Wednesday, July 15, 2009

Botanical Xenophobia?





There’s a long-standing belief that dense riverbank stands of Himalayan balsam Impatiens glandulifera – like these plants flowering beside the river Tyne at Corbridge - represent a major threat to our native riverbank plants. The press loves a good ‘alien invader’ story that predicts the extermination of our native wildlife by some botanical ‘foreigner’, and it’s true that some introduced plant species that have escaped into the wild, like Japanese knotweed and Australian swamp stonecrop for example, have damaged natural habitats, but in the case of Himalayan balsam the threat is more imagined than real. Although it’s a tall, fast-growing and very conspicuous plant it’s an annual, and doesn’t compete very effectively with established riverbank perennial native plants that have dense root systems. Two recent research projects have demonstrated that it isn’t a dire threat to our flora and one very recent one has shown that it’s a positive asset as far as bees are concerned. It’s a rich source of nectar and pollen and bees visit it constantly. Sadly, once these 'alien invader' stories take root they tend persist, no matter how good the scientific evidence is that shows they are a myth. So I dare say well-meaning but misguided ‘conservationists’ will organise many more ‘balsam bashing’ events to tackle this imagined threat. Still, the plant is well established almost everywhere now, and its exploding seed pods disperse its seeds very effectively, so it will probably continue to provide a valuable food source for our hard-pressed bees for a long time to come.

22 comments:

  1. I'll whisper this, but I quite like Himalayan Balsam...

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  2. Great shots Phil. Love that first with the bee about to visit.
    I like these plants too; a splash of colour along a riverbank. I saw a few white ones last year also, proudly standing amongst the usual red and pink.

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  3. I'm sorry but I have to disagree, having watched balsam taking over my local stream bank, displacing comfrey, bittersweet, grasses, buttercups etc. Although it is pretty in summer, the bare, muddy banks in winter are not attractive, and also allow the beck to erode its banks more readily. Also, I doubt whether there is much more nectar in the balsam than there was in the comfrey etc.

    Also I very rarely see balsam leaves attacked, which suggests it is not a good food plant for insects. I may be biased, but to me any invasive alien is bad news.

    Gill

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  4. A lovely looking plant Phil. Let's face it, when did the media ever let the truth get in the way of a good 'disaster' story.

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  5. I'm afraid I disagree. Over the past 5 years or so I have watched balsam invade the banks of my local beck, replacing comfrey, bittersweet, buttercups and grasses. In winter the banks are now bare mud, which means the beck can erode them more easily.

    I would also doubt whether the balsam provides much more nectar than the flowers it replaced.

    I have rarely seen any damage to balsam leaves, which suggests it is not a good food source for insect larvae etc. I may be biased but I view any invasive alien with alarm - and certainly round here it is invasive: not only along watercourses, but also beside roads and tracks and at the edge of woods.

    Gill (Ryedale)

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  6. I was looking forward to welcoming a positive view on balsam, which we've found attractive and nicely-perfumed.
    Gill's experience seems, however, to compromise that welcome. An interesting post, as ever.

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  7. The fact that Himalayan Balsam has 'become well established almost everywhere' confirms the fact that it's invasive, if only by its mere presence where something else used to grow. If you examine the leaves, roots and stems of HB you will see that no native fungus or insect uses them as food. However, the native plant species that have been ousted by the HB certainly have species that depend on them. I include here sawflies, hoverfly larvae, beetle larvae and adults, moths - both macro and micro, butterflies, fungal rusts, smuts and ascomycetes, aphids and assorted diptera. Then we need to consider the secondary feeders - other insects, fungi, mites, spiders and birds that would have benefited from the presence of the primary species, not to mention the parasitic hymenoptera and Tachinid flies. So the presence of HB removes vast swathes of habitat from our native flora, fungi, insects and birds.

    Yes, bees benefit from the presence of a rich nectar and pollen source, but we cannot praise the benefit to part of one insect family to the detriment of dozens of other species.

    Today I drove along a road that I haven't travelled for a few months and I saw HB in dozens of new locations.

    Yes, it's pretty, but it's extremely dangerous for our wildlife, if only due to habitat removal, which continues at a high enough pace without help from HB.

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  8. Your secret is safe with me Steve. Saying anything positive about this plant is rather like saying that magpies aren't all bad...

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  9. Hi Keith, I think the fact that it is so showy counts against it in the eyes of some.....

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  10. Thanks for your comments Gill. One recent scientific research paper suggest that attempts to remove balsam may now do more harm than good, leaving exposed stream banks that become more prone to erosion. Interesting that you should mention comfrey, as it was growing in amongst the plants where I photographed the bee visiting. The comfrey had almost finished flowering, while the balsam was just beginning to flower, so as the season progresses the latter seemed to be taking over from the former as a nectar source here in Durham....

