Great stuff Ben. I have read that Blue Tits are responsible for much predation of PE larvae.
I wonder if predation increases in Spring when the birds have young to feed.
Like you I am hoping for a bumper PE emergence this year.
Stay safe and well,
Trevor.
Benjamin
Re: Benjamin
Cheers Trevor. Tits certainly seem responsible for the majority of winter losses. Of course numbers fluctuate, but in good years flocks of various passerines (mainly a few tit species) sweep through the sallows apparently systematically checking forks and other likely spots for winter sustenance.
I don’t lose many to predation once the leaves burst though. There are very few iris caterpillars left at this stage and they are expertly camouflaged amongst the leaves and catkins. Larvae of various moth and sawfly species explode with leaf burst and I guess foraging birds quickly become accustomed to targeting them.
So a reduced population of tits does seem (as anyone would guess) to be positive for overwintering iris caterpillars, although perhaps also positive for bronze shieldbug numbers! Most likely some confirmation bias is at play here, but I am noticing lots of them this year! Cold weather reduces their activity though, so a combination of low tit numbers and cold weather should give us the result we’re looking for.
I don’t lose many to predation once the leaves burst though. There are very few iris caterpillars left at this stage and they are expertly camouflaged amongst the leaves and catkins. Larvae of various moth and sawfly species explode with leaf burst and I guess foraging birds quickly become accustomed to targeting them.
So a reduced population of tits does seem (as anyone would guess) to be positive for overwintering iris caterpillars, although perhaps also positive for bronze shieldbug numbers! Most likely some confirmation bias is at play here, but I am noticing lots of them this year! Cold weather reduces their activity though, so a combination of low tit numbers and cold weather should give us the result we’re looking for.
Re: Benjamin
Speaking of cold weather…..
Yesterday (6th) I took the opportunity of an expected heavy frost to have a go at some wintry caterpillar photos.
Unfortunately I couldn’t get to the woods until after the school run, and with temperatures rapidly rising my window for icy pics was very small. I managed to get a few though, and will try again if I’m free next time the opportunity arises.
Yesterday (6th) I took the opportunity of an expected heavy frost to have a go at some wintry caterpillar photos.
Unfortunately I couldn’t get to the woods until after the school run, and with temperatures rapidly rising my window for icy pics was very small. I managed to get a few though, and will try again if I’m free next time the opportunity arises.
- Fatima
- David
- Linford
- Not taken at the right angle but you get the idea
- Many dangles still remain - most caterpillars do not settle as close to them as this!
- Back to a muddy bog within an hour unfortunately
Re: Benjamin
That second frost covered Emperor larvae shot is quite superb Benjamin ! one of my favourite shots that i,ve seen since i,ve been posting !
Brilliant .. .. And enjoying you,re superb diary also Regards Allan.W.
Brilliant .. .. And enjoying you,re superb diary also Regards Allan.W.
-
Neil Hulme
- Posts: 3595
- Joined: Fri Feb 08, 2008 8:27 pm
Re: Benjamin
I'll second that! A rare and stunningly beautiful image.
I hope to catch up soon for a beer.
BWs, Neil
I hope to catch up soon for a beer.
BWs, Neil
Re: Benjamin
Many thanks Allan for your very kind comments.
Thanks also Neil, and yes - beers before things begin to stir….
Thanks also Neil, and yes - beers before things begin to stir….
Re: Benjamin
Just following on from my previous thoughts regarding the functioning of this particular iris site I did a few doodles to show what is really just common sense. Having collected all the data though, it’s satisfying to represent it in some way.
The structure here is that of a mainland-island metapopulation (at least according to a quick Google search). I know these terms are useful when comparing and contrasting population structures but I can’t help but have a natural aversion to them. I’m sure it’s not the term itself I dislike though - more likely the people that like to hide behind an impenetrable web of them (or perhaps I just have a chip on my shoulder having not been educated in this area!).
Anyway - this certainly isn’t a difficult concept to get hold of. A big breeding ground with some smaller satellite/island sites supporting it. Actually according to my pupae searches, in recent years these satellite sites have hosted so few larvae that very little, if any contribution has been made from them to the adult population. But it needn’t necessarily be like that, and in good years there’s certainly enough habitat available in these satellite/island sites for a contribution to be made. Not enough to support a population without the central hub though, which is I suppose, what the term describes, and why I have to admit that it is useful.
