, 62 forbs (flowering plants) and several grasses are considered invasive alien plants, 15 of which pose serious economic and ecological threats. Many of the invasive shrubs are cactus species that flourish in Namibia's arid environments, while aquatic weeds can choke the few permanent water sources in the country. Dry riverbeds and other disturbed areas are particularly vulnerable to invasion, which diminishes groundwater.

Several invasive alien animal species have been recorded in Namibia, including seven fishes, three reptiles, four birds, four snails, one crayfish, two mussels and seven mammals. Eleven of these are considered serious threats to our native species. Besides outcompeting local species for space and resources (often the main threat), some invasive alien animals are similar enough to our native animals to interbreed with them and thus pose a genetic problem.

Winning the battle against invasive alien species requires a multi-pronged strategy that involves the public, government, and non-governmental partners. Most of the work until now has focused on controlling plant species, since these are currently a bigger problem for Namibia than the animal species. The four main prongs of this strategy are:

1. creating public awareness of the problem;
2. mapping the current distribution of invasive aliens;
3. manually removing problematic species;
4. deploying biological control agents.

Creating awareness

Public awareness requires providing accurate information to the public using as many outlets and formats as possible. Popular articles, books and book chapters, posters and lectures have all been used to explain why invasive alien species are a problem and what the public can do to help. It is especially important that people do not unwittingly plant invasives in their gardens or release invasive animals into the wild (e.g. releasing fish into a dam or river).

The Botanical Society of Namibia has taken the lead in creating awareness by producing the Nasty Nine – Alien Invasive Species poster and a series of factsheets containing more detailed information. The Tree Atlas of Namibia, Trees and Shrubs of Namibia (2009 and 2018 editions) and the 2022 edition of the Atlas of Namibia all include sections on invasive alien plants. The Botanical Society is currently preparing information and maps on their website to highlight some of the most problematic plant species and areas of Namibia that are particularly vulnerable to plant invasion.

Identifying and mapping invasive alien plants

Another way in which the public can help in the battle against invasive species is to record sightings of these plants by using the Atlasing in Namibia mobile application. The app includes clear photographs and brief descriptions of 36 invasive alien plant species to help users identify the species of concern. Verified records are then shown on maps on the atlasing part of the Namibian Environmental Information Service. These maps allow conservationists to identify invasive hotspots, keep track of the spread of species and develop plans for controlling them.

To complement the app, a clearly illustrated field guide with preliminary maps of an expanded list of the 50 most invasive or potentially invasive alien plants in Namibia was prepared by Petra Mutota, an Honours student at the Namibia University of Science and Technology (NUST), with the assistance of Coleen Mannheimer and Ben Strobach. A poster covering the same species has also been produced. The field guide and poster will be published once funding is secured.

Manually controlling invasive plants

The Cactus Clean-Up team, led by Gunhild Voigts, regularly clears stands of cacti and other alien plants around Windhoek. This enthusiastic team has managed to clear invasive cacti in several areas and has persuaded many gardeners to plant non-invasive indigenous plants rather than cacti (see text box for more information).

Other more sporadic initiatives in the city and elsewhere in Namibia tend to be undertaken by volunteers, rangers in our national parks, municipalities and youth from schools, environmental clubs and environmental education centres, often as part of the President's annual clean-up day. The 3rd Windhoek Scouts have organised several invasive alien clearing days around Avis Dam.

Biological Control

Biological control (or biocontrol) involves introducing specifically identified natural enemies of the targeted invasive alien species. To ensure that these natural enemies do not become invasive, they are rigorously tested in hermetically sealed laboratory conditions, usually for at least three years, before they are introduced to the alien plants in the field. For plants, most biological control agents are insects that attack particular plant species, thus preventing the plant's growth and reducing its ability to reproduce and spread.

The earliest attempt at biological control of an invasive alien plant in Namibia targeted Kariba weed (Salvinia molesta), first recorded in the Zambezi River in 1948. By the 1960s it made headlines when dense mats formed on the newly impounded Kariba Dam. Ten years later similar mats were blocking channels in the Eastern Zambezi (formerly Caprivi) wetlands, interfering with the movement of boats and reducing fish catches. These mats block the sunlight needed by submerged plants and as the weeds decay they deplete the oxygen available for fish and other aquatic species.

