By Caroline Kibii
Restoration of most African ecosystems has been impeded by the emergence and dominance of alien plant species. This introduction of invasive plant species, whether intentional or unintentional, has threatened the ability of native flora and fauna to thrive and is also a hindrance to the progression of human livelihoods.
The UN Food and Agriculture Organization (FAO) estimates that two-thirds of Africa’s arable land will be lost by 2030 due to desertification. The fact that invasive plant species do not give a chance to other plants to germinate or grow, coupled with climate change and intense land-uses, expands the scale of arid lands and accelerates desertification in some parts.
One current example is in Kenya, where landscapes have been invaded by notorious plants that have obstructed agricultural activities, degraded soils, and replaced native vegetation and trees. In 2010, Kenya declared the annual herb Parthenium hysterophorus as being noxious, as it has predominated crucial areas such as the Nairobi National Park and surrounding ecosystems.
According to the Kenya Wildlife Service, Nairobi National Park, which is home to over 100 mammal species and about 400 migratory and endemic bird species, should be natural and free of alien plants to promote biological diversity.
However, the park has been heavily invaded by the Parthenium weed, which is currently controlled through hand-weeding – a time-consuming and laborious process that requires regular uprooting to be eradicated.
The weeds must be uprooted before they flower and seed, as once they flower, they will have produced pollen grains that aid their reproduction, and once they seed, the seeds can easily be dispersed by wind, animals, water, people and vehicles. A single plant is estimated to produce 10,000 to 25,000 seeds.
Research shows that Parthenium weed colonizes mostly disturbed and degraded areas, making such environments even more difficult to restore. Paradoxically, to reduce Parthenium seed dispersal and seed loads, the areas should be restored.
Similarly, Prosopis juliflora, a woody weed – often a thorny shrub or tree – that can grow up to 15 meters and is famously known as Mathenge plant in Kenya, has invaded the country’s arid and semi-arid regions, such as Isiolo, Marsabit, Baringo and Samburu. In the wetland and grassland areas surrounding Lake Baringo, Mathenge has heavily colonized the landscape, limiting the chances of other trees, plants or crops to grow, reducing tree cover. The Mathenge plant has deep roots, is fast-maturing and has resistance to harsh conditions; hence it can survive through drought and regenerate afterward.
The plant is not only an environmental problem but also a deterrent to the social and economic well-being of the residents within the invaded areas. The aggression and dominance of the species in Baringo has promoted intensive charcoal business as a source of livelihood because the residents cannot grow crops in invaded lands. Charcoal burning leads to the destruction of the few available Indigenous trees, such as the acacia tree species.
Over time, Baringo residents are learning to embrace the invasive plant species as a source of shade, poles, fencing (with its twigs) and animal feeds (from its seeds, pods, and leaves), notwithstanding the controversies surrounding its weakening of dental condition in goats.
But environmental tradeoffs are hard to reconcile with these benefits. Mathenge makes the land prone to floods and soil erosion in the lowlands and areas around wetlands as it cannot hold the soil firmly. In the grasslands, it hampers the restoration, as its removal is necessary to pave the way for reforestation using Indigenous trees, but doing so is cumbersome.
The lack of the capacity to eradicate invasive plants in Kenya and other African countries threatens already degraded ecosystems, making restoration of rangelands and arable lands difficult. However, acquiring knowledge about the invasive plants and devising ways to cope with them is the first step toward eradication of alien species.
Caroline Kibii is an environmental scientist, researcher and freelance science writer. She is the founder of Enviro Wild Initiative, an organization that inform and engage on environmental issues.