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  11. I Take your point John. In the world of wildlife conservation, as elsewhere, the only good news is bad news..

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  12. Hi Kingsdowner, and thanks for the comment. I find attitudes to alien plants interesting. Horse chestnut is an alien, widely introduction from the Balkans, that has been here for less than 400 years but is now intimately woven into our folklore - thanks to its conkers - and there is now widespread alarm (making front page news in the Independent,a prolific perpetrator of wildlife scare stories) that this wonderful tree is now supposedly threatened by the leaf mining moth Cameraria ohridella. I've long suspected that wildlife onservation in Britain is as much about sentiment as science...

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  13. Hi Steve, thanks for your comments. "The fact that Himalayan Balsam has 'become well established almost everywhere' confirms the fact that it's invasive, if only by its mere presence where something else used to grow." - maybe, maybe not - thanks to glaciation and then separateion by sea from continental Europe, our islands have a relatively poor flora (only about one third of the species found in France), so it might also mean that it's monopolising an underexploited ecological niche. The most recent research suggests that it is not a particularly strong competitor against native perennial species and its success (as is so often the case) is also attributable to human folly (eutrophication of our rivers with agricultural run-off; it thrives on nitrogen pollution. Interestingly, aresearch published in German suggests that this Colonist (which seems to be spreading over much of Europe) is beginning to acquire its own fauna of phytophagous insects - recently a sawfly called Siobla sturmi and a geometrid moth called Xanthorhoe birivata, so maybe its enemies are catching up with it.......

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  14. Sorry Stuart, I called you Steve..........it's been a long day!

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  15. I'm on Stuart's side in this one. But I would like to apologise for the double posting - I thought the first one had disappeared into the ether....

    I think it is the rapid spread of balsam that unnerves me as much as anything - which is not true of horse chestnut. An interesting point that cutting it might be counter-productive, In fact all the advice I have seen is that if you want to control it you should pull it up by the roots when it is small and then dispose of it properly, preferably by burning - not cut and leave. If you do this early in the season it gives other plants a chance to (re)colonise the ground.

    Gill

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  16. Hi Gill,It's interesting that (according to the Royal Horticultural Society) Himalayan balsam was first introduced into Britain in 1839 so it's taken 170 years to reach its present problematic status. I suspect that nutrient enrichment of our waterways might have been a contributory factor in its spread over the last few decades, as it thrives in high-nitrogen situations. Some alien species lie relatively dormant for quite a long time and then suddenly spread when conditions change; it seems likely that we can expect further spread of introduced horticultural species as a result of a warmer climate...

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  17. I have to disagree It is no better a nectar source then other plants. There is a discussion about this blog subject on the yahoo botany groups.

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  18. Hi Colin, according to this source http://www.open2.net/naturalhistory/invaders/wanted/himalayan_balsam.html
    and several others on the web (with all the usual caveats about the accuracy of information on the web) it produces more nectar than any European native species. This site http://www.gccv.org.uk/gccv_balsam.html quotes a figure that it produces 23 times more nectar than our native purple loosestrife (which has itself become an invasive weed in the US, after being introduced by the Brits). There are reputable published scientific studies that show that, given the choice, bees vote with their wings and preferentially visit Himalayan balsam, on account of its high nectar volume with a high sugar concentration. There is some evidence that it diverts pollinators away from native plants but, on the other hand, if it's abundant resources boost bumblebee populations that effect might be counterbalanced, so it isn't really clear, overall, whether it has a positive or negative effect on the surrounding flora's pollination success .... it seems that more research is needed.

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  19. I like the picture of the plant with the sky behind it very much, Phil.

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  20. Thanks Emma, one of the virtues of small pocket digital cameras like the one that I used is that their small sensors give them amazing depth of focus, which is really useful for showing close-ups of plants in their natural surroundings

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  21. "...quotes a figure that it produces 23 times more nectar than our native purple loosestrife" I don't want to get into details here (e.g. is that per flower? per plant? per flowering season?) but it does seem balsam is genuinely a good nectar source. I would have no problem with its being grown as a 'honey crop' like say borage - so long as it had a cordon sanitaire to prevent escapes!

    I just don't want to see it taking over my local woods and watercourses. I do take the point about eutrophication - indeed there is plenty of evidence of habitat degradation around farms with all the chemical runoff, including on roadside verges which are generally becoming rank and coarse at least here in Ryedale.

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  22. Hi Gill,I suspect that floods, of the kinds that we've had recently in the North East, are another way in which this species moves around and spreads from riverbanks into other damp locations. Eutrophication and habitat degradation certainly provide opportunities for introduced species to spread but I recently came across an interesting research paper which indicates that these degraded habitats are just as likely to be invaded by agressive native species - e.g. nettles, rank grasses and bracken - as by 'alien' species....

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