Re: Benjamin
Posted: Mon Jul 24, 2023 12:25 pm
It seems like a long time since you commented on the WLH post but many thanks both.
Finally back from trips and turning my attention to the things that really matter, starting with my thoughts regarding one of the few species I think I have a pretty good understanding of - the plucky old purple emperor.
‘Plucky’ might seem to undersell such a celebrated butterfly but it works for me. The wonderful hyperbole notwithstanding I think I’ve seen emperors being the victim of other wildlife and weather just as much as I’ve seen them dominate it. They certainly do have a go though and I still readily concede that there is no more beautiful a sight in all of nature than a freshly emerged and grounded purple emperor flashing his scale perfect iridescence around like some nymphet mischievously revelling in her newly acquired powers.
The season started with a bang at Knepp, and to some degree took everybody by surprise. After a wet and slow spring emperor larvae were a little behind schedule but the incredible period of warm, settled, dry and sunny weather that commenced around the 22nd of May (presumably triggered by my questionable decision to lay a new lawn so late in the spring) and lasted well into the second half of June had a big impact on larval/pupal development. Under these conditions larval development speeds up significantly and importantly the development range is reduced (or at least doesn’t increase). I think this is because the effects of microclimatic conditions around a site are reduced when the whole site experiences excellent overall conditions for prolonged periods. In the usual mixed spring weather, for example, some particularly well situated caterpillars may still benefit from the small windows of sun and warmth that speed up development whereas others that remain shaded and cool will fall further behind. Under excellent stable conditions everything races along at max speed regardless of location.
At Knepp numbers seemed very high initially and the season has generally been celebrated as being very good. The fine settled weather during the pupal period has been highlighted as the likely cause of the boost in numbers - a reduction in predation due to a shortened pupal period is a major factor being imagined here. There are other benefits that settled weather brings at this delicate time - serious storms for example will certainly cause casualties, but I’ll focus on the idea of reduced predation for now.
I can’t fault the thinking around this, but I tend to think the effect would be smaller than is being imagined.
I’ll quickly lay out my reasoning using the huge population at Knepp as an example.
If we imagine that Knepp produces something like 500 butterflies each year (Neil has suggested this previously - who would argue?) and that pupal predation is around two thirds (the last few years at SW support this) then we give ourselves a starting point of 1500 pupae. This does feel like quite a lot, having spent quite a few hours UV searching there, but the area is vast so ok….
So 1500 are reduced to 500 over the course of the average pupal period, of let’s say, 20 days. If we assume predation to be spread evenly over the 20 days of standard pupation then we can conclude that in a population of this size, 50 pupae are lost to predation each day.
Clearly we now need to look at just how short the pupal period might become under excellent developmental conditions. I think the shortest record in captivity is something like 14 days for a male at the extreme end of the spectrum. In the wild the effect of the conditions will be somewhat reduced compared to captivity (still some mixed weather and cold nights etc) so I think it’s reasonable to suggest a figure somewhere between the average and this extreme record in captivity. If we go down the middle and suggest 17 days as the average under excellent conditions in the wild then we can conclude that three days will have been saved compared to the average pupal period in a standard year.
Saving three days should save 150 butterflies according to the figures we’re working with for Knepp.
That does sound significant, however, my experience (data) and certainly common sense dictates that predation is certainly not spread evenly over the 20 days, and curves away from a high point on day one, to a low point on day 20. Poorly positioned pupae (highly visible) or pupae in areas where birds forage regularly will be picked off early, and those that survive the first few days are much more likely to make it all the way. A pupa that has made it to day 17 is clearly in a very good spot and so predation becomes far less likely over the final three days.
So shaving off these final three days will save far fewer butterflies than the 150 I suggested. How many would still be predated so late on is anyone’s guess, but if we allow 50 (and personally I think it’s likely even fewer than that) then the effect of the shortened pupal period is to produce 550 butterflies instead of 500. A boost for sure, and one that may be noticeable to the most fastidious observers, but not IMO the key factor that I think is being imagined.
Of course at the other end of the spectrum, in a cold year, we can add a few days on to the average 20. This will increase the difference at the extremes but again the extra days will be of increasingly low predation so the effect is again reduced.
As I’ve suggested in previous posts, I believe the impression of a good year has far more to do with weather conditions during the flight period than anything else. This year I think the emergence was condensed following excellent developmental conditions, and this, combined with the excellent weather in the early part of the flight season ensured that everything that emerged was instantly very visible.
