Subspecies of Speckled Wood in Europe

[Padfield]

A thread continuing a conversation begun in my diary pages, if anyone has any further information to add, to retain the continuity of my diary:

Why are your Swiss Speckled Woods like our British ones, rather than the rich orange brown spotted French versions?

Switzerland is on the dividing line, David, which runs from about Brittany to Italy. Both forms fly here, and some intermediates too, like this one, photographed this April:



Guy

Interesting stuff, Guy.

What happens if they interbreed? Do you just get a mélange?

Anywhere along the 'border' you can see intermediate individuals, from the Channel Islands, through France, to Switzerland, and for that reason I have always naturally assumed they do interbreed. But once you get well within the territories of each form you only see one kind (in my experience). It's a strange situation, I admit! Presumably it is maintained in part by the very sedentary behaviour of this species.

Guy

Anywhere along the 'border' you can see intermediate individuals, from the Channel Islands, through France, to Switzerland, and for that reason I have always naturally assumed they do interbreed. But once you get well within the territories of each form you only see one kind (in my experience). It's a strange situation, I admit! Presumably it is maintained in part by the very sedentary behaviour of this species.

Guy

They can't be that sedentary, Guy; they've recently colonised the Isle of Man (presumably from Cumbria). I was going to ask you if you'd ever seen an example of one form mating with another, but then I suddenly realised that I've never seen Speckled Woods mating, which I find odd given that other members of the 'Brown' family are commonly observable (Grayling, Meadow Brown, etc). Do they go up to the canopy for this?

It's true that speckled woods readily colonise new areas, and have been particularly active in that respect of late, but that doesn't count against the predominant behaviour, within an area where the species is well established, being sedentary. It has been observed that colonising speckled woods are larger than sedentary ones and there may be a trade-off between fecundity and flight (see http://www.jstor.org/pss/3592338, though you have to pay to get the full article). If so, it would mean that where one form attempted to colonise a previously colonised area it would be outmatched, reproductively, by the established, non-dispersive population. Those same dispersive individuals moving to an unpopulated area would be able to reproduce with less competition, when the genes for non-dispersive individuals would be expressed in a new, sedentary colony.

The species never flies terribly far, though, even in dispersal mode. You never (to the best of my knowledge) get migrant speckled woods of the 'other' form very far away from the boundary.

All good questions, and I don't know most of the answers; but there is a lot of research going on in these areas at the moment, as if there is continued climate change the ability of various species to move into newly suitable habitat will be very important.

Guy

PS - Here is the abstract from the article linked to above:

"During recent climate warming, some species have expanded their ranges northwards to keep track of climate changes. Evolutionary changes in dispersal have been demonstrated in these expanding populations and here we show that increased dispersal is associated with reduced investment in reproduction in populations of the speckled wood butterfly, Pararge aegeria. Evolutionary changes in flight versus reproduction will affect the pattern and rate of expansion at range boundaries in the future, and understanding these responses will be crucial for predicting the distribution of species in the future as climates continue to warm".

...it would mean that where one form attempted to colonise a previously colonised area it would be outmatched, reproductively, by the established, non-dispersive population. Those same dispersive individuals moving to an unpopulated area would be able to reproduce with less competition, when the genes for non-dispersive individuals would be expressed in a new, sedentary colony.

That sounds highly plausible, though surely there would be occasions when one form mates with another? In the case of continental Speckled Woods though (where thera are no sea borders) is there some kind of Maginot line which separates the two populations? Does anybody know why there are two forms in such close proximity which are quite different to one another in coloration?

[Gibster]

Slightly at a tangent, but worth repeating...a few months ago Piers made the comment (sorry, I can't recall which thread) that it has been scientifically proven that the Scottish "subspecies" oblita will after just 3 or so generations, revert to bog-standard tircis, an incredibly rapid response! If true (and I don't doubt Piers' word) this sort of blows away the whole concept of oblita being a valid subspecies. I wonder if "subspecies" insula would readily revert too? Taking the idea further - put oblita in southern Spain..?

Gibster.

[Matsukaze]

We know the range of the species has shifted north apparently in response to rising temperatures. Is there any evidence that the line dividing the two forms has shifted north as well, as might be expected if climate is the determining factor?

[David M]

We know the range of the species has shifted north apparently in response to rising temperatures. Is there any evidence that the line dividing the two forms has shifted north as well, as might be expected if climate is the determining factor?

Good question.

If the range of the northern European form has expanded then one would expect the same to be true of the more southerly form (unless, of course, there's no such thing as climate change).

[Padfield]

I've tried to find the answer to this (the question about a possible shift in the dividing line) in the literature available to me but can't. The Climate Risk Atlas of European Butterflies models speckled wood but doesn't distinguish between subspecies. Another recent document, entitled 'Impacts of Europe's changing climate - 2008 indicator-based assessment' doesn't deal with it either. So I'm left with my own observations, after living about 20 years in Switzerland. During that time, the speckled wood has enormously increased its range in the UK, not just south-north but also west-east (in my childhood there were no speckled woods in East Suffolk - now it is a common butterfly there). But Switzerland has remained poised on the dividing line between tircis and aegeria. I've seen intermediate individuals since I arrived here.

I find it interesting that there is only one species of Pararge in Europe, or indeed, the Palearctic (if you don't count two island species, xiphia and xiphioides, in Madeira and the Canaries respectively). The present distribution, with a NW/SE dividing line, doesn't seem to be ecological, as there are very hot regions where tircis flies and relatively cool, northern regions where aegeria flies. An alternative explanation, for which I have absolutely no evidence at all, is that the Ice Age left two distinct populations, in the NE and in the SW, which subsequently expanded and met at the present dividing line, which would be, therefore, ecologically arbitrary.

As I'm not a great fan of groundless speculation I will stop there!

Guy

[David M]

Excellent explanation again, Guy, but (and I'm not doubting your brainwork for a millisecond) if that theory were true wouldn't there be other European species similarly affected?

[celery]

Here's a picture of a speckled wood I took in Madeira back in 2007.



Now, I'd always assumed it was just the southern form of speckled wood - until I read about the Madeiran Speckled Wood Pararge xiphia in 'Butterflies of Britain and Europe' (Tan Haahtela, Kimmo Saarinen, et al.) - and how it was endemic to the island and completely separate species from the British and mainland europe populations.

But then I saw somewhere that somehow the mainland species had managed to arrive on Madeira and was threatening the continued survival of the Madeiran Speckled Wood. So now I'm thoroughly confused... could anyone offer an ID of the butterfly in my photo and/or any of the latest thinking on what's actually happening on the island?

[Padfield]

Hi Celery. Both species fly on Madeira and yours is aegeria (speckled wood), not xiphia.

I don't know anything about the relation between the two there, but it is not a very recent introduction. I have a book from 1983 which mentions speckled wood on Madeira, so it's been there at least thirty years.

Guy

EDIT - Tolman (in his photographic guide) notes that speckled wood was probably introduced to Madeira in 1976.

[celery]

Thanks Guy

I have another picture of a different individual taken at the same time which looks a lot like your 'intermediate' photo shown above. Trouble is it's a terrible picture which is completely out of focus. I'll see what I can do to 'correct' it and maybe post it here later. Thanks again.

[Pieter Vantieghem]

An interesting article on Pararge species can be found here: http://www.nymphalidae.net/Weingartneretal2006b.pdf

Contrary to what might be expected from morphology, we have not discovered any indication that P. a. aegeria and P. a. tircis are separate evolutionary entities based on a
mitochondrial gene and a nuclear gene. Instead, we found two separate clades within P. aegeria corresponding to the European and African + Madeiran populations.