This site is a little bit more complicated however, as Knepp has recently emerged on the doorstep. Knepp was no doubt colonised from butterflies originating from my mainland hub (or whatever it’s supposed to be called) and now completely dwarfs it. No doubt there’s a term for this! So the apparent offspring becomes bigger than the parent - hmmm - cuckoo colony? But there’s no foul play here so perhaps not. Perhaps the ever swelling new site reaches some kind of tipping point (they do love to name a threshold) where matriphagy becomes inevitable - I’m sure you could get some juicy terms out of that process. Whatever you call it it’s unlikely to happen here though, because the Knepp megahub/cuckoo colony/swollen baby monster hub, or whatever, is separated from the original mainland by a couple of miles of largely unsuitable habitat - the ‘matriphagal defence zone’ perhaps.
Forgetting Knepp for a moment, the mainland-island structure is clearly dictated by the surrounding countryside and were there another, similarly sized, block of sallow rich woodland nearby then we’d no doubt see a two hub system with a two way flow of individuals surrounded by small satellite/island sites - I guess this would be called something else. In fact I think this particular species will likely utilise whatever island/mainland combination happens to be available, and so any number of metapopulation variants might be encountered, perhaps even (as long as there are enough useable islands), without the need for a self sustaining central hub.
As I’ve said before, a vast forest could and probably would produce exactly the same structure when only occasional pockets of regrowth provide the correct conditions for early stages, so it is hardly surprising that iris seems apparently perfectly adapted to exploit this habitat structure when it encounters it in our fragmented modern countryside.
- Distribution of overwintering larvae (2019-2021)
- Distribution of pupae (2019-2021)
Anyway - this certainly isn’t a difficult concept to get hold of. A big breeding ground with some smaller satellite/island sites supporting it. Actually according to my pupae searches, in recent years these satellite sites have hosted so few larvae that very little, if any contribution has been made from them to the adult population. But it needn’t necessarily be like that, and in good years there’s certainly enough habitat available in these satellite/island sites for a contribution to be made. Not enough to support a population without the central hub though, which is I suppose, what the term describes, and why I have to admit that it is useful.
This site is a little bit more complicated however, as Knepp has recently emerged on the doorstep. Knepp was no doubt colonised from butterflies originating from my mainland hub (or whatever it’s supposed to be called) and now completely dwarfs it. No doubt there’s a term for this! So the apparent offspring becomes bigger than the parent - hmmm - cuckoo colony? But there’s no foul play here so perhaps not. Perhaps the ever swelling new site reaches some kind of tipping point (they do love to name a threshold) where matriphagy becomes inevitable - I’m sure you could get some juicy terms out of that process. Whatever you call it it’s unlikely to happen here though, because the Knepp megahub/cuckoo colony/swollen baby monster hub, or whatever, is separated from the original mainland by a couple of miles of largely unsuitable habitat - the ‘matriphagal defence zone’ perhaps.
Forgetting Knepp for a moment, the mainland-island structure is clearly dictated by the surrounding countryside and were there another, similarly sized, block of sallow rich woodland nearby then we’d no doubt see a two hub system with a two way flow of individuals surrounded by small satellite/island sites - I guess this would be called something else. In fact I think this particular species will likely utilise whatever island/mainland combination happens to be available, and so any number of metapopulation variants might be encountered, perhaps even (as long as there are enough useable islands), without the need for a self sustaining central hub.
As I’ve said before, a vast forest could and probably would produce exactly the same structure when only occasional pockets of regrowth provide the correct conditions for early stages, so it is hardly surprising that iris seems apparently perfectly adapted to exploit this habitat structure when it encounters it in our fragmented modern countryside.
Re: Benjamin
So now I’ll move on to the tricky topic of population size.