In the early 1980s, 644 weevils of the specific species known to attack Kariba weed were imported from Australia, where they had been thoroughly tested, and used effectively. By 1998, 98% of the sites were free of the weed. Today, some plants are still found on the edges of lakes and rivers, but they are no longer a problem and insects can be quickly released if the weed gets out of control again.

In the mid-1980s, a cochineal insect was introduced to Namibia from South Africa to control the sweet prickly pear cactus (Opuntia ficus-indica), which had become a major problem particularly in the Otavi mountain area. The cochineal was highly effective and today this cactus is at such a low population level in that area that it is not of ecological concern.

More than thirty years after these success stories, in 2019, the Botanical Society and the Namibian Chamber of the Environment (NCE) received government permission (environmental clearance certificate and phytosanitary import permit) to import from South Africa and release three biocontrol agents to target three particularly invasive cactus species. Two cochineal insect species targeted the imbricated or devil's rope cactus (Cylindropuntia imbricata) and the common pest pear cactus (Opuntia stricta) respectively, while a mealybug species was released on the midnight lady or snake cactus (Harrisia pomanensis).

Dr Iain Paterson from the Centre of Biological Control of Rhodes University in South Africa brought the biocontrol agents and demonstrated how to release these species at two selected sites in Windhoek. In the three years since then, Lucrensia Ndeilitunga, Aktofel Elago and Monica Nande, NUST students funded by the Botanical Society and Rhodes University, have been monitoring the effectiveness of the cochineal insects and moving them from one cactus stand to another when necessary (these insects cannot move over large cactus-free areas).

The results for the cochineal biocontrol agent on the devil's rope cactus have been mixed. The insect takes time to become established, especially on woody old cacti, on plants growing on steep slopes and in areas that are too exposed or are vulnerable to flooding. After two years, 90-100% of the cladodes (the fleshy cactus leaves) on most of this cactus species were covered by the cochineal insects at seven of the ten study sites. The number of small cactus plants had been drastically reduced and any remaining small plants were covered in cochineal. By mid-2022, most plants were reduced in height by 5-20 cm at the seven sites where the cochineal is well established.

Despite the success recorded to date for most of the study sites, we cannot afford to become complacent. In August 2022, the researchers were surprised and disappointed to find that the cactus stands at the original release site had started to recover and were growing strongly with low infestation rates. The likely causes are a combination of intense rain early in 2022 and abnormally severe frost this winter. The absence of small plant regrowth from dropped cladodes indicates that the biocontrol is working. It also shows that ongoing monitoring is important and that reinfestation with the biocontrol insect may be necessary from time to time.

Now that many of the cacti at the survey sites are well covered by cochineal, the next step is to combine manual and biological control by felling the large cacti and leaving their cochineal infested remains at the site. Thus, the biocontrol insect will continue infecting the smaller plants, while our monitoring exercises continue to keep track of their progress.

Next steps: developing a strategy and action plan

A more concerted, integrated approach is needed to fully control the spread of invasive alien species across Namibia. NCE therefore invited those with experience and interest in the control of invasive alien species, from government, local authorities, NGOs and academia, to discuss a strategy to address invasive alien species.

All agreed on the need for an Action Plan for invasive alien species with four clear components:

1. preventing the import of invasive and potentially invasive alien species into Namibia;
2. stopping the spread of existing invasive species within Namibia;
3. actively managing, controlling or eradicating (where possible) these species; and
4. establishing an inclusive institutional mechanism to implement the plan.

The Invasive Alien Species Working Group will be established to allow cooperation among all of the key role players that will be implementing these activities. As it involves the government and all non-governmental organisations working on this problem, the Working Group will coordinate activities and seek joint funding to implement the action plan. By joining forces and reaching out to the public for help, we are taking the battle against invasive alien species to the next level.