Decline of the Wall - fascinating article.

[bugboy]

http://butterfly-conservation.org/3114- ... -wall.html

Many of you have probably already seen this but it does make a facsinating read. I remember growing up in north west london green belt and used to spend many a weekend/summer holiday out and about where the Wall was one of the commonest summer butterflies. Haven't seen one for over 20 years.

Will be interesting to see if this hypothesis affects other natives in the future.

[Roger Gibbons]

The distribution of the Wall has been monitored very closely in Hertfordshire over the past 50 years when it was relatively common and appearing in most tetrads of the county. It is now believed to be extinct.

In France it is widespread and common, occurring in nearly every département. In the far south, in Var, it is very common and has three generations, being on the wing well into late October, but there, of course, there is no problem of a weather-curtailed third brood.

[millerd]

Fascinating. I grew up in Southgate (North London: N14) in the Sixties, and Walls turned up in the garden far more often than Speckled Woods and Commas.

The discussions in this article are very interesting, and may apply equally to the Holly Blue - well this year, certainly. In fact they mirror somewhat my earlier unscientific speculations (on 30th October) about that species and concerns for their numbers next spring. Common Blue and Brown Argus too have both had third broods around here more than once in the last five years.
viewtopic.php?f=29&t=6157&start=700

Nice to be possibly on the right lines for once!

Dave

[David M]

Really enjoyed reading this article and every bit of it makes perfect sense.

Like many, my childhood was one where Wall Browns weren't worth a second look as they were SO common (and this was a good 20 miles inland in the NW of England).

Today, even here in south Wales, they're not terribly common and almost exclusively coastal (I tend to think of them as bedfellows of the Grayling).

The hypothesis of warming fits in very well with their recent expansion further north into Scotland as well as the 'doomed' third brood theory. Perhaps that's why their range hasn't contracted anything like so much in the north of England as in the south, as here one suspects, like several other species, cooler temperatures might suppress the instinct to generate an additional brood.

[Jack Harrison]

Yes a good article that makes sense. But let me throw in a thought that I have harboured for a long time.

The Wall Brown's closely relative in Britain is then Speckled Wood. During the last 50 years or so Speckled Wood has done very well and expanded its range and abundance, more or less the same period during which the Wall has declined. The two might be interconnected. Could it be that the Speckled Wood has an immunity to say a virus or parasite to which the Wall Brown is vulnerable?

I am not suggesting that this is a better hypothesis than the one in the article but is worth a thought.

Jack

[Pete Eeles]

That's an interesting thought, Jack. On a related note, given that the Speckled Wood can overwinter as both a larva and pupa (and is unique, in the British Isles, in this regard), I wonder if the ratio of larvae / pupae overwintering has shifted - although I guess this would be hard to determine.

Cheers,

- Pete

[Padfield]

I agree - there might well be something in Jack's suggestion. The article makes sense, as David says, but it is at least strange for an adaptive response to changing climate (i.e., sticking in a third brood) to be effectively suicidal. One suspects there are additional factors at play.

Guy

[ChrisC]

just a thought...... does the decline in Wall correlate with the increase of Rabbits after myxomatosis. food plant competion.
and if it was climate, is there a reason why coastal populations remain?
Chris

[bugboy]

just a thought...... does the decline in Wall correlate with the increase of Rabbits after myxomatosis. food plant competion.
and if it was climate, is there a reason why coastal populations remain?
Chris

The theory is that coastal temperatures are generally a few degrees cooler than inland sites so the Wall is less likely to produce a third (failed) brood and the populations therefore remain stable. This is also why northern populations have fared better than the southern inland populations.

As to the link with myxy, I would have thought that since they like bare ground an increase in rabbits would benefit the Wall rather than be implicated in it's decline. Certainly the areas where I used to see them as a kid were mainly horse fields where the grass was perminantly cropped very short

[David M]

It'd be interesting to know whether a similar decline has occurred inland in the northern third of France too. There is no mention of any contraction in Lafranchis's Papillons de jour de France, Belgique et Luxembourg.

[Padfield]

It'd be interesting to know whether a similar decline has occurred inland in the northern third of France too. There is no mention of any contraction in Lafranchis's Papillons de jour de France, Belgique et Luxembourg.

The only regions of France I have specific information for are the coastal départements of Loire Atlantique and Vendée (see: http://www.amazon.co.uk/Biohistoire-pap ... 2753519684). Here, the species has increased since 1990 and now occupies every 10km square in both départements, as opposed to about half of them before 1990. Perrein records it as trivoltine or quadrivoltine there: the third generation flies from mid-August to the middle/end of September and the partial fourth generation flies from the end of September to the beginning of November. In Switzerland, I have seen adults as late as mid-December. These opportunistic late-season generations clearly have no negative impact in these more southerly regions (surely, a partial fourth generation in Loire Atlantique et Vendée should be expected to have the same kind of effect as a partial third generation in the UK), which is why, without doubting the authors of that report, I wonder if something else is at work too.