In an area this kind of size (my site is something like 100 hectares) it might be reasonable to imagine that the adult population consists of perhaps a couple of hundred butterflies. This would be consistent with much that it written about this butterfly - primarily that its elusiveness means that we only ever catch sight of a tiny minority of the total population. Also when you imagine that the emergence may be drawn out over the course of 4 weeks or more, and an individual lifespan will be some fraction of this, then a couple of hundred starts to seem modest for an area where sightings have historically been relatively reliable throughout the flight period. Perhaps you might even be tempted to lean more towards Heslop’s (possibly the only person brave enough to have put a number to it) estimate of 1000 individuals being the minimum requirement for a self sustaining colony.
I think at the start of this project, and having visited the site on and off for several years during the flight period, I would have been happy to assume that ‘several hundred’ would be a reasonable way to describe the probable population.
I am no longer of that view, and right from year 1, I found it hard to make the numbers work. Initially I assumed I was simply suffering from the severely limited access to the full picture that should be expected from my traditional fieldwork approach. It seemed so surprising to many in the know, that I was finding anything at all during winter, that I had to conclude that my discoveries must only account for the tiniest fraction of the overwintering population. I was also warned that despite my success in the first year, I should prepare myself for the possibility that I find nothing at all in year 2. It seemed that to hope for anything more than a glimpse into the ecology of this species would be to set oneself up for a fall. Over the last 3 years of fieldwork I’ve gradually broken this assumption down and have written previously about the various factors that have given me the confidence that I now have. The use of UV pupae searches was crucial in this regard, not because they revealed something previously unknown, but because they confirmed my already firm views and gave me the confidence to air them.
I’ve put together some tables that give an idea of how I think the population at my site works, and then run the numbers for various adult population values. The loss rate at various stages is based sometimes on good data, and sometimes largely guessed. I have studied the early stages population from L3 diapause onwards far more comprehensively than anything earlier than this, so the tables were developed by first inserting the high confidence values and data, and then filling in the low confidence values in such a way as to
a)fit in with my observations however limited
b)allow the table to be coherent in terms of the relationship between adult population and expected egg lay.
My early stages study will lean towards testing these low confidence early instar values in coming years.
The model tables represent relatively stable populations ie the adult population can be reasonably expected to produce enough eggs to produce the same number of adults in the following year.
As we know, population can fluctuate quite dramatically, and although significant changes to any of the values could produce this, I suggest losses during diapause as the most crucial. Weighting this so heavily is pure speculation, but I’m hopeful the very low winter predation I have observed so far this year might continue and produce the sort of result seen in the low winter predation tables below. These tables do not represent stable populations but show how a reduction in winter predation from 70% - 30% would lead to an increasing population and an escape from the stable bases of 30 & 10 adults.
Of course I realise that if you tweak one value then another may change in response. For example you might suggest that if more caterpillars survive winter predation, then a higher % of losses will occur in later instars. This could be the result of predators encountering them more often and learning to target them, but at the kind of low numbers I will suggest for my site, I can’t see this occurring. I would imagine this to be more of a factor in early instars when abundance is high. I guess it could come into play with pupae - especially if their fluorescence makes them relatively easy to target.
Currently I see this factor (winter predation) coupled with egg lay conditions (obviously not represented in my tables) as the most crucial in dictating local population fluctuations. Much has always been made of spring conditions for larval development and weather during the pupal phase, but I’m unconvinced that these variables are crucial. Of course everything has an effect, but in my experience larvae seem very able to cope with everything spring has to throw at them, and pupae suffer relatively high predation regardless of whether the period is shortened by a few days in warm weather or lengthened by cold (if they are to be predated it often happens within the first few days). In terms of ‘spring conditions’ I suppose timing of leaf burst can have a strong effect as caterpillars are more secure once able to hide amongst unfurling leaves, but in my calculations this is lumped into winter predation.
So having laid out some possible models the following is a suggestion of what I believe occurred at my site last year (the 2nd of two particularly bad years) and what I hope might be the basis of a recovery this year.
