GEORGE NEWS - Alien vegetation has a devastating impact on the environment with repercussions for human existence. 

Even if a contractor does a poor job at clearing alien invasives, taking little care of the land, it is still better than doing nothing, says Wilderness resident and international expert on invasive alien species (IAS), Dr Arne Witt.

He warns that we are facing a massive extinction crisis unless invasive plants are managed. 

"I can say, based on my experience from working around the world, that we are heading to a situation where most of our landscapes will be dominated by invasive plants. I call it the Homogynene. The whole of the Garden Route will be dominated by pines, wattles, lantana, etc. There will be very little biodiversity, there will be much less water, and we will have frequent and very intensive fires."

Witt responded to a Wilderness Heights resident who aired concern over indiscriminate clearance of alien trees on a municipal property where no vegetation was left behind.

The bigger picture: Green deserts

Witt says critics of invasive species management need to look at the bigger picture. "Invasive alien plants cost the country billions in terms of their impacts and management, affecting biodiversity, tourism, crop and livestock production, human and animal health, water resources, and economic development in general.

"Furthermore, alien trees are preventing the regeneration of large native trees that have been lost due to deforestation. Very little to nothing grows beneath alien trees such as gum, pine or wattle, so they do not necessarily prevent erosion. 

"Although gums are known to be a source of nectar and pollen for bees, wattle is considered to be a very poor resource, and often results in the production of very bitter honey. There are a myriad of native plant species that produce copious amounts of pollen / nectar that are themselves displaced by wattles and gums."

It has been postulated that colony collapse disorder (CCD) among bees could be attributed to a loss of diversity in pollen / nectar sources. "Just like we need a diverse diet to be healthy and build up our resistance to disease, so do bees. A landscape dominated by wattles and gums cannot provide that. Natural vegetation will provide a variety of sources, all flowering at different times of the year."

Wattles, pines, lantana, etc. are invasive because they have no natural enemies. Although gums are known to be a source of nectar and pollen for bees, wattle is considered to be a very poor resource, and often results in the production of very bitter honey. There are a myriad of native plant species that produce copious amounts of pollen / nectar that are themselves displaced by wattles and gums."

This means that there are very few plant-eating insects associated with them, which most birds, especially hatchlings, even of fruit-eating birds, depend on for their growth and survival. 

"Invaded areas are in effect green deserts. The wildlife do not feed on the leaves of most aliens. In fact, many alien plants are toxic. Most birds hardly, if ever, depend solely on alien trees for nesting - I have plenty of birds nesting on my property and I have no aliens. Natural habitats, free of aliens, provide for much better opportunities for wildlife in terms of food and shelter than invaded landscapes," says Witt.After: Indigenous vegetation made a comeback on this property after Dr Arne Witt and his team had cleared a forest of alien trees.

Biodiversity, fire and water

In terms of biodiversity conservation, IAS are often ranked as having the biggest negative impact on biodiversity in the world. Since the year 1600, 39% of animal extinctions arose mainly from the introduction of alien species, according to Witt.

Aliens also intensify wildfires and are sucking the country dry. "Most of our streams and rivers, that used to be barriers to fire, have become like wicks in the landscape due to invasion by aliens," says Witt.

Losses in water runoff in South Africa, attributed to infestations of black wattle, amount to an estimated 577 million cubic metres of water annually.

In the Western Cape, stream-flow from invaded fynbos water catchments has declined by 55%.

Dr Arne Witt has extensive experience researching the biological control of invasive alien species and has a team that is clearing land in the Garden Route on an ongoing basis.

Dr Arne Witt is IAS coordinator for Cabi (Centre for Agriculture and Bioscience International) and an international project coordinator and technical advisor on a number of Global Environment Facility / United Nations Environment Facility (Gef/Unep) projects in Africa, Asia and the Caribbean.

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