B. W. van Wilgen
6 minute read
10 Nov 2010

Be afraid, the alien invasion is real

B. W. van Wilgen

THE opinion put forward by your correspondent Gloria Keverne (“Destruction of aliens is misguided”, The Witness, November 4) needs to be challenged as it contains statements that are at best misleading, and at worst plain wrong. Although the article is obviously well intentioned, it is in the public...

THE opinion put forward by your correspondent Gloria Keverne (“Destruction of aliens is misguided”, The Witness, November 4) needs to be challenged as it contains statements that are at best misleading, and at worst plain wrong. Although the article is obviously well intentioned, it is in the public interest to challenge these views and to provide some of the facts of the case. It is true that South Africa has invested considerable sums in containing the spread of invasive alien trees, but this is based on good science that strongly demonstrates the damage that invasive species do to the environment, and the considerable environmental benefits associated with control. The statement that “invasion biology’s claims are unproved” can be countered by a host of studies that have demonstrated the declines in insects, birds, small mammals and native plants that accompany alien invasions, not to mention the decreases in water flow from our catchments, depletion of our groundwater resources, and deterioration of our rangelands. Invasive alien species are widely recognised as the second largest threat, after direct habitat destruction, to the conservation of global biodiversity. The claim that “exotics host the normal mix of insects, avian life and understory” is incorrect, and many detailed studies have demonstrated the degree to which this “normal mix” is severely depleted by invasive alien plants.

While the ongoing and accelerating destruction of forests across the globe is certainly a cause for concern, if not alarm, it is misleading to compare this with clearing invasive alien trees that threaten to convert our species-rich grasslands, fynbos shrublands and riparian corridors to relatively sterile monocultures of alien trees. These areas were never forests to start with, and the loss of services they provide when invaded, in terms of grazing, water, harvested products and recreation, is as alarming as rainforest destruction. It is important to note that most alien trees are not invasive, and clearing programmes target only those alien species that are invasive — that is, the ones that produce copious offspring, spread at rapid rates, and swiftly come to dominate the landscape. To suggest that “millions are squandered just so that native plants can evolve without normal competition” suggests a remarkable lack of understanding of the significance of the problem of alien invasions. The exponential increases in human populations and the accelerating volume of trade over the past 100 years have been accompanied by a global mass redistribution of species that is unprecedented in the history of life on Earth. To dismiss the consequences of this phenomenon as “normal competition” is astounding.

Oversimplification of these issues is dangerous, as they are often complex. Take the issue of eucalyptus trees (gums) as an example. The claim that South Africa has embarked on a “eucalyptus eradication campaign” is simply untrue. Gums form an important component of the forestry industry, and they cover over 540 000 hectares in formal plantations. Gums are also found in many informal plantations, as well as in thousands of woodlots and other plantings across the country. A crude analysis of the extent of non-forest plantation gums in South Africa came up with a figure of 2,5 million hectares in 1998. However, with the notable exception of Red River gum (Eucalyptus camaldulensis), gum trees are not aggressively invasive. Clearing efforts therefore aim to remove gums from specific areas only, such as along rivers where they use too much water, or in nature reserves where they compete with native biodiversity. However, millions of hectares of gums will remain a feature of our rural landscapes for a very long time. There is no campaign aimed at eradicating these trees, nor has there ever been one. Eucalypts also demonstrably use more water than indigenous vegetation, and the claim that “eucalypts tested as highly efficient water users in eighties studies” is taken completely out of context. The research concerned showed that eucalypts produced valuable timber in return for their additional water use, and were thus arguably efficient in comparison with other forms of land use that convert water into paying crops, for example maize or sugarcane. They certainly use more water (and are thus less efficient) than indigenous vegetation, for which the forestry industry has to pay a charge for streamflow reduction (currently they are the only form of land use levied in this fashion). The estimate of relative efficiency will also change as the value of water increases with rising demand in a water-stressed country.

Another surprising claim is that “99,9% of small raptors breed in alien trees”, and that clearing aliens “threatens to empty our skies of birds”. While birds do make use of alien trees as nesting sites, it is nowhere near the truth that 99,9% of them do so. Another aspect to this issue is that raptors such as rufous-chested and black sparrowhawks have expanded their distribution and numbers as a result of a huge increase in alien trees, mainly in forestry plantations and well-wooded suburbs (both of which are not targeted for clearing). As their name implies, sparrowhawks prey on other birds, bringing their numbers down, and possibly threatening some bird species as a result. From the point of view of these birds, a decline in sparrowhawk numbers would be a good thing.

The use of biological control to contain alien invasions is also portrayed as dangerous by suggesting that they could become “failed experiments”. Weed biological control, as practised in South Africa, is a safe, environmentally benign, cheap and fully sustainable way of reducing the invasive potential of many alien species. The selected control agents (insects, mites or pathogens) are all what is known as “host-specific” — that is, they attack only the target plant, and are incapable of damaging crops or indeed any species other than the selected target-plant species. Recent studies by the Council for Scientific and Industrial Research (CSIR) have estimated that biological control has effectively protected ecosystem services (such as streamflow runoff, grazing and harvested products) with a combined value of tens of billions of rands in South Africa. The fact that “South Africa boasts the world record for using foreign insects to kill alien plants” is indeed something to be proud of.

Finally, the suggestion that invasive alien plant-clearing programmes are motivated by “huge potential profits”, and influenced by “herbicide and regulatory industries” (whatever a regulatory industry may be) cannot go unchallenged. Having been involved in invasion ecology and management for almost four decades, I can assure readers that there is no truth in these accusations. The motivation for the clearing programme is provided by strong evidence of significant environmental damage associated with these species, and driven by dedicated people whose primary concern, like your correspondent, is for the wellbeing of the planet and for the continued delivery of the ecosystem services that it provides to humanity. Working for Water, the prime mover behind the clearing of invasive alien plants, is a multiple award-winning poverty-relief programme that addresses a significant environmental problem and provides employment to the rural poor. It is based on solid scientific evidence about the detrimental effects of invasive alien trees, and deserves widespread support.


• Dr B. W. van Wilgen is the chief ecologist at CSIR Natural Resources and the Environment.