I realise suggesting a population of 10 butterflies might test the patience of even the most forgiving ecologist, but it is entirely consistent with my thorough investigations. Something like 80% (including all the prime areas) of my mainland hub was surveyed with UV light over several nights throughout the pupation phase. A high quality UV light represents a very efficient and high accuracy survey method, and revealed 13 pupae, 5 of which survived to emerge. I’m allowing 5 others, from the other 20% of unlikely habitat (including surrounding islands) or pupae that I somehow missed in my surveyed area. In light of these pupae surveys we should look back at the stable population models:
I also spent something like 10 days on site during what I knew to the peak of the flight period and managed a total of 4 sightings. I don’t consider iris a particularly difficult butterfly to spot - they’re big and obvious, take up known territories, wheel around treetops against clear skies, fly low down along human walkways, attack everything in sight and don’t care a jot whether you see them or not - well, the boys anyway! I suspect that it’s not the behaviour, but the numbers that often lead to them going unnoticed, but let’s not take that on now.
With a population as low as 10 my tables start to finally feel like they represent the true early stages picture that I have witnessed.
If I am correct that I have witnessed 2 particularity bad years at my site, then I would suggest the 30 adult model to be the norm here, and the good years to look like the associated low winter predation escape from this model (70 adults).
Lastly I should just repeat that I’m using this PD to keep track of my thoughts as I go along, and these entries only summarise my theories as they currently exist (i.e. i’m aware this could all be total bollocks). The project continues, and no doubt new data will continue to enhance my understanding. It could well be the case, for example, that winter predation ends up at the normal level (70%) this year, and the low predation (30%) boom years that I’m suggesting in my tables fail to materialise.
In an area this kind of size (my site is something like 100 hectares) it might be reasonable to imagine that the adult population consists of perhaps a couple of hundred butterflies. This would be consistent with much that it written about this butterfly - primarily that its elusiveness means that we only ever catch sight of a tiny minority of the total population. Also when you imagine that the emergence may be drawn out over the course of 4 weeks or more, and an individual lifespan will be some fraction of this, then a couple of hundred starts to seem modest for an area where sightings have historically been relatively reliable throughout the flight period. Perhaps you might even be tempted to lean more towards Heslop’s (possibly the only person brave enough to have put a number to it) estimate of 1000 individuals being the minimum requirement for a self sustaining colony.
I think at the start of this project, and having visited the site on and off for several years during the flight period, I would have been happy to assume that ‘several hundred’ would be a reasonable way to describe the probable population.
I am no longer of that view, and right from year 1, I found it hard to make the numbers work. Initially I assumed I was simply suffering from the severely limited access to the full picture that should be expected from my traditional fieldwork approach. It seemed so surprising to many in the know, that I was finding anything at all during winter, that I had to conclude that my discoveries must only account for the tiniest fraction of the overwintering population. I was also warned that despite my success in the first year, I should prepare myself for the possibility that I find nothing at all in year 2. It seemed that to hope for anything more than a glimpse into the ecology of this species would be to set oneself up for a fall. Over the last 3 years of fieldwork I’ve gradually broken this assumption down and have written previously about the various factors that have given me the confidence that I now have. The use of UV pupae searches was crucial in this regard, not because they revealed something previously unknown, but because they confirmed my already firm views and gave me the confidence to air them.
I’ve put together some tables that give an idea of how I think the population at my site works, and then run the numbers for various adult population values. The loss rate at various stages is based sometimes on good data, and sometimes largely guessed. I have studied the early stages population from L3 diapause onwards far more comprehensively than anything earlier than this, so the tables were developed by first inserting the high confidence values and data, and then filling in the low confidence values in such a way as to
a)fit in with my observations however limited
b)allow the table to be coherent in terms of the relationship between adult population and expected egg lay.
My early stages study will lean towards testing these low confidence early instar values in coming years.
As we know, population can fluctuate quite dramatically, and although significant changes to any of the values could produce this, I suggest losses during diapause as the most crucial. Weighting this so heavily is pure speculation, but I’m hopeful the very low winter predation I have observed so far this year might continue and produce the sort of result seen in the low winter predation tables below. These tables do not represent stable populations but show how a reduction in winter predation from 70% - 30% would lead to an increasing population and an escape from the stable bases of 30 & 10 adults.
Currently I see this factor (winter predation) coupled with egg lay conditions (obviously not represented in my tables) as the most crucial in dictating local population fluctuations. Much has always been made of spring conditions for larval development and weather during the pupal phase, but I’m unconvinced that these variables are crucial. Of course everything has an effect, but in my experience larvae seem very able to cope with everything spring has to throw at them, and pupae suffer relatively high predation regardless of whether the period is shortened by a few days in warm weather or lengthened by cold (if they are to be predated it often happens within the first few days). In terms of ‘spring conditions’ I suppose timing of leaf burst can have a strong effect as caterpillars are more secure once able to hide amongst unfurling leaves, but in my calculations this is lumped into winter predation.
