Sunday, August 04, 2013

It's Himalayan Balsam flowering season again.

Himalayan Balsam, Impatiens glandulifera. Do you rush out and pull it up or spray it with glyphosate or 2,4-Damine as DEFRA recommend or do you say, "That's a pretty flower and the bees love it, where's the harm?"

Himalayan balsam is listed under Schedule 9 to the Wildlife and Countryside Act 1981 with respect to England and Wales. As such, it is an offence to plant or otherwise allow this species to grow in the wild.

Many folk seem willing to take the government advice and rush out on a killing spree. Others prefer to read the scientific literature first. For those I can recommend a couple of papers.

There's this one:

Martin Hejda and Petr Pyšek
What is the impact of Impatiens glandulifera on species diversity of invaded riparian vegetation?
Biological Conservation Volume 132, Issue 2, October 2006, Pages 143–152

in which the authors say:
 "It is concluded that I. glandulifera exerts negligible effect on the characteristics of invaded riparian communities, hence it does not represent threat to the plant diversity of invaded areas."
and there's this one: 

Philip E. Hulme and Eleanor T. Bremner
Assessing the impact of Impatiens glandulifera on riparian habitats: partitioning diversity components following species removal
Journal of Applied Ecology Volume 43, Issue 1, pages 43–50, February 2006

in which a more complex set of conclusions are drawn but can be roughly summed up as saying that in most cases it's probably best not to bother interfering.

Certainly these papers, and others in the scientific literature, do not support the simplistic advice of government agencies to kill Himalayan Balsam wherever possible and by whatever means even including herbicides.

It seems that Himalayan Balsam can be quick to invade disturbed ground but struggles to compete with established swards of grass or vigorous native species such as Stinging Nettle, Great Willowherb and Bracken but can dominate other non-native 'invasive' ruderal species.  Overall biodiversity is not adversely affected.

The question must be raised as to why government sponsored advice is cast in such simplistic terms and why so many individuals and agencies act of such advice without checking the scientific literature.  It is easy to accept, uncritically, advice from a competent authority but perhaps a piece of advice, initially not rigorously researched, has been repeated so often that it has become the accepted custom and practice.

It may, of course, be challenging for people to accept that their long-held beliefs have little foundation in science and that their hard work may have had little beneficial effect.  Is objectivity clouded by prejudice against the alien? I leave it to my readers to consider whether Himalayan Balsam might be regarded as an allegory, the lessons learnt applied to other areas of public policy.


Anonymous Anonymous said...

Mostly it is just another plant among the riparian mix of willowherb, nettle, meadowsweet ... and does no harm. Sometimes it can dominate, for example in wet woods ... but then so can nettle. And I know which I would rather walk into. I suspect that both HB and Japanese knotweed are good at providing riparian cover for otters, water voles, water fowl, where otherwise lacking. Another important benefit.
Oliver Tickell

9:42 pm  

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