BRITAIN: A NATION TIED IN KNOTWEED - The New Wild: Why Invasive Species will be Natures Salvation - page 49

Himalayan balsam ( Impatiens glandulifera ), the last of the “big four” botanic villains, fits a similar
template. Introduced by the wild gardeners for what advertisements of the time called its “Herculean
proportions” and “splendid invasiveness,” Himalayan balsam spread along rivers and waterways. But
how far?
In late 2014, the Environment Agency claimed it covered 13 percent of the riverbanks of England
and Wales, and CABI trumpeted the stat while releasing a rust fungus to wipe it out. But that 13 percent
figure appears rather inflated. When I checked out the source, an old River Habitat Survey conducted by
the agency, it turned out that to qualify for “occupying” a riverbank, it was only necessary for surveyors
to find “a single occurrence” of the plant along a five-hundred-meter stretch of sampled riverbank. That
is hardly “covering.” Moreover, the agency had separately tallied where the weed occurred along more
than 33 percent of the bank stretch. The figure for that—surely a much more sensible measure of how
much the bank was occupied by the weed—was just 3 percent.
But while the Environmental Agency and CABI clearly have it in for Himalayan balsam, it has
friends. The weed produces excellent pollen that attracts both honeybees and bumblebees. The British
Beekeepers' Association says it “can be crucial in helping honey bees provision their colonies to survive
overwinter.” The association advises its members to keep a few bushes in their gardens. 17
Such alien plants like disturbed land, and so they congregate in urban areas and other places where
we notice them. But they rarely invade the countryside. On a ranking list of common species in the
British countryside, the much-hated rhododendron came 233rd, with Himalayan balsam, Japanese knot-
weed, and giant hogweed lagging further behind. 18 In terms of their potential to invade, some natives
are a much greater nuisance. Bracken ( Pteridium aquilinum ), blackberry ( Rubus fruticosus ), stinging
nettle, and ivy are all natives and love disturbed ground and nutrient-rich soils often created by hu-
man occupancy. Other native species—including holly, gorse, hawthorn, blackthorn, and birch—invade
grassland, heaths, moors, and bogs. If they were foreign there would be a hue and cry. Just imagine if
the stinging nettle was an alien.
Rob Marrs of the University of Liverpool looked at the prominence in the National Woodland Sur-
vey of three local “thugs”—bramble, bracken, and ivy—and three “aliens”—rhododendron, sycamore,
and Himalayan balsam. He found that the locals caused four times more damage to local woodland biod-
iversity than the aliens. In other words, we have got it all wrong. Invaders are not a big cause of change
in woodlands, and native invaders are far more of an issue than aliens. 19
David Pearman, former president of the Botanical Society of the British Isles, edited a recent botan-
ical atlas of the British Isles that collates eighteen million records of four thousand species. He says
that despite the hype and fear, “most aliens are rare, and occur overwhelmingly in and around towns
and transport networks. They are generally uncommon in the semi-natural habitats that we most want to
preserve.” 20 They like disturbed ground, whether graveyards or fields, herbaceous borders, or railway
cuttings. In such places, they keep nature going where other less hardy and less adventurous types falter.
As Mabey puts it in his topic Weeds , “Many of them may be holding the bruised parts of the planet from
falling apart.” 21 If we don't like them, it may often be because we associate them with the messy things
we do to nature, rather than for their inherent beastliness.
Britain gives the lie to the conservationists' mantra that aliens are self-evidently bad and natives ob-
viously more at home. In the real world it is often impossible to tell the difference. Many aliens are so
well integrated that they are assumed to be native. The snowdrop, usually the first wildflower to blossom
each year in Britain, arrived from Brittany in the sixteenth century. It began as a garden flower but soon
went feral. It is much more widespread than the weeds we love to hate. But few in Britain know they are
foreign, and nobody calls for them to be eradicated. Or consider the horse chestnut tree ( Aesculus hip-