So having laid out some possible models the following is a suggestion of what I believe occurred at my site last year (the 2nd of two particularly bad years) and what I hope might be the basis of a recovery this year.
- The graffiti should really speak for itself.
With a population as low as 10 my tables start to finally feel like they represent the true early stages picture that I have witnessed.
If I am correct that I have witnessed 2 particularity bad years at my site, then I would suggest the 30 adult model to be the norm here, and the good years to look like the associated low winter predation escape from this model (70 adults).
Re: Benjamin
I'm a little late viewing your later posts, but I have to agree with others
that those frosted comparison images are rare and stunning. Real textbook stuff!.
I think you've earned a beer from Neil ( and from me sometime ).
Looking forward to the adults,
Trevor.
that those frosted comparison images are rare and stunning. Real textbook stuff!.
I think you've earned a beer from Neil ( and from me sometime ).
Looking forward to the adults,
Trevor.
Re: Benjamin
Cracking shots of the cats in cryogenic suspension Ben, another reminder that butterflies aren't as fragile as we often imagine them 
I'll need a little time to read through your report on the population distribution but it's something I'm looking forward to, Science nerd that I am 
Have a goodun and stay safe
Wurzel
I'll need a little time to read through your report on the population distribution but it's something I'm looking forward to, Science nerd that I am Have a goodun and stay safe
Wurzel
Re: Benjamin
Fascinating stuff, Ben. Good to see scientific rigour, along with your hypotheses.
Jon
Jon
Re: Benjamin
Impressive and eye-catching images of the frozen larvae, Ben. Just shows how tough these little critters need to be to make it through to spring.
Even more impressive is the amount of work you are putting in. Those theories regarding the interaction between sites is thought-provoking. I’d love it if an extremely light GPS transmitter could be attached to one of these butterflies. It would be hugely interesting to see their movements over a period of time.
Even more impressive is the amount of work you are putting in. Those theories regarding the interaction between sites is thought-provoking. I’d love it if an extremely light GPS transmitter could be attached to one of these butterflies. It would be hugely interesting to see their movements over a period of time.
Re: Benjamin
Thanks all - it’s hard to go too far wrong with caterpillars surrounded by ice but the small window of opportunity does make it a frantic race against time. If we get some really cold weather then I can have some fun.
I’ve often fantasised about GPS tracking of butterflies David - with other species even more than iris. I keep an eye on the state of the necessary technology, and although something workable will no doubt arrive before we imagine, it does seem a little way off as it stands. The more limited alternatives in use offer reasonable results within the much narrower applications that they are used for, but none of these are the breakthrough that we are imagining. I must confess to mixed feelings though - we’ll learn so much with relatively little effort and many of the mysteries that captivate us will be solved overnight. Much will be gained but something will also be lost. On the surface that might sound a bit backwards, but I think it drills down into much deeper considerations. Anyway…..I’m sure I’d be unable to resist peeking into ‘The Big Book of Answers’ were it presented to me!
Yesterday (14th) I had another go at some early morning icy photos. Again the frost was suitably heavy, but again I had precious little time to zip around and take advantage.
After the thaw I checked on a nearby island area and found evidence of caterpillar presence (definite dangle) but no hibernating cats. A female flew through here, but didn’t stay long. It’ll be checked at night for pupae in June, but i don’t expect to find anything.
Predation by tits has now started unfortunately - an entire tree (4 cats) wiped out in a single visit (I guess). We may suffer heavy losses this month. Poor Linford amongst the first wave of casualties (typically after I’d said I fancy his survival chances).