Guy

[David M]

...and the partial fourth generation flies from the end of September to the beginning of November...

You have a point regarding the partial fourth generation equating to a partial third in England & Wales, but if the species is over-generating itself to extinction here (and we know it has become extinct in many inland areas) then surely this is not due to a partial additional brood. It must be a total additional one, otherwise the insurance 'kick-back' from adopting the partial approach would continue to favour those early stages from the second brood which didn't emerge to form the third?

Like you, I'm uncomfortable with the idea that nature could evolve a process which is self-defeating to the point of extinction in a period of less than 30 years!

[Matsukaze]

The Normandy butterfly atlas (1991-2004) shows no sign of decline and describes the species as widespread and abundant.

On the coast around Malaga, the species flies into January, if not in particularly large numbers, and is at the very least close to being continuously brooded there.

[bugboy]

Thats the thing with evolution though, it is a fluid thing, constantly in process, experimenting. Extinction is a natural part of evolution. It would seem the Wall, giving the right conditions is a continuous brooded butterfly, it's in it's genetic makup. Warm weather extending into the autumn simply triggers this natural adaptation and it would only take a few seasons of this for the population to crash irreversably, no time for evolution to change it's genetic makeup. Evolution experimented and failed on this occasion which is as natural (and probably much more common) as it succeeding. If global warming continues and we get warmer autumns extending into october and november then this natural adaptation will succeed and inland sites will almost certainly become repopulated and coastal sites may go extinct as their micro climates become what we see inland at present, i.e. they are tricked into producing a failed third generation.

[David M]

Thats the thing with evolution though, it is a fluid thing, constantly in process, experimenting. Extinction is a natural part of evolution. It would seem the Wall, giving the right conditions is a continuous brooded butterfly, it's in it's genetic makup. Warm weather extending into the autumn simply triggers this natural adaptation and it would only take a few seasons of this for the population to crash irreversably, no time for evolution to change it's genetic makeup. Evolution experimented and failed on this occasion which is as natural (and probably much more common) as it succeeding. If global warming continues and we get warmer autumns extending into october and november then this natural adaptation will succeed and inland sites will almost certainly become repopulated and coastal sites may go extinct as their micro climates become what we see inland at present, i.e. they are tricked into producing a failed third generation.

I'm not sure I agree that the Wall is per se a continuously brooded species, because if it was it wouldn't be present in such northerly latitudes.

That said I take on board your comment regarding experimental 'triggers' but am concerned that it seems to only apply to this species amongst the 59 considered to be UK residents. Why does the Common Blue, for example, not find itself in the same predicament...or the Small Copper? Both of these are more numerous near to the coast than they are inland yet they have strangely resisted the same instinctive temptation to evolve towards generating an unnatural (as far as conditions allow) additional brood.

The progressive rise in temperature is the same for all creatures whether they be butterflies or anything else, so why is it that the Wall Brown alone is the one dealing with this catastrophe?

I think Guy is right in pontificating that other factors may well be instrumental, but sadly no-one has (as yet) set forth a convincing explanation of what these might be.

[millerd]

Thinking out loud here...

In warmer climes further south, certain species have additional broods over their cousins further north. Somewhere in between, there must be a transitional area, where sometimes there is an extra brood, sometimes not. Or where it is partial. As the climate warms, this transitional area will move north. However, it will not be smooth transition, and the vagaries of weather during a given year may tempt an additional brood out in a long summer only to be much less kind in the winter - killing any offspring of that brood or preventing them from breeding altogether. On the face of it, this transitional period may very likely temporarily reduce numbers until the new climatic regime becomes properly established.

The transitional period might be longer for some species than others. It will vary geographically as it is affected by microclimates (coastal vs. inland, for example). The relative abundance of certain affected species may reduce the risk of local extinction during the transition and also obscure what is actually happening. Unwitting intervention by human activity may interfere with the process (as it invariably does).

More specifically, in my fairly short (about seven years) experience of taking a close interest in butterflies local to me, I can say that a third brood of Common Blues is evident one year in two on average. This is based on seeing fresh individuals in late September or October. There has also been a definite third brood of Holly Blues this year, and I have seen it here before - the books do say it is not that unusual.

Dave

[Paul Harfield]

I'm not sure I agree that the Wall is per se a continuously brooded species, because if it was it wouldn't be present in such northerly latitudes

Is it not true that the Walls close relative the Speckled Wood is continuously brooded in mainland Europe and is edging towards that in the UK as well. Yet that species seems to thrive pretty much everywhere.

I also think there must be other factors at work.

I live in Hampshire where Wall is particularly thin on the ground. I find it strange that the surrounding counties of Dorset, Sussex and IOW have higher numbers of Wall. Just thinking out loud, but I hope/feel sure the Wall will bounce back at some point due to some similarly unknown reason.

