The metres-high shrub Chromolaena odorata is a major threat to humans and the environment in South Africa. The South American shrub species has turned into what would be an unimaginable plague for Europeans – it’s overwhelming complete savannas and river beds, consuming vast amounts of the scarce water and making massive tracts of grasslands unusable. This has been revealed by research conducted by biologist Mariska te Beest, who will be awarded a PhD by the University of Groningen on 18 June 2010.
How the exotic species Chromolaena odorata (also known as triffid weed) got to South Africa is a mystery. Perhaps somewhere around 1940 some seeds hitched a ride in animal feed from South America, but it’s also possible that the shrub escaped from the Cape Town botanical gardens and spread across the rest of South Africa. The whole of the east coast is covered with it in the meantime, and it has also been spotted in Mozambique and Tanzania. The invasion is a disaster, because wherever chromolaena is, you find a two-metre high, impenetrable ‘wall’ of branches and virtually nothing else can grow.
University of Groningen PhD student Mariska te Beest investigated why chromolaena is so successful in South Africa. During field studies in the Hluhluwe-iMfolozi Park, she discovered that the semi-woody shrub is not able to overcome the native grasses on its own. It needs the ‘help’ of large herbivores such as the African buffalo and the white rhino. Only once these animals graze a piece of ground bare can the shrub grab its chance.
Once chromolaena has rooted properly, virtually nothing can stop it. Te Beest’s research has revealed that the triffid bush has few natural enemies or competitors. The plant produces an abundance of seeds that can be spread on the wind, it grows relatively fast and has a great capacity to rejuvenate from root stock.
In South Africa, the bush has adapted itself genetically to its environment, which is much drier than its original habitat. Organisms in the soil also play a role in the successful invasion of chromolaena. These organisms appear to stimulate the investment in stalk growth, which means that chromolaena gains a better competitive position for light compared to indigenous plants.This has created a species that needs to spend little energy on defence against enemies and which can concentrate entirely on growth and reproduction, casting the indigenous species literally and figuratively into the shade.
Chromolaena is tackled by chopping, poisoning and then burning the bushes. However, Te Beest has discovered that the approach has to be precise. Only a very intense, hot fire is able to destroy the stalks and seeds still in the ground. If an overwhelmed savanna wood is burnt down by a medium-hot fire, then chromolaena is much better able to recover than the indigenous species and will be a worse problem than ever. Park wardens must therefore be careful about the weather conditions when they want to burn off a piece of overwhelmed ground.
The South African government is well aware of the dangers of chromolaena. It is taking over grasslands, affecting the population in those areas because they no longer have anywhere to graze their stock. The income from tourism is also dropping – the shrub ruins the tourists’ view of and access to the savanna. In addition, it is taking over river beds and consuming vast amounts of precious water. Under the name Working for Water, the government is mobilizing large numbers of the unemployed in the fight against this shrub and the many other invasive exotics in the country.
As long as there is enough money to mobilize large numbers of ‘weed fighters’, the expansion of chromolaena in the savannas can be kept reasonably under control. But the fight will continue, Mariska te Beest states. It will never be possible to eradicate it completely.
Mariska te Beest (Dinxperlo, 1976) studied biology in Wageningen. She conducted her research at the department of Community and Conservation Biology (COCON) of the University of Groningen and was supervised by Prof. Han Olff. She will be awarded a PhD by the Faculty of Mathematics and Natural Sciences. Te Beest currently works as a postdoc at the University of Stellenbosch in South Africa. The title of her thesis is The ideal weed? Understanding the invasion of Chromolaena odorata in a South African savanna.
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