Decline of the Wall - fascinating article.

[Essex Bertie]

Bob,
I'm sure that I speak for most in that we are amazed by the level of searching that you do for this species. Thanks for sharing with us.
I am a keen exponent of the development trap theory, as it's a huge mechanism that would have a large impact and seems to fit the experience at many Uk and North European location. However, I'm still not comfortable with it as there never seems to be a year when the second brood really bounces back in numbers; certainly not on the Essex Coast. You would just expect a good year at some point in the sequence. your observation on the nocturnal feeding of second brood larvae feels as if it could be significant, but then this would always have been the case. The 2 parasited pupae are intriguing. If possible, please send such pupae to Dr Mark Shaw at the Museum of Scotland. He has worked with BC in the past and may be able to identify the culprit to see if it could be host-specific. The stage at which it lays its eggs and the time of year might be significant.
Yesterday, 3rd Oct, I saw 2 Wall (m & f) on the north side of the Thames. Both fresh, so a small partial third brood seems likely again. I also searched at Rainham Marshes which used to be a reliable site. But I didn't find any and haven't seen any there since 2011, so I think they could still be contracting their range in Essex.

[Essex Bertie]

I attended the National Recorders' meeting last week and Dirk Maes did a talk on the Wall Brown paper. He was asked what was going wrong at the 'third brood' stage and he didn't pinpoint anything. So it could be any aspect of the life cycle, pupae not emerging, poor mating success, eggs failing to hatch, larvae not at right stage to hibernate successfully. He hasn't been able to get hold of enough data across Europe - some Phenographs might be interesting.

[Jack Harrison]

Maybe people are perhaps trying to be too clever with explanations such as 3rd brood failure.

If I understand correctly The Wall is expanding its range northwards into Scotland. In England except on some high ground, it is now predominantly a coastal species. The common factor here is that temperatures in such places are generally lower than in inland low ground England.

Perhaps our British Wall is simply not adapted to higher temperatures. Agreed that the Wall does occur in hot climates such as that of Malta. However, just because a species thrives in certain conditions in one part of its range, it does not follow that it will always thrive in similar conditions in another part of its range. “Our” Wall might have evolved as a specialist in the British climate and habitat. Compare for example the Swallowtail and just how specialised it has become in England.

I throw in another thought. The decline of The Wall has to an extent coincided with the spread of its close relative, the Speckled Wood. Could it be that Speckled Wood harbours a virus, disease or parasite to which it has considerable immunity? But when the Wall has suddenly come in greatly increased contact with the Speckled Wood and its illnesses, the Wall simply hasn’t the immunity – yet.

Jack

[Essex Bertie]

I've wondered about over-heating and lack of humidity in the past as I tended to only see Walls in proximity to gravel pit lakes in Essex. On hot days, the Wall can completely disappear during the middle of the day, perhaps only coming out at 5 or 6pm. So I can see that the ‘British race adaptation’ idea could have merit in this respect, but regulating moisture loss is an issue for all butterflies and the Ringlet doesn’t seem to be suffering.
The ‘retreat to the coast’ idea doesn’t always work as there are now large sections of the Essex North Sea coast where the Wall is scarce or absent and these were former strongholds with good habitat. So i think the 'coastal' thing can be a bit misleading. Dirk Maes' experiments 'merely' showed that a small difference in microclimate at the same latitude can produce a critical difference in phenology. He doesn't regard the Wall as a coastal species in Flanders. There are two strongholds there - in the north-west corner, near and along the coast, but also on the chalk on the inland eastern border with Germany, with almost complete absence elsewhere including the southern coastal section . So any European race is experiencing the same problem/s as any British adaptation and we know that the Wall remains strong in southern Europe where 3 or 4 broods occur and the development trap risk is better spread.
I’m not sure about the Speckled Wood parasite idea as on long, tree-less stretches of the Essex coast, there is virtually no interaction with the Speckled Wood, but Walls are largely absent.

[peterc]

Jack, you raised an interesting point but has the habitat ranges of Speckled Wood and Wall overlapped sufficiently for 'competition' to be an issue? Of course, we can't rule out the possibility of a parasite or virus being responsible.

Anyway, I was reading some related material some days ago which got me thinking.

Wickman et al. found a positive relationship between egg size and offspring fitness for the Small Heath, a near relative of the Wall, in the temperate regions in Europe, although phenology was also a factor. The smaller the eggs the less likely that they will survive through to adulthood due partly, if not mainly, to fewer or poorer nutrients like proteins contained in the eggs. This raises the question if smaller eggs is a major factor in the so-called lost third generation for the Wall. Could third brood Wall females be under selection pressure to lay their eggs as quickly as possible due to supposedly less favourable environmental and climatic conditions later in the year? If so, the eggs may not have matured enough by the time they are laid and hence are smaller in size. In addition, the lower quality and/or quantity of adult feeding sources like nectar during this time could mean a lack of sufficient nutrients for the eggs and subsequent stages. Size perhaps does matter after all .

Third generation Small Heaths are very rare in the UK and their life cycle appears to be longer so the pressure or propensity for most females to lay eggs quickly probably doesn't arise.

Just an idea which might be worth investigating further.

ATB

Peter

[Essex Bertie]

I recall Jack making the observation several years ago that, just when you think the Small Heath first brood is tailing off, it kicks on again. It's something that I look out for each year and Dirk Maes has also noticed some weird phenology with this species in Flanders. So there could be some protracted development problems with late second brood progeny during the previous autumn.

[peterc]

I recall Jack making the observation several years ago that, just when you think the Small Heath first brood is tailing off, it kicks on again. It's something that I look out for each year and Dirk Maes has also noticed some weird phenology with this species in Flanders. So there could be some protracted development problems with late second brood progeny during the previous autumn.

Bertie, good point which may partly explain why the Small Heath is in decline as well but to a much lesser extent. It was a protracted second brood for the Small Heath locally last year (due to a cool August?) and quite a few were still flying well into October.

ATB

Peter