Cheers Dave - I do agree that a lot of the pleasure lies in the unknown. If I ever start to feel like I’m closing in on the truth I’ll have to consider stepping back. Luckily, for now, and as you rightly say, the more I look the further from the truth I seem to end up!
Thanks Pete - yes, Linford and Big Sal were big losses for me - I was sure I’d be following them on their journeys to pupation come summer.
I should just clarify Wurzel, that in dismissing the statements people make about how certain species are faring, I wasn’t meaning to suggest that I know any better, but just that having made the step to realise just how much I don’t know, I can’t help but view all these statements through the same lens. Of course I don’t dismiss everyone - genuine experts or people who spend a lot of time making observations (everyone on here for example) are always worth listening to!
But anyway - I’ll persist with thinking out loud as it does at least organise my thoughts (however far from the truth they might be!). It’s MY truth, and that’s something these days isn’t it? Far more important than THE truth if I’m understanding current trends correctly. Dear me - things will get rather complicated very quickly if we all go down that path…..
Right - firstly a quick graph to illustrate the point I was trying to make in the previous post - not really necessary, but here it is anyway. It does also illustrate the later case studies well.
![EAA2A084-9AFD-4410-9133-6D7A1AB2251D.jpeg (182.4 KiB) Viewed 4571 times EAA2A084-9AFD-4410-9133-6D7A1AB2251D.jpeg](./files/thumb_17917_981b2d27309517199b665f06de4cba01)
If the purple line is the true population at a given site, then the margin for error is shown by the two fine black lines. Population can easily be overestimated by erroneous extrapolation of sightings as well as underestimated due to observational difficulty. Pretty obvious stuff - flight season weather, and various other factors cause a large margin for error and play a key role in the conclusions we come to for any given year. A big population change will certainly still be picked up amongst the noise (1st graph) but small fluctuations in a relatively stable environment clearly won’t (2nd graph). These graphs assume that habitat provision at a given site is absolutely key to population size, and all the other things we fuss about; egg lay conditions; leaf quality in August; conditions for spring larvae; conditions throughout pupation; predation over winter; how many frosts we have; what mood the tits are in etc etc etc all amounts to not a fat lot. Ok - some effect, but nothing like the dramatic difference that habitat provision makes.
Personally I’m starting to lean strongly towards this way of thinking, even after I spent all that time on those tables! To be fair, I still stand by the numbers, but find it less likely that low winter predation is the cause of local population spikes. I’m certainly not confident about this though, and I’ll continue to look for some evidence that one of these variables plays a crucial role; but the longer I go without finding it, the more I’m inclined to place all the emphasis on habitat provision.
So, a butterfly of natural regen……
PE numbers can certainly build rapidly in response to the creation of prime new habitat. As a forest species (we’ll leave the discussion about ‘wood pasture primal Europe’ for now) breeding on light-loving sallow, females must be highly specialised to seek out the sallow rich areas of regrowth that may only occur in relatively small pockets spread far and wide throughout a dense forest of otherwise largely unsuitable habitat.
It is no surprise therefore, that any sallow rich rewilding site within reasonable distance of a PE colony becomes a new breeding ground for PE very quickly.
As we’ve seen at Fermyn, Southwater and most recently Knepp, sallow regrowth unsurprisingly corresponds with a spike in PE abundance (see graph again). From my limited experience, and no doubt this varies from site to site, peak breeding conditions for PE occur something like 10/15 years after the onset of new growth, or maybe 20 years after the land is first cleared. Various factors shift this figure to some degree but generally sallows somewhere between 5-15 years old seem to be favoured. I don’t imagine this is so much to do with their age, but just reflects the fact that in areas of natural sallow regen, trees often become crowded after this point.
At Southwater large blocks of conifer blew down in the 1987 storms and the area was allowed to regenerate. I think some grazing may have delayed/staggered the onset of the regrowth, but essentially a large amount of sallow and birch regrowth occurred soon afterwards. 35 years later it is clear that the main blocks of sallow regrowth are now far too dense to support PE. The trees have grown together shading out all the lower branches, lifting their crowns and generally leaving nothing of any interest to egg laying PEs.