[bugboy]

Thats the thing with evolution though, it is a fluid thing, constantly in process, experimenting. Extinction is a natural part of evolution. It would seem the Wall, giving the right conditions is a continuous brooded butterfly, it's in it's genetic makup. Warm weather extending into the autumn simply triggers this natural adaptation and it would only take a few seasons of this for the population to crash irreversably, no time for evolution to change it's genetic makeup. Evolution experimented and failed on this occasion which is as natural (and probably much more common) as it succeeding. If global warming continues and we get warmer autumns extending into october and november then this natural adaptation will succeed and inland sites will almost certainly become repopulated and coastal sites may go extinct as their micro climates become what we see inland at present, i.e. they are tricked into producing a failed third generation.

I'm not sure I agree that the Wall is per se a continuously brooded species, because if it was it wouldn't be present in such northerly latitudes.

That said I take on board your comment regarding experimental 'triggers' but am concerned that it seems to only apply to this species amongst the 59 considered to be UK residents. Why does the Common Blue, for example, not find itself in the same predicament...or the Small Copper? Both of these are more numerous near to the coast than they are inland yet they have strangely resisted the same instinctive temptation to evolve towards generating an unnatural (as far as conditions allow) additional brood.

The progressive rise in temperature is the same for all creatures whether they be butterflies or anything else, so why is it that the Wall Brown alone is the one dealing with this catastrophe?

I think Guy is right in pontificating that other factors may well be instrumental, but sadly no-one has (as yet) set forth a convincing explanation of what these might be.

Padfields comments certainly backup my suggestion that the Wall does pocess in its genetics the ability to be continuously brooded if environmental conditions allow. Probably much like others of our natives again proved in Pads personal observations in Europe. Inhabiting such a variable climate as we have (even without man-made global warming, the part of the world we live in is naturally unpredictable) so it would stand to reason that species inhabiting it would have the ability to adapt to changes in the climate, evolutionary pressures in the past would have forced them too. The thing with the Wall is that in the inland extinctions it is on the cusp of being able to successfully produce a third brood (assuming this theory is correct and to my mind it seems to fit all the facts), it's just that unlike other species it seems to throw all its eggs in one basket and boom, extinct. Other species would have slightly different environmental triggers which is what the phrase 'survival of the fittest' is all about. I doubt very much personally that there is a parasite involved, I'm quite sure that would have been found by now and if it was viral why is the decline only in certain areas, that doesn't make sense either. If I were a betting man, my money would definately be on the theory in the article, for it ticks all the boxes and makes perfect sense. Whatever the reason, i do find this branch of science endlessly fascination.

[David M]

Thinking out loud here...

In warmer climes further south, certain species have additional broods over their cousins further north. Somewhere in between, there must be a transitional area, where sometimes there is an extra brood, sometimes not. Or where it is partial. As the climate warms, this transitional area will move north. However, it will not be smooth transition, and the vagaries of weather during a given year may tempt an additional brood out in a long summer only to be much less kind in the winter - killing any offspring of that brood or preventing them from breeding altogether. On the face of it, this transitional period may very likely temporarily reduce numbers until the new climatic regime becomes properly established.

The transitional period might be longer for some species than others. It will vary geographically as it is affected by microclimates (coastal vs. inland, for example). The relative abundance of certain affected species may reduce the risk of local extinction during the transition and also obscure what is actually happening. Unwitting intervention by human activity may interfere with the process (as it invariably does).

More specifically, in my fairly short (about seven years) experience of taking a close interest in butterflies local to me, I can say that a third brood of Common Blues is evident one year in two on average. This is based on seeing fresh individuals in late September or October. There has also been a definite third brood of Holly Blues this year, and I have seen it here before - the books do say it is not that unusual.

Dave

I agree that certain species in notably warm years have additional broods (that much is backed up by copious evidence); but how does a butterfly suddenly jump from an occasional additional brood when conditions are abnormally beneficial to conducting such behaviour as a matter of permanent routine, which would appear to be the hypothesis made in the report regarding the Wall Brown?

Surely, there HAS to be a transitional (if you like, experimental) period where the butterfly 'dabbles' in an additional brood where conditions aren't really suitable whilst still retaining the default, historic option of not doing so to ensure the option of survival if things go wrong?

Given nature's in-built 'security measures', one would postulate that such a transitional stage would eventually lean towards a more 'conservative' approach, i.e. that an additional brood is generally not feasible and that natural selection would favour those individuals that were programmed towards hibernating in an earlier stage rather than going on to their adult stage prematurely early?

[bugboy]

Ok well I would argue that the Wall is a more recent coloniser to britain than some of our others. It's love of 'hotspots' and warm micro climates would suggest that. Spreading from the south where it evolved as a continuously brooded species, as its distribution moved north it evolved an ability to hibernate to survive the northern winter (maybe that was already programmed into it's genetics from previous ice ages etc). This would be a secondary piece of evolution which, when conditions permit, is cancelled out by its primary continuously brooded traits. I suspect, based on the theory, that it will always push towards an extra brood if at all possible. The 'saftey net' is that it is widely distributed and any local extinctions can be filled from surrounding areas as environmental conditions permit.