The invasive alien prosopis species is spreading at an alarmingly rapid pace. It is already covering 1,8 million hectares of grazing rangelands, and based on the present spreading rate of eighteen percent per year, the extent of the invasion can double every five to eight years.
“The rapid spread of prosopis in South Africa is a major catastrophe,” says Ken Coetzee of Conservation Management Services. “It is equally clear that the problem is not being as effectively addressed as it should be. Many millions of hectares of grazing rangelands, rivers, wetlands, and groundwater are at risk. Without human intervention, much of our valuable rangelands will forever be transformed into useless, environmentally damaged prosopis monocultures.”
Based in George in the Western Cape, Ken recently travelled to Loe-riesfontein in the Northern Cape with the purpose of assessing the state of vegetation. “Travelling anywhere north and west of Beaufort West reveals an increasingly dominant feature in the landscape, a feature that does not belong, namely the invasive alien prosopis or honey mesquite tree (Prosopis glandulosa var. torreyana).
“Previously this plant could be observed as occasional trees along some of the watercourses, but now they are completely dominating the drainages and the land far beyond them.”
According to Ken, these plants have benefitted from degraded veld conditions, and exacerbated by drought, they are here to stay. They form dense thickets to the almost complete exclusion of the locally indigenous vegetation, and in some areas, they are the only plants to be seen along the roads.
A young Prosopis glandulosa shrub that has taken root in the unforgiving soil away from the dense infestation along a watercourse. Photo: Ken Coetzee
Origins of prosopis
Prosopis, which has about 45 species, is a native plant of north-eastern Mexico and southwestern USA. The plant was introduced to many countries and has become a global problem because of its invasion of rangelands.
Prosopis is an extremely successful invader. Originating from an arid region, it is well equipped to survive drought and it flourishes on overgrazed land, especially during a drought.
It has invaded rangeland in the USA, Australia, India, Hawaii, and the Middle East, as well as in East and northeast Africa. Some species are indigenous to Africa, and especially in Kenya, Prosopis juliflora is a problem. Prosopis africanus, or djembe, is also a native prosopis to Africa.
Prosopis was introduced to Southwest Africa (now Namibia) in 1897 to provide shade and feed for livestock. By 1912, it had spread to the veld, and by 1960, dense stands that were difficult to control, were recognised as a problem.
In South Africa prosopis was planted for shade and as windbreaks in Upington in the Northern Cape, from where it spread to the Kalahari Thornveld and the Great Karoo.
Prosopis glandulosa var. torreyana (honey mesquite) is the main invasive Prosopis species in Southern Africa, but it can also hybridize with Prosopis velutina, which makes accurate identification difficult.
Both species and the hybrids are equally invasive. In South Africa, prosopis is classified as a Class 1 invader species because of its aggressive spread in natural rangeland and the threat to water resources in arid areas.
Prosopis glandulosa leaves and sizes. (Source: swbiodiversity.org)
Impact on the environment
The whole ecosystem, including water, vegetation and therefore grazing, as well as pollination, is seriously affected by the prosopis invasion.
The most important negative impact of prosopis invasion is the loss of soil moisture, which endangers indigenous vegetation and leave less food and water for livestock and game.
Encroachment reduces the density, species richness and diversity of indigenous woody plants, as well as the cover of indigenous herbaceous plants. Grasses and herbs are affected by the allelopathy characteristic of prosopis. Allelopathy is the releasing of a substances by one plant that inhibit the germination and growth of another species.
The loss of species richness has a negative impact on the diversity of birds and invertebrates, including pollinating species.
Soil erosion is typical of prosopis invasions. The loss of protective indigenous groundcover underneath prosopis invasions accelerate soil erosion as the bare soil is more easily washed away by run-off water whenever it rains.
Prosopis invasions have been known to cause the demise of keystone tree species like kameeldoring (Acacia erioloba) and soetdoring (Vachellia karroo).
