Author: Boudine Kruger | Article adapted from Ken Coetzee’s “Prosopis, a Deadly Green Cancer”
The invasion caused by the Prosopis tree damages the ecosystem by affecting resources like water supply and grazing. The diversity and biology of smaller animals like birds and invertebrates are also negatively affected by the Prosopis invasion, ultimately affecting pollination in these areas.
The invasion reduces the density and diversity of indigenous species. Some important tree species like Vachellia karroo and Acacia erioloba have also succumbed to the invasion. Both of these are important keystone plants for farming in arid systems, and the loss of grazing and browsing for livestock irreversibly changes the fortunes of the people who traditionally depended on these natural resources.
However, the greatest negative impact of the Prosopis invasion is unquestionably the loss of soil moisture, making conditions drier for native plants and reducing water availability for livestock and game. This loss of ground cover also creates an opportunity for accelerated soil erosion as runoff water carries off soil surfaces.
How can we stop the invasion
To effectively control the spread of Prosopis, awareness of the invasion must be made more prevalent. If landowners are encouraged to prevent and stop infestations before they become widespread, these invasions will be easier to eradicate. Once land is widely infested with Prosopis, it can be too difficult and expensive to remove them all effectively.
Landowners must know how to effectively remove these plants to avoid inadequate removal, which will lead to resprouting that will form a dense multi-stemmed tree with two or three times as many flowers as the original.
Some of the best eradication methods include:
This method uses manual efforts, such as clear-cutting the trees as close to the ground as possible. The tree stumps will then be treated with a suitable herbicide to ensure that the trees can’t grow again. The herbicide used to treat the stumps is applied with a paintbrush carefully to prevent it from affecting the indigenous plant life around the stump. This works well for smaller density infestations. This is an effective way of keeping areas clear of the Prosopis invasion for extended periods. In denser areas of infestation, it is beneficial to clear out the lightly invaded areas first, as this causes the highly infested areas to become isolated patches that are easier to deal with.
With this method, it is also crucial that the occasional resprouts be monitored and newly germinated seeds are eradicated. The initial control of the denser mature Prosopis will be labour intensive and expensive, but the maintenance level of control will reduce in cost with every follow-up action. If, however, there is no follow-up clearing plan, the initial eradication would have been for nought.
One of the biggest reasons invasive plants flourish so well is that they grow in the absence of natural enemies. Biological control refers to managing these plants by reintroducing them to natural enemies collected from the area where the plant originated from.
Using natural plant enemies, such as insects, can be very effective to control invasive plant species. There have been several successful instances in South Africa where natural enemies have been introduced and invasive plants have been almost entirely eradicated.
Of course, this type of eradication must be closely monitored. The chosen biocontrol agent must be host-specific, meaning that it won’t eradicate any closely related indigenous species. If this biocontrol agent isn’t closely monitored, it stands a chance of becoming a new invasive species of its own.
Choosing the proper biocontrol agent can be very time-consuming and expensive. The Plant Protection Research Institution of South Africa must do extensive research to clear any potential biocontrol agents. Once cleared, this can be a very affordable and effective method of eradicating an invasive plant species, especially when introduced alongside chemical control methods and mechanical methods.
Additionally, goats can also be an effective domesticated control agent – it has been found that goats eat more of the Prosopis plant’s seed than any other livestock and game. The majority of the seeds also don’t survive the goat’s digestive system.
Clearly, the rapid spread of Prosopis in South Africa is a major catastrophe. Equally clear is that the problem is not being effectively addressed as it should be and that many millions of hectares of grazing rangelands, rivers, wetlands and groundwaters are at risk.There are, however, methods of tackling this major threat, as detailed above, but it all starts with raising awareness and educating landowners and the broader public. Please share this article and tell others to help claim back our landscapes from Prosopis.