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Botanical Mysteries 2: the Strange Case of White-Flowered Marsh Thistles
Marsh thistle Cirsium palustre flowers are generally deep purple (bottom photo) but, wherever you go in the North Pennines, you begin to notice white-flowered plants (top photo) once you climb more than about 1000 feet above sea level. As you go higher, the white flowered forms become more prevalent. Why? One suggestion, put forward by Dave Mogford at Oxford University back in 1974, was that white-flowered forms tend to attract more bumblebee pollinators, so it pays to be white at higher altitudes, where bumblebees are fewer. If that's the case, why aren't there white-flowered populations of other thistle species at higher altitudes? There’s scope for amateur naturalists to make some observations to check out this explanation....................
Marsh thistle Cirsium palustre flowers are generally deep purple (bottom photo) but, wherever you go in the North Pennines, you begin to notice white-flowered plants (top photo) once you climb more than about 1000 feet above sea level. As you go higher, the white flowered forms become more prevalent. Why? One suggestion, put forward by Dave Mogford at Oxford University back in 1974, was that white-flowered forms tend to attract more bumblebee pollinators, so it pays to be white at higher altitudes, where bumblebees are fewer. If that's the case, why aren't there white-flowered populations of other thistle species at higher altitudes? There’s scope for amateur naturalists to make some observations to check out this explanation.................... 
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I feel another PhD thesis coming on, Phil. I have to say that I had no idea that there were white thistles.
ReplyDeleteIt certainly would make an interesting project for a student, Emma.
ReplyDeleteHi Phil
ReplyDeleteI took a walk at Berrow Downs (Malvern) yesterday and immediately noticed the large numbers of white marsh thistle; on the hills but also on the commons at lower altitudes. I also wondered about the reasons for the noticeable flower-colour polymorphism at this site. Not just altitude then? Local pollinator diversity (old unimproved site) or preference? Some other (unknown) selective advantage linked to pigment production? Am just writing a short blog on it and your page came up in a search! There is a suggestion it could be due to relative costs of producing pigments at altitude but again these thistles are not at altitude (although seeds could disperse from the hills I guess). Short conf abstract here: http://eco.confex.com/eco/2008/techprogram/P14546.HTM. Most interesting!
Hi Mel, Dave Mogford published a couple of papers on the phenomenon back in 1974 and I think they are both freely available on the web, at http://www.nature.com/hdy/journal/v33/n2/pdf/hdy197491a.pdf
ReplyDeleteand
http://www.nature.com/hdy/journal/v33/n2/pdf/hdy197490a.pdf
It's a long time since I've read these but I think exposure was part of the stoty too - he also looked at sea cliff populations, I think. Reciprocal transplantation experiments would be interesting, wouldn't they? kind regards, Phil. P.S. Love your blog!
Hi. Thanks. Yes managed to find his 2 papers this morning. Got them ready to read! Mel. PS I enjoy your blog too, esp. the digital botanic garden, which I've not fully explored yet.
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