The loss of biodiversity impacts on the livelihood of rural communities. The loss of abovementioned keystone trees in arid systems, as well as the loss of grazing and browsing for livestock and game, irreversibly affects the sustainability of people who depend on natural resources.
Prosopis glandulosa seed pods. (Source: veerubberus.com)
Urgent action needed
Ken believes a new awareness and a fresh approach to the invasion is needed and that landowners need to be advised, helped, and encouraged to act, especially with new and spreading invasions. Organised agriculture, including farmers associations and unions should create awareness of the threat.
“One gets the feeling that nobody seems to be really concerned or to care enough about this problem,” says Ken. “This can be because of ignorance, a lack of awareness, or simply an apathetic attitude to an apparently unsolvable problem.
“I have observed that many landowners who have low-density, scattered prosopis on their land fail to recognise the impending threat or to be moved to control these precursors to the invasion that will follow.”
Low-density prosopis trees need to be quickly and continually removed before they become dense invasions that are extremely costly and nearly impossible to control.
“It is quite clear that a fresh approach to the Prosopis problem is urgent. It must be an approach that will inspire both land management authorities and individual landowners to act.
“We need videos about the problem, the threat, and the control actions required. Future implications of doing nothing must be clearly and visually expressed in the media and to those that have the responsibility of guiding agriculture, nature conservation and ultimately governing the country.
More aggressive biocontrol
“Also, research and implementation of additional, more aggressive biocontrol agents is urgently required as this is probably the most practical way to effectively address the areas that are already densely invaded.”
Value adding options for the manufacture of charcoal, biochar, activated charcoal and animal feed must be investigated, researched, and implemented. “Attaching a value to the plant will certainly help control the invasion,” Ken believes.
Research has shown that local rural communities prefer to use indigenous plants for firewood, because the coals last longer. The thorns of the prosopis also make it more difficult to harvest, and the smoke has an unpleasant smell. The commercial use as firewood is clearly not a viable option.
Neither is the use in silviculture, as the wood is vulnerable to wood-boring beetles, and the timber is not as widely used as elsewhere, such as in India.
“In a nutshell, a more effective biocontrol solution to the rampant spread of prosopis is really urgent,” Ken concludes.
In the next issue we shall look at the origins and distribution of prosopis, as well as the urgent reasons why these infestations must be restrained.
For more information contact Ken Coetzee at 076-227-5056 or consken@mweb.co.za
References
Bromilow,C. (2018) Problem plants and alien weeds of Southern Africa. 4th Edition. Briza Publications, Pretoria.
Coetzee,K. (2005) Caring for natural rangelands. University of Kwa-Zulu Natal Press, Scottsville.
Henderson, L. (2001) Alien weeds and invasive plants. Plant Protection Research Institute, Handbook No.12, Agricultural Research Council.
Shackleton,R.T., Le Maitre,D.C., van Wilgen, B.W. & Richardson,D.M. (2015)The impact of invasive alien Prosopis species (mesquite) on native plants in different environments in South Africa. S. African J. Botany, Vol.97 (25-31).
Shackleton,R.T., le Maitre,D.C. & Richardson,D.M. (2015) Use of non-timber forest products from invasive alien Prosopis sp. (mesquite) and native trees in South Africa: Implications for Management. Forest Ecosystems, Open Access, Springer Nature.
Strohbach,B.J., Ntesa,C., Kabajani, M.W., Shidolo, E.K. & DÁlton,C.D. (2015) Prosopis encroachment along the Fish River at Gibeon, Namibia. Dinteria, No. 35(53-73).
Van Wilgen, B.W. & Richardson, D.M. (2014) Challenges and tradeoffs in the management of invasive alien trees. Biol. Invasions. 16 (721-734). Springer Nature, Forest Ecosystems, open access on-line.
Zachariades,C. Hoffman, J.H. & Roberts, A.P. (2011) Biological control of mesquite (Prosopis species) (Fabaceae) in South Africa. African entomology,19 (2): 402 -415).