I’ve often fantasised about GPS tracking of butterflies David - with other species even more than iris. I keep an eye on the state of the necessary technology, and although something workable will no doubt arrive before we imagine, it does seem a little way off as it stands. The more limited alternatives in use offer reasonable results within the much narrower applications that they are used for, but none of these are the breakthrough that we are imagining. I must confess to mixed feelings though - we’ll learn so much with relatively little effort and many of the mysteries that captivate us will be solved overnight. Much will be gained but something will also be lost. On the surface that might sound a bit backwards, but I think it drills down into much deeper considerations. Anyway…..I’m sure I’d be unable to resist peeking into ‘The Big Book of Answers’ were it presented to me!
Yesterday (14th) I had another go at some early morning icy photos. Again the frost was suitably heavy, but again I had precious little time to zip around and take advantage.
- Just a reminder of scale
- Alistair
Re: Benjamin
Some beautiful winter images there, Ben. Those poor cats have to suffer being frozen regularly as well as being picked off by predators. 
It makes the appearance of the adults all the more precious, I suppose, knowing what they've had to put up with in their earlier stages.
Sad to hear about Linford, although I suppose there is a certain irony in him ending up being a lunchbox for a bird!
It makes the appearance of the adults all the more precious, I suppose, knowing what they've had to put up with in their earlier stages.Sad to hear about Linford, although I suppose there is a certain irony in him ending up being a lunchbox for a bird!
Re: Benjamin
Your fascinating study of overwintering cats continues, sorry to read of losses.
Even PE larvae are part of the food chain. I have a large Acer tree in the garden,
which is regularly visited by Blue and Long Tailed Tits during the Winter.
It is amazing to watch them giving the tree a thorough once over, and it becomes
obvious how easy it would be for them to locate a dormant PE cat.
Keep up the good work, stay well.
Trevor.
Even PE larvae are part of the food chain. I have a large Acer tree in the garden,
which is regularly visited by Blue and Long Tailed Tits during the Winter.
It is amazing to watch them giving the tree a thorough once over, and it becomes
obvious how easy it would be for them to locate a dormant PE cat.
Keep up the good work, stay well.
Trevor.
Re: Benjamin
Many thanks David and Trevor. Losses are disappointing but we must try not to begrudge these evil little razor-beaked bastards their lunch - ok, I’ve still got some work to do on that front.
As I disturb them on their foraging trips they sometimes stop en mass and look at me. I’m always reminded of this video I saw some years ago - I’m sure if one started they’d all pile in - might have to invest in a tin hat.
https://youtu.be/sGpqOGWpK3E
January check complete and it was a brutal month in the woods.
I’m not sure why the tits took December off but I’m sure they had their reasons (best not to think about it!). Whatever the truth they certainly made up for lost time by removing 44.2% of the field in January.
8 of the Olympians were retired, with a further 11 from the wider sample joining them.
Only one was lost in December so the running total of losses is 20/44 - 45.5%.
I have quite a few extras that are higher up that require too much disruption to check on, or else have only been located via dangling leaves so I’ll ignore those until spring.
I had hoped that the lack of predation in December pointed towards a winter of increased survival and possibly a population boost at my site (detailed in tables in previous post). This now looks to be unlikely, as we appear to be heading towards the 70% winter predation that we’ve seen previously (75% year 1, 65% year 2).
Also with 24 caterpillars currently remaining it’s not unreasonable to think that we might end up with a similar number of survivors to previous years (14 year 1, 13 year 2).
This kind of consistency certainly gets you thinking. I’m waiting for a year where something dramatically different happens (population explosion ideally), so I can try to figure out what caused it, but I’m starting to wonder whether such a thing could even happen.
As I’ve suggested previously it might just be that the current population size is very tightly tied to the quality and quantity of breeding habitat available. I know that sounds pretty obvious, but if that is the case then there will be no dramatic change in numbers, unless a dramatic change in habitat occurs. But we do see numbers fluctuate from year to year - don’t we? I’m not really sure if we do. Flight season weather plays such a huge role in the visibility of PE, and butterflies generally, that two years of exactly the same numbers could be reported in quite opposite ways, purely due to weather.
Unsurprisingly I’ve noticed this a lot more since being freed up to spend so much time out in the field. Everyone I meet in the woods who knows anything about butterflies has something to say about how a particular species might be doing in the current season compared to the last, and literally none of it ever bears any relationship to what I see as I plod around (obviously I don’t tell them that - I just nod along and say whatever I think might be key to getting them to shuffle off with a vaguely positive feeling about our interaction). By the way, if any of the lovely people I meet and chat to are reading this (unlikely) then obviously I don’t mean you - it’s those others I’m on about!
