The alien invasion myth
Words: David Pearman, former President of the Botanical Society of the British Isles
Gardeners are in danger of becoming scapegoats for the perceived threat of alien plants spreading in the countryside. The picture is hopelessly confused by unsubstantiated myths: that alien plants are deemed to be one of the great threats to biodiversity, and that it is gardeners who are responsible (and if we only planted from an ‘approved list’ all would be well).
The truth is that nobody knows. For the last 20 years I have co-ordinated our Society’s largest network of botanical recorders (from the Botanical Society of the British Isles): together we have accumulated about 18 million records, covering the entire area and 4,000 species. In simple terms, the data shows that most aliens are rare; that they occur overwhelmingly in and around towns and transport networks; and they are generally uncommon in the semi-natural habitats that we most want to preserve. Recording alien plants is, broadly, a recent phenomenon, and all extrapolations are to be taken with a pinch of salt.
The trouble is that scare stories attract headlines, funding and a large industry – from the Government’s Non-native Species Secretariat to the ‘invasive species officers’ of conservation charities.
There has been scientific research to show that Himalayan balsam Impatiens glandulifera largely only displaces other aliens or thuggish natives; that native bluebells Hyacinthoides non-scripta are under only a minute and local threat from garden outcasts; and even Japanese knotweed Fallopia japonica harbours both a spring flora and late pollen for bees.
But the real, inescapable line is that our countryside and flora is changing like never before, mainly from abandonment of traditional management practices and pollution. It is largely ‘native’ plants that are to blame – look at the chalk downlands of southern England (choked with gorse on the dip slopes) or our rivers (a vast increase in reeds, due to higher nutrients).
Yes, some gardeners are irresponsible, and yes, many ponds and some rivers have been harmed by garden plants dumped in the wild. But even taking those into account, the gardener’s contribution to the spread of aliens is the smallest of beers in the wider picture.
Sources
Myth: Alien plants are the second greatest threat to biodiversity
Cited in many papers, seeming to originate from a comment by Wilson, 1992, (on animals). For an elegant summary and discussion on this view see particularly Preston (2009) ‘Alien Plants in Scotland – Is There a Problem’ in JM Baxter & CA Galbraith Species Management: Challenges and Solutions for the 21st Century; Stationary Office Books, Edinburgh; ISBN 9780114973483.
Wilson, EO (1992). The Diversity of Life; Belknap Press, Cambridge, Massachusetts; ISBN 9780674212985.
Myth: they cost £1.8 billion a year to control
It is very difficult to find any hard evidence to support any of these figures. DEFRA produced a report in 2003, which included on p84 ‘a range of estimates’ for control or eradication for ‘selected established invasive non-natives’.
This was the famous report that estimated £1.56 billion for Japanese knotweed alone. DEFRA were unable or unwilling to produce coherent supporting figures for these estimates, and we suspect they can only extrapolations of estimates. The yearly costs of control, cited by various conservation organisations, seem to be extrapolations of extrapolations. DEFRA 2003. Review of non-native species policy; search for the report at DEFRA website.
Myth: Gardeners are responsible, and if only we planted from a sanitised approved list all would be well
There is little doubt that many (or most) of the alien plants widely established in Britain have arisen from introductions by farmers, foresters, seed merchants, botanic gardens, gardeners and the general public.
It is probable that the magnitude of the scale of introductions is approximately in the order given in the preceding sentence. Plantlife and the RHS have collaborated in a booklet, which I see does contain Michaelmas daisy, a pernicious weed now widely established throughout Britain, especially in urban and ruderal areas.
Search for ‘Gardening without harmful invasive plants’ at Plantlife website.
Himalayan balsam Impatiens glandulifera
See, for instance: Hulme, EH & Bremner, ET 2006. ‘Assessing the Impact of Impatiens glandulifera on Riparian Habitats: Partitioning Diversity Components Following Species Removal’. Journal of Applied Ecology 43: 43–50. Visit: www.journalofappliedecology.org
Bluebells Hyacinthoides spp.
Search for ‘bluebells’ at www.rhs.org.uk, or ‘Bluebells of Britain’ (2006 leaflet) at www.plantlife.org.uk
Japanese knotweed Fallopia japonica
See, for instance: Gilbert, O (2001). ‘Figs, Japanese Knotweed and Himalayan Balsam Enhance the Urban Ecology of Sheffield’. Glasgow Naturalist 23, suppl: 52–56. Visit www.glasgownaturalhistory.org.uk