Essentially it all happened at once and we got an excellent view of it.
Finally back from trips and turning my attention to the things that really matter, starting with my thoughts regarding one of the few species I think I have a pretty good understanding of - the plucky old purple emperor.
‘Plucky’ might seem to undersell such a celebrated butterfly but it works for me. The wonderful hyperbole notwithstanding I think I’ve seen emperors being the victim of other wildlife and weather just as much as I’ve seen them dominate it. They certainly do have a go though and I still readily concede that there is no more beautiful a sight in all of nature than a freshly emerged and grounded purple emperor flashing his scale perfect iridescence around like some nymphet mischievously revelling in her newly acquired powers.
The season started with a bang at Knepp, and to some degree took everybody by surprise. After a wet and slow spring emperor larvae were a little behind schedule but the incredible period of warm, settled, dry and sunny weather that commenced around the 22nd of May (presumably triggered by my questionable decision to lay a new lawn so late in the spring) and lasted well into the second half of June had a big impact on larval/pupal development. Under these conditions larval development speeds up significantly and importantly the development range is reduced (or at least doesn’t increase). I think this is because the effects of microclimatic conditions around a site are reduced when the whole site experiences excellent overall conditions for prolonged periods. In the usual mixed spring weather, for example, some particularly well situated caterpillars may still benefit from the small windows of sun and warmth that speed up development whereas others that remain shaded and cool will fall further behind. Under excellent stable conditions everything races along at max speed regardless of location.
At Knepp numbers seemed very high initially and the season has generally been celebrated as being very good. The fine settled weather during the pupal period has been highlighted as the likely cause of the boost in numbers - a reduction in predation due to a shortened pupal period is a major factor being imagined here. There are other benefits that settled weather brings at this delicate time - serious storms for example will certainly cause casualties, but I’ll focus on the idea of reduced predation for now.
I can’t fault the thinking around this, but I tend to think the effect would be smaller than is being imagined.
I’ll quickly lay out my reasoning using the huge population at Knepp as an example.
If we imagine that Knepp produces something like 500 butterflies each year (Neil has suggested this previously - who would argue?) and that pupal predation is around two thirds (the last few years at SW support this) then we give ourselves a starting point of 1500 pupae. This does feel like quite a lot, having spent quite a few hours UV searching there, but the area is vast so ok….
So 1500 are reduced to 500 over the course of the average pupal period, of let’s say, 20 days. If we assume predation to be spread evenly over the 20 days of standard pupation then we can conclude that in a population of this size, 50 pupae are lost to predation each day.
Clearly we now need to look at just how short the pupal period might become under excellent developmental conditions. I think the shortest record in captivity is something like 14 days for a male at the extreme end of the spectrum. In the wild the effect of the conditions will be somewhat reduced compared to captivity (still some mixed weather and cold nights etc) so I think it’s reasonable to suggest a figure somewhere between the average and this extreme record in captivity. If we go down the middle and suggest 17 days as the average under excellent conditions in the wild then we can conclude that three days will have been saved compared to the average pupal period in a standard year.
Saving three days should save 150 butterflies according to the figures we’re working with for Knepp.
That does sound significant, however, my experience (data) and certainly common sense dictates that predation is certainly not spread evenly over the 20 days, and curves away from a high point on day one, to a low point on day 20. Poorly positioned pupae (highly visible) or pupae in areas where birds forage regularly will be picked off early, and those that survive the first few days are much more likely to make it all the way. A pupa that has made it to day 17 is clearly in a very good spot and so predation becomes far less likely over the final three days.
So shaving off these final three days will save far fewer butterflies than the 150 I suggested. How many would still be predated so late on is anyone’s guess, but if we allow 50 (and personally I think it’s likely even fewer than that) then the effect of the shortened pupal period is to produce 550 butterflies instead of 500. A boost for sure, and one that may be noticeable to the most fastidious observers, but not IMO the key factor that I think is being imagined.
Of course at the other end of the spectrum, in a cold year, we can add a few days on to the average 20. This will increase the difference at the extremes but again the extra days will be of increasingly low predation so the effect is again reduced.
As I’ve suggested in previous posts, I believe the impression of a good year has far more to do with weather conditions during the flight period than anything else. This year I think the emergence was condensed following excellent developmental conditions, and this, combined with the excellent weather in the early part of the flight season ensured that everything that emerged was instantly very visible.
Essentially it all happened at once and we got an excellent view of it.