*oh right - so that’s how you italicise text - should really have figured that out sooner. No point changing now - I’ll stick to not bothering for scientific names.
Of course butterfly populations do fluctuate from one year to the next - dramatically so in certain species and for all sorts of reasons, but in many cases, especially for mobile species it’s almost impossible to get a realistic handle on numbers in any year. For these species I’m sure that weather over a few key weekends of the flight season plays a bigger role in the conclusions we draw than anything else.
PE must be particularly prone to this observation error given its elusive nature and habits. Yes, I know in my previous post I was making the case for exactly the opposite, but in that moment I was reflecting upon their visibility to someone who studies the butterfly; who knows the site; the territories; flight season timings etc etc. I’m well aware that someone out for a pleasant woodland ramble (even if they have a general interest in butterflies) could spend all summer in a PE wood and never see one.
My feeling for PE is that the state of the habitat at a given site dictates numbers but flight season weather probably dictates the conclusions we draw about the quality of the season. With that in mind, I think the site at Southwater is currently returning to an area of low carrying capacity having enjoyed a decade or so of increased population and I’ll dedicate the next post to having a closer look at this.
As I disturb them on their foraging trips they sometimes stop en mass and look at me. I’m always reminded of this video I saw some years ago - I’m sure if one started they’d all pile in - might have to invest in a tin hat.
https://youtu.be/sGpqOGWpK3E
January check complete and it was a brutal month in the woods.
I’m not sure why the tits took December off but I’m sure they had their reasons (best not to think about it!). Whatever the truth they certainly made up for lost time by removing 44.2% of the field in January.
8 of the Olympians were retired, with a further 11 from the wider sample joining them.
I have quite a few extras that are higher up that require too much disruption to check on, or else have only been located via dangling leaves so I’ll ignore those until spring.
I had hoped that the lack of predation in December pointed towards a winter of increased survival and possibly a population boost at my site (detailed in tables in previous post). This now looks to be unlikely, as we appear to be heading towards the 70% winter predation that we’ve seen previously (75% year 1, 65% year 2).
Also with 24 caterpillars currently remaining it’s not unreasonable to think that we might end up with a similar number of survivors to previous years (14 year 1, 13 year 2).
- Still in the game for now….
- Still in the game for now….
As I’ve suggested previously it might just be that the current population size is very tightly tied to the quality and quantity of breeding habitat available. I know that sounds pretty obvious, but if that is the case then there will be no dramatic change in numbers, unless a dramatic change in habitat occurs. But we do see numbers fluctuate from year to year - don’t we? I’m not really sure if we do. Flight season weather plays such a huge role in the visibility of PE, and butterflies generally, that two years of exactly the same numbers could be reported in quite opposite ways, purely due to weather.
Unsurprisingly I’ve noticed this a lot more since being freed up to spend so much time out in the field. Everyone I meet in the woods who knows anything about butterflies has something to say about how a particular species might be doing in the current season compared to the last, and literally none of it ever bears any relationship to what I see as I plod around (obviously I don’t tell them that - I just nod along and say whatever I think might be key to getting them to shuffle off with a vaguely positive feeling about our interaction). By the way, if any of the lovely people I meet and chat to are reading this (unlikely) then obviously I don’t mean you - it’s those others I’m on about!
*oh right - so that’s how you italicise text - should really have figured that out sooner. No point changing now - I’ll stick to not bothering for scientific names.
Of course butterfly populations do fluctuate from one year to the next - dramatically so in certain species and for all sorts of reasons, but in many cases, especially for mobile species it’s almost impossible to get a realistic handle on numbers in any year. For these species I’m sure that weather over a few key weekends of the flight season plays a bigger role in the conclusions we draw than anything else.
PE must be particularly prone to this observation error given its elusive nature and habits. Yes, I know in my previous post I was making the case for exactly the opposite, but in that moment I was reflecting upon their visibility to someone who studies the butterfly; who knows the site; the territories; flight season timings etc etc. I’m well aware that someone out for a pleasant woodland ramble (even if they have a general interest in butterflies) could spend all summer in a PE wood and never see one.
