Is the Clearing of Alien/Invasive Species in South Africa Cost-Effective?

funny looking cartoon alien in his flying saucer; click for larger cute alien pilot in flying saucer free sci-fi clipart

An alien invasion?! Of course not. Just an alien/invasive species, the Acacia mearnsii.

           Alien species, also known as invasive species, are species that are not native to a particular region. Alien species can cause problems in the region’s ecosystem such as the modification of niches, the unique roles specific organisms play in the ecosystem. This can lead to some organisms becoming extinct if their niche no longer exists and they are unable to adapt to the new environment and create a new niche. Alien species can have an adaptation that allows them to outcompete native species and cause them to become extinct. Because of this, alien species are a major threat to the biodiversity and proper functioning of an ecosystem.

Four researchers, Matthew M. McConnachie (Nelson Mandela Metropolitan University, Port Elizabeth, South Africa), Richard M. Cowling (Nelson Mandela Metropolitan University), Brian W. van Wilgen (CSIR Natural Resources and the Environment, Stellenbosch, South Africa), and Dominic A. McConnachie (Massachusetts Institute of Technology, Cambridge, MA, USA) analyzed data for a period of six years (2002-2008) from the government-funded invasive alien plant control program in South Africa called “Working for Water” (WfW). They found that the current methods of alien plant clearing have low cost-effectiveness values. This means that these methods are not efficiently controlling the alien species populations in the areas of South Africa that were tested, and improvements need to be made to protect the ecosystem and native species.

It is an important task of conservation biology to control the presence of alien species once they have been introduced to an ecosystem. The best method of controlling alien species is to prevent their introduction to an ecosystem in the first place, but often, biologists and ecologists must find ways to control alien species that are already present in the ecosystem. The plans that are created must take into account economical, social, and political factors in addition to the biological and ecological factors that relate to the ecosystem. The WfW program in South Africa is trying to control the alien species Acacia mearnsii, which is commonly known as the black wattle tree. To do this, project managers for each section of South Africa where the black wattle tree is present hire unemployed people for their teams. Unemployed people are used because one of the goals of WfW is to reduce poverty. The teams go to the designated section and chop down all of the black wattle trees and then apply herbicide to the cut stems to kill the tree and prevent coppicing, which is the growth of new trees from the cut stem. In most cases, additional treatments of herbicide are necessary to kill any coppicing and new seedlings.

The researchers compared initial alien species plant cover (pre-treatment), final alien species plant cover (post-treatment), and the cost of the project to determine the cost-effectiveness of this method. In the two regions that they analyzed, the method was found to be extremely inefficient. Previous estimates had suggested that most of the black wattle trees could be removed from one area in about one year, while this study found that anywhere from 50-700 years would be needed if the removal of the black wattle trees proceeded at the current rate. In terms of the estimated cost for these projects, this study found that the cost would be 2.4 times greater than the cost that had been estimated in the past. Clearly, this method is not an efficient way to remove the invasive black wattle trees. It is only slowing down the spread of the black wattle trees, instead of completely preventing it.

Some ways to improve this method involve prioritizing which areas have the highest risk of ecosystem destruction and loss of biodiversity and target these areas first. Another improvement would be to use biological control solutions, such as insects or fungi that can prevent the reproduction of the black wattle trees. A third improvement would be to improve the management of the program. Much of the procedures were not performed thoroughly, so the black wattle trees were able to grow back after the treatments had been performed. And lastly, how to deal with private lands needs to be determined. Land owners do not always continue the preventative measures that have been put in place, and as a result, the black wattle trees return to the area.

Preventing the spread of alien species is a very important goal because the spread of alien species can result in habitat degradation and species extinctions. Preventing habitat degradation and species extinctions are two of the major goals of conservation biology. If a successful method of controlling the invasive black wattle tree is found in South Africa, this method could be applied to other invasive species around the world. An example of an invasive species that is a tree in Texas is the Chinese tallow tree. If the population of the Chinese tallow tree is reduced, it is possible that native species whose populations have become smaller since the introduction of the Chinese tallow tree will be able to expand their populations. This could also potentially restore the previous habitat conditions. Controlling alien species leads to healthier native populations and ecosystems.

By Sarah Ali

Works Cited


McConnachie, Matthew M., Richard M. Cowling, Brian W. Van Wilgen, and Dominic A. McConnachie. “Evaluating the Cost-effectiveness of Invasive Alien Plant Clearing: A Case Study from South Africa.” Biological Conservation 155 (2012): 128-35.


Alien Spaceship Clipart. Digital image. Aliens & Their Spacecraft Art Collection of Clipart Alien   Spaceships, Earth Invaders Flying Saucers, Strange Cool Outer Space ETs. Advantage Business Designs, n.d. Web. 18 Sept. 2012. <;.

Starr, Forest, and Kim Starr. File:Acacia mearnsii blossoms.jpg. Digital image. Wikipedia. N.p., 24 Dec. 2005. Web. 18 Sept. 2012.


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