Arne Witt, coordinator for invasive species at the Centre for Agriculture and Bioscience (CABI), Africa. Photo credit: CABI

Q+A: The backstory on invasive plant species with CABI’s Arne Witt

"Homogynene" a risk

NAIROBI (Landscape News) — Invasive plant species pose a major threat to biodiversity, water resources, food security, economic development and human health – and this threat is expected to increase with climate change and the additional trade and travel associated with globalizing economies.

How can we prevent invasions and protect the livelihoods of the rural poor who have most to lose from the destructive impacts of these plants?

The solution lies in integrated approaches, according to Arne Witt, coordinator for invasive species at the Center for Agriculture and Bioscience International (CABI) Africa, who will speak at the upcoming Global Landscapes Forum in Nairobi. Holistic efforts developed in coordination with governments and local communities, he suggests, will be critical as invasive plant species become more pervasive.

Arne spoke with Landscape News about prevention, early detection, and control strategies, drawing on his 25 years of experience researching the management of invasive species.

Q: How do invasive plant species spread to colonize new areas, and what advantages do they have over indigenous plants? 

 A: Exotic species can be introduced intentionally or accidentally. For example, through the importation of plants for ornamental purposes and agroforestry, or as contaminants in imported goods, mainly fruit and vegetables. Once introduced, they displace crops by competing for resources. In the absence of natural enemies, invasive plants can grow rapidly and they produce copious amounts of propagules, such as seeds and corms. They are often hardy, tolerate a wide range of soil and climatic conditions, and make efficient use of limited resources. Many invasive plant species are also unpalatable or toxic – meaning they are not eaten by livestock or wildlife – and they do not require specialized pollinators since their flowers are easily pollinated by a multitude of birds, insects, and other organisms. Finally, some can also release chemicals into the soil – a process known as allelopathy – which prevents other plants from growing.

Q: How can invasive species negatively impact rural communities?

A: Invasive alien plants pose a menacing threat to livelihoods and economic progress, especially in the developing world, where most people are dependent on natural resources for their survival. They negatively alter natural ecosystems and the services and benefits they provide; they act as secondary hosts for crop pests – harboring pests and diseases that attack crops; and they also interfere with harvesting – thorny or woody weeds can make it extremely difficult to harvest crops. Studies have suggested that the effects on farmer productivity can be significant: weeds could cause crop-yield losses of about 10 percent in less developed countries, and 25 per cent in the least developed countries. Their effects can also be devastating for livestock. One study found that without the management of invasive plants, natural grazing capacity could be reduced by 71 percent in South Africa.

There are additional impacts on human health. Paper mulberry, a tree that is invasive in Ghana, Uganda, Pakistan, and elsewhere, for instance, produces vast amounts of pollen that exacerbates asthma in sufferers, often resulting in fatalities. Other invasive plants use copious amounts of water or block water courses, often leading to flooding, while others cause riverbank erosion. Finally, invasive plant species reduce biodiversity – in fact many consider them to pose the second biggest threat to biodiversity worldwide, after direct habitat destruction.

Q: What are the most practical and effective defenses against invasive species?

Prevention is the first line of defence and the most cost-effective strategy. As the saying goes “an ounce of prevention is worth a pound of cure.” Undertaking risk assessments is important: authorities should consider the biology of the species, the characteristics of the environment it’s being introduced to, and whether the target species is recorded as invasive elsewhere. Improving measures at ports to prevent the accidental introduction of exotic plants that may become invasive is also sensible. If preventative measures fail, however, good surveillance or early detection and rapid response programs (EDRR) are needed to detect new invasions and quickly eradicate them.

In situations where invasive alien species are already widely distributed, other options need to be considered: physical removal; cultural control methods such as flooding, grazing or burning; and chemical control. Where possible, these strategies should be followed by land restoration/rehabilitation. Biological control, one of the most cost-effective interventions should also be considered, especially in resource-poor countries. This involves sourcing a potential agent in the invasive plant’s country of origin – an insect or pathogen – testing the agent to ensure it is not a threat to native plants or crops, and then releasing the agent into the field.

Studies in the United States and South Africa suggest the most effective approaches are likely to be a combination of physical, chemical and biological control strategies – commonly known as integrated pest management. Efforts need to be strategic: demarcate target areas, calculate costs associated with control activities, and develop baseline data on the density, distribution, and impacts of target species to monitor the effectiveness of control operations. Also, contain less dense or isolated populations first; treat higher areas before lower-lying areas; and remove plants before they flower and produce seeds.

Q: A lack of support from local communities could limit the long-term success of management activities. How important is local involvement?

A: It is extremely important. Local communities have a good understanding of the plants growing within their environment, and they can recognize new species when they arrive and become established. They are our ears and eyes, and if they are provided with the necessary tools, their capacity could be enhanced considerably. We also need to raise public awareness about unsustainable practices such as over-grazing which make areas more vulnerable to invasion; the over-use of herbicides which encourages herbicide resistance and threatens the environment and human health; and the risks associated with some of the plants we purchase for our gardens – some of the most invasive species are actually grown as ornamentals and hedge plants.

Q: How can farmers and rural communities be incentivized to participate in preventative measures and strengthen defenses against invasion?

A: The key is reliable information. People need to be aware of the impacts of invasive species and how best to manage them, and we need to be consistent in our messaging on the costs and benefits of invasive plants. For instance, some well-meaning NGOs (non-governmental organizations) and development agencies have encouraged the use of woody weeds as a source of fuelwood, but communities have not been informed about their negative impacts on grazing and water resources. It is also important to provide people with the tools they need to identify invasive alien plants and reduce poor land-use practices that leave their lands vulnerable to invasion. If there is disagreement among the community as to the costs and benefits of the target species, it is recommended that a cost–benefit analysis be undertaken.

Q: What role can government play?

A: Communities can only do so much. It is ultimately up to governments to develop and implement an action plan which details how awareness is going to be created, how capacity is going to be built, and what authorities are going to do with regard to prevention, EDRR and control. Governments also need to develop and implement biosecurity legislation to prevent the introduction of invasive plant species. Both Australia and New Zealand are considered leaders in this regard. They have well developed invasive species plans that prevent the importation of animals or plants without first declaring them, which is not the case, for instance, in much of Africa. Governments also need to look at how they are going to finance management interventions – potentially through taxes or levies.

Q: Is the threat of invasive species becoming more significant as a result of climate change? If so, why? 

A: Yes. Climate change will drive more invasions. Increased disturbance as a result of fires, droughts and floods, for instance, will make habitats more prone to invasion. Increased carbon dioxide levels will also benefit many invasive species. This, in conjunction with increased trade, travel and transport, is going to make the situation a lot worse. We are rapidly heading towards the “homogynene,” whereby most landscapes around the world will be dominated by only a handful of species.

Learn more about invasive species at Global Landscapes Forum in Nairobi on Thurs. Aug. 30 at 9:00 a.m. local time (6:00 a.m. GMT). Attend discussion forum Invasive alien plants, land degradation and restoration in person or online. Learn more here.

Article tags

agriculturealien speciesaliensArne WittCABICenter for Agriculture and Bioscience Internationalclimate changeforestsGLFNairobi2018homogyneneinvasive specieslandscapesrestorationseeds



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