My feeling for PE is that the state of the habitat at a given site dictates numbers but flight season weather probably dictates the conclusions we draw about the quality of the season. With that in mind, I think the site at Southwater is currently returning to an area of low carrying capacity having enjoyed a decade or so of increased population and I’ll dedicate the next post to having a closer look at this.
Re: Benjamin
That is all wonderfully fascinating speculation, Ben. It seems the more you observe and go into the detail of what you see, the more unanswered questions there are. Purple Emperors appear to be a law unto themselves (the clue is in their Imperial name, perhaps - they are answerable to no one! 
). My own relatively meagre experience over a number of years of the species at Chiddingfold has to be honest provided no hard-and-fast predictive rules as to how a season will pan out - but I've come to realise that the uncertainty is actually part of the fun. It will always be a butterfly that provides a great deal of pleasure - entertainment, even - when it does appear, and all kinds of quirky moments. In 2021, I had already put in a lot of hours of as yet unrewarded watching when I received a message (plus photo) from a friend in Naseby (Northants) asking what the large butterfly was that had just invaded her dining room... You can guess. 
I shall be continuing to follow the evolving story as you move into spring among the sallows... Many thanks for this, Ben, I wish I had your patience!
Cheers,
Dave
). My own relatively meagre experience over a number of years of the species at Chiddingfold has to be honest provided no hard-and-fast predictive rules as to how a season will pan out - but I've come to realise that the uncertainty is actually part of the fun. It will always be a butterfly that provides a great deal of pleasure - entertainment, even - when it does appear, and all kinds of quirky moments. In 2021, I had already put in a lot of hours of as yet unrewarded watching when I received a message (plus photo) from a friend in Naseby (Northants) asking what the large butterfly was that had just invaded her dining room... You can guess. I shall be continuing to follow the evolving story as you move into spring among the sallows... Many thanks for this, Ben, I wish I had your patience!
Cheers,
Dave
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Pete Eeles
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Re: Benjamin
I have to admit that I'm absolutely gutted (not as much as your good self, I'm sure, Ben) that some of the iconic larvae (e.g. Linford) have been lost. Like many others, I'm in awe of your diligence And look forward to the next update - good work, sir!
Cheers,
- Pete
And look forward to the next update - good work, sir!Cheers,
- Pete
Life Cycles of British & Irish Butterflies: http://www.butterflylifecycles.com
British & Irish Butterflies Rarities: http://www.butterflyrarities.com
British & Irish Butterflies Rarities: http://www.butterflyrarities.com
Re: Benjamin
Some really interesting thoughts there Ben, plenty of food for thought although one bit did stick out for me in particular:
" I’m well aware that someone out for a pleasant woodland ramble (even if they have a general interest in butterflies) could spend all summer in a PE wood and never see one."...I've often found that these are the very same people that the Purple Emperor will land on, generally when I'm on the other side of the wood patieintly watching one that refuses to come down from the canopy!
But seriously you made me consider how easy it is to make sweeping statements such as "species x is having a good year" based on very little evidence/a few trips out...something I'll bear in mind in future. Keep up the great work!
Have a goodun
Wurzel
" I’m well aware that someone out for a pleasant woodland ramble (even if they have a general interest in butterflies) could spend all summer in a PE wood and never see one."...I've often found that these are the very same people that the Purple Emperor will land on, generally when I'm on the other side of the wood patieintly watching one that refuses to come down from the canopy!
But seriously you made me consider how easy it is to make sweeping statements such as "species x is having a good year" based on very little evidence/a few trips out...something I'll bear in mind in future. Keep up the great work!
Have a goodun
Wurzel
Re: Benjamin
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.
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.
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.
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.
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!
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.
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.
- Almost entirely ignored block of dense sallow at SW
- SW evolution post 1987 storm
- Comparison of prime breeding habitat 2012 (close to peak) & 2020 (well past peak)
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.
- Areas of regen that were dense, largely unsuitable sallow jungles when I visited in 2019
- Prime regen habitat developing and reaching a peak soon after 2010
- Close up of one such block showing superb breeding habitat in 2010, and largely unsuitable conditions by 2021
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!
