In a nature reserve near you, hardy work parties of wardens
and volunteers will have ‘bashed’ the dreaded exotic invader Himalayan Balsam
(Impatiens glandulifera or HB) all summer. Introduced to Britain in 1839, it is
our fastest growing annual plant with beautiful flowers and explosive seed pods
loved by children. But according to Natural England and the Environment Agency
it:
·
Is a highly invasive annual weed, which can
reduce biological diversity by outcompeting native plants for space, light and nutrients
·
Restricts access to the river, leaves autumn river
banks bare of supporting vegetation and prone to erosion and the dead plant
material increases flood risk.
· Attracts pollinators away from native species, reducing
the genetic diversity of native species and fitness by reducing seed set.
But the charge sheet feels a bit hysterical. Can any plant
be this bad? What is the evidence? Once you start to dig a bit, there’s not
much. A few short term studies, one that looks at the effect on just a single
native species, some that don’t really support the conclusions put about by our
conservation bodies at all. Few seem to have been subjected to peer review, a
scientific process that allows us to be fairly certain about the results.
Indeed given the urgency of the calls to eradicate HB, not much at all. But
what do the experts actually say?A study by Hulme and Brenner[i] showed that more species are present if you remove Himalayan Balsam (HB), but noted ‘in open and frequently disturbed riparian vegetation, many of the species negatively influenced by Impatiens are widespread ruderal[1] species. Thus while several authors recommend its removal such action may only lead to a compensatory increase in the abundance of other non-native species and thus fail to achieve desired conservation goals.’ This doesn’t suggest that there is much effect on the sort of native plants that we might want to conserve. TIckner[ii] compared the competitive ability of nettle and Balsam sometimes our native nettle loses out. But nettle only grows where there are a lot of nutrients – the sort of sites that have little diversity of other native British plants anyway.
Chittka and Schürkens[iii]
reported that suggested competition for pollinators might reduce the amount of
viable seed set. But they only looked at the effects on Stachys palustris, which is pretty common anyway, and even then it
was a one season trial. This is a long way from showing that in a real habitat HB
affects the viability of even this one species which is of no real conservation
interest, let alone other species. We know that HB has high-sugar nectar and
flowers for a long time. But of course it might just support more bees in the medium
term leaving the same number of bees available to pollinate other flowers. And
finally Tanner[iv]
looked at the effect on invertebrates. The evidence was inconclusive and fertile
ground for a follow-up study.
I can’t find a scientific paper that looks at the alleged
erosion-enhancing effects of HB; all the evidence seems to be anecdotal. HB often
inhabits urban river banks where flows are unusually peaked, favouring the sort
of erosion that HB is blamed for. It might of course be that HB is just good at
colonising river banks that are already prone to erosion and that this is
nothing to do with HB.So there you have it. The studies are generally short-term and focus on the disturbed, urban and the sort of nutrient-rich habitats that HB favours rather than more important plant communities. None of the studies ‘prove’ that HB reduces biological diversity in the long or medium term, or that it is responsible for erosion, or for reducing genetic diversity. Of course, the scientists themselves are honest about pointing these issues out in their paper, and state that more research is needed. But these caveats are never reported in the campaign against HB.
That there is so little hard evidence that HB harms conservation is bad enough. Even worse than this, Wadsworth looked at control strategies and concluded that they are ‘rarely effective in the long term’. Now I’ve ‘bashed Balsam’ myself, enjoyed it, and even met some nice people. It is certainly true that HB gets in the way and takes up a lot of space. There may be a need to remove the plant where it blocks a path. But in an age of austerity should we really be spending millions of hours and millions of pounds fighting an enemy when the case is so weak?
[1]
Scientific name for plants that tolerate disturbance. Examples include docks,
dandelions and thistles.
[i]
Hulme and Brenner 2006: Assessing the impact of Impatiens glandulifera on
riparian habitats: partitioning diversity components following species removal
(Journal of Applied Ecology2006 43, 43–50)
[ii]
Tickner et al 2001: Hydrology as an influence on invasion
[iii]
Chittka and Schürkens 2001: Successful invasion of a floral market
[iv]
Tanner et Al 2013: Impacts of an Invasive Non-Native Annual Weed, Impatiens
glandulifera, on Above- and Below-Ground Invertebrate Communities in the United
Kingdom
