Examples of expressed public perception
Position adopted by authorities responsible for invasive alien plant
Soil and water resources
Trees bring rain, and felling them will exacerbate droughts and desiccation
(Dix 2005, Wickham 2005).
There is no evidence that increased tree cover has increased rainfall
(Wicht et al. 1969, Calder 1999). In contrast, long-term research indicates that
invasion by alien trees and shrubs increases evapotranspiration and decreases
streamflow (Görgens and van Wilgen 2004).
Felling trees will lead to excessive flooding (Ellis 2009).
Flooding events that triggered the expression of this concern were caused
by abnormally high rainfall (20% of the mean annual rainfall in 48 hours). This
exceeded the previous record rainfall intensity in a 52-year record. The level
of flooding was in line with the intensity of the rainfall event, and would have
taken place regardless of the vegetation cover in the catchment (Hewlett and
Bosch 1984), and should not be attributed to the felling of trees.
Forests, and tree cover in general, are the best way of ensuring soil
conservation and of preventing erosion (Yeld 2004).
There is no evidence of excessive erosion from sites with a cover of
natural (nonforest) fynbos shrublands (Versfeld 1981). The naturally treeless
fynbos shrublands are regarded as the best form of vegetation cover to ensure
sustained yields of high-quality water (Kruger 1977b). Healthy fynbos is
therefore considered sufficient to prevent erosion.Related to this is
the evidence that invasion by alien trees, or the establishment of plantations,
will lead to increases in fire intensity when the areas burn. This can damage
the soil and increase erosion (Lindley et al. 1988, van Wilgen and Scott
2001). As fires are inevitable in these environments (van Wilgen et al. 2010),
any effort to reduce the cover of plantations, and invasions away from
plantations, will reduce the risk of erosion in the longer term.
Felling trees will expose the soil and increase the risk of erosion (Ellis
Felling of trees in plantations will expose the soil to erosion for a
limited time. Experience has shown that sites where plantations have been
cleared do develop a cover of vegetation within a year or two (although the full
complement of species does not establish) (Holmes and Richardson 1999). The risk
of erosion is therefore relatively short-lived, and needs to be assessed against
the risks associated with not clearing the plantations. There is a risk that the
plantations could burn in a wildfire, which will also lead to erosion (Scott et
al. 1998). In addition, and more importantly, leaving the plantations in place
will provide an ongoing source of seeds that will invade adjacent natural
vegetation, perpetuating the problem of invasive alien plant control, and
leading to more widespread risks of erosion over larger areas (van Wilgen and
Biodiversity and ecosystem health
Trees provide the habitat for a range of animal species, including
squirrels and raptors (nesting sites), and should not be cleared.
Some of the species that benefit from trees are themselves introduced alien
species (e.g., the American grey squirrel, Scurius carolinensis), and their
protection or conservation is not a priority given the focus on conserving
indigenous species. Many raptors have colonized areas from which they were
previously absent because of increases in tree cover. These changes would have
cascading negative effects on local biota (Curtis et al. 2005). Because
of the need to protect the highly diverse and often unique original biota,
clearing of invasive trees should receive priority.
Earth’s ecosystems are self-healing, but their ability to continue to
do this is compromised by widespread destruction. Clearing invasive alien plants
only adds to this destruction, and is unwarranted and dangerous (Keverne
Scientific evidence of loss of biodiversity associated with alien plant
invasion, and of the ability of ecosystems to recover after clearing does not
support this view. The evidence shows that invasion can lead to reductions in
species richness (Richardson et al. 1989, van den Berckt 2000, Pryke and Samways
2009), species extinctions (Raimondo et al. 2009), and trophic cascades in which
reductions or extinctions in one species lead to the loss of others (Johnson
Aesthetics and recreational opportunities
Plantations of alien trees provide shade, and are often the only acceptable
areas where picnics can be held on hot days (Ellis 2004).
Plantations do provide this service, but the plantations provide a large
seed source from which invasive populations are continually recruited. The
negative impacts of invasions outweigh the benefits of shade provision, and a
trade-off has to be made.
Trees are beautiful, and they enhance the environment. In particular,
plantations of century-old trees define the character of certain suburbs that
border on the park (Hallauer 2000).
Plantations of noninvasive species (for example, eucalypts) will be
retained in certain areas, but invasive species should be cleared for the same
reasons outlined under “recreation and shade” above.
Trees counter climate change through carbon sequestration, and should not
be felled (Yeld 2009).
The advantages of carbon sequestration need to be assessed against the
negative consequences of both plantations and invasive trees. The gains in
carbon sequestration from alien plant invasions and from tree plantations on the
Cape Peninsula will be very small. For example, the carbon offset of all
plantation forestry in South Africa is only 3% of the country’s carbon
emissions (DEAT 2009). Losses of water resources and biodiversity are a far
greater cause for concern, and models suggest that climate feedbacks associated
with carbon sequestration are unlikely to offset water losses, and could even
exacerbate them (Jackson et al. 2005).
Alien species should be protected, especially when they have been present
for centuries. The control of alien species is a form of xenophobia or racism
(Simberloff 2003), or ethnic cleansing (Todeschini 2000). The enthusiastic
campaigns to clear alien trees and shrubs are viewed by some as being carried
out by “ecofascists” (Packenham 2007).
The motivation put forward by conservationists and ecologists is not
xenophobia or racism, but rather a desire to prevent environmental or ecological
harm through the removal of a significant threat (Simberloff 2003).
Today’s natural vegetation probably replaced some other form of
vegetation in the distant past, and could thus also be classified as invasive.
The invasive alien trees replacing the natural fynbos vegetation were introduced
over 300 years ago, and it would be incorrect to draw an “arbitrary”
line 300 years ago, and to treat species introduced then in any different way to
the natural vegetation (Ellis 2004).
South African National Parks (SANParks) have a primary mandate to conserve,
on the land that they manage, South Africa’s biodiversity, landscapes, and
associated heritage assets (SANParks 2008b). This is interpreted as protecting
the natural fynbos vegetation, which evolved in situ over millions of years, from
invasive species that are relatively recent introductions, and which threaten to
overrun the fynbos (Richardson et al. 1996). A further responsibility to
conserve the original vegetation is placed on SANParks through the declaration
of the Table Mountain National Park as a World Heritage Site in recognition of
the global significance of its unique biodiversity
Alien trees are grown in plantations as a crop. Their economic value needs
to be recognized. Removal of plantations will lead to loss of economic and
employment opportunities (Zietsman 2000, Du Toit and Ackerman 2009).
Trade-offs have to be made. Where the economic value of forestry is
outweighed by negative impacts (De Wit et al. 2001), a solid case can be made
for removal of plantations, as the benefits gained will more than compensate for
economic sacrifices (van Wilgen and Richardson, in press).
Eucalypt trees (Eucalyptus species) are a critical resource that
sustains captive bee populations. These bees are in turn critical pollinators of
deciduous fruit trees, a major agricultural activity in the region. Reductions
in eucalypt tree populations will have serious negative impacts on the
beekeeping and deciduous fruit industries (Allsopp and Cherry 2004).
Eucalyptus species do have negative impacts on ecosystems where they
are planted (especially on water resources) (van Lill et al. 1980). However, in
contrast to the policy of removing all pine plantations, the Table Mountain
National Park will retain many eucalypt plantations for their historic and
aesthetic values. The difference in approach is due to the fact that, in
contrast to pines, eucalypts are not highly invasive (Forsyth et al.
2004), so their impacts can be more easily contained.