![55925D51-23F2-4C57-9274-FD3A2B0FEC51.jpeg (4.25 MiB) Viewed 4571 times Almost entirely ignored block of dense sallow at SW](./files/thumb_17917_0ece7f97bbbe1a58141e6f4bf52c874c)
- Almost entirely ignored block of dense sallow at SW
There still exists a limited amount of useable habitat, along the many rides that are maintained by regular cutting, at junctions, and around pools etc where sallows are still able to maintain the lower, horizontal branches that produce the best conditions for young larvae. From my own understanding of the site, and the imagery available from Google Earth, I would guess that PE should have done very well here around the period 2005-2015.
![07FC552C-5552-4773-B4E6-95A2FDE0E96E.jpeg (1.43 MiB) Viewed 4571 times SW evolution post 1987 storm](./files/thumb_17917_4d531df05d0debfb0f46c48d2a97cff7)
- SW evolution post 1987 storm
![1E8267CB-B77A-4CD6-8E00-F5F397F35853.jpeg (2.45 MiB) Viewed 4571 times Comparison of prime breeding habitat 2012 (close to peak) & 2020 (well past peak)](./files/thumb_17917_21f517579a65b3afe1b36a458561a07e)
- Comparison of prime breeding habitat 2012 (close to peak) & 2020 (well past peak)
In short, in this kind of woodland habitat the butterfly ticks along at low density making do with ride side sallows, until new habitat is suddenly created and numbers are allowed to build. The quality of the new habitat peaks when most of the new sallows are substantial young trees that crucially still have enough room around them to allow them to reach out horizontally. Habitat quality reduces as the new trees become crowded and eventually we return to the situation where only the ride side sallows that rely upon regular ride cutting remain useable. This process is now largely complete at Southwater and we are back to pre spike levels. Something now needs to change, or I’m going to end up with 14 post hibernation caterpillars a year for the rest of my days…..
Fermyn has recently been through exactly the same process but on a much greater scale. A couple of years ago I searched for larvae there and found acre after acre of dense sallow jungle with very few egg laying opportunities. Clearly there is still good habitat around Fermyn (recent ride widening will certainly help) as the butterfly continues to do well there, but the sallow jungles I searched were certainly a few years past their peak. My general impression according to the sallow regrowth was that peak Fermyn was 5-10 years ago. Looking back through the historic imagery it’s very easy to identify peak conditions occurring somewhere between 2010 and 2015.
I’m not entirely sure when the land was cleared as 2004 is the first good image, but somewhere around the mid 90’s would fit in with my understanding of peak condition timings.
![357A6DB6-6740-458B-9DA5-06EFB65073FB.jpeg (556.75 KiB) Viewed 4571 times Areas of regen that were dense, largely unsuitable sallow jungles when I visited in 2019](./files/thumb_17917_53010ec7a46532af9aa98ce2462105fc)
- Areas of regen that were dense, largely unsuitable sallow jungles when I visited in 2019
![A7FFAEE3-2DEB-4E01-87B2-5655EDF03598.jpeg (1022.56 KiB) Viewed 4571 times Prime regen habitat developing and reaching a peak soon after 2010](./files/thumb_17917_a4c19dd13e1b63cc26ec21a57e69340f)
- Prime regen habitat developing and reaching a peak soon after 2010
![FE49C396-A298-414C-B659-C3772299D8B9.jpeg (807.2 KiB) Viewed 4571 times Close up of one such block showing superb breeding habitat in 2010, and largely unsuitable conditions by 2021](./files/thumb_17917_80b3794f70e0fe31034b16da83c774d0)
- Close up of one such block showing superb breeding habitat in 2010, and largely unsuitable conditions by 2021
Knepp is more complicated as it is a controlled project that seeks to balance natural regen with various herbivores in such a way as to create a more open, biodiverse landscape. How that plays out over time in terms of PE population will be very interesting. Large blocks of sallow have already become so dense that again, they are past their peak in terms of supporting PE, but other areas are much earlier in the process and will provide peak breeding conditions for several years to come. What kind of longer term egg laying opportunities will exist in the imagined wood pasture landscape one can only guess, but it seems very likely that they will be far greater than currently exist along the few rides in our woodlands where sallows are able to persist.
Ok - that’ll do for this week!
What with my guesstimated tables and now some made up graphs I’m building a nice collection of convincing looking nonsense - I just need to get the hysterical media on board and I’ll be in a position to lock down the entire country!
![🤭](//cdn.jsdelivr.net/gh/twitter/twemoji@latest/assets/svg/1f92d.svg)