Hi guys, as the more regular readers of my PD may know I do a couple of UKBMS transects at my place of work near central London. As part of it I produce an end of year report for my bosses, as much to show I'm not just going off on a jolly for an hour once a week as anything else. This year was my fifth year of doing it and so I was able to produce a few graphs focusing on how recent summers have had impacts on local populations and I thought I might share some of my findings/theories with you guys. It's still work in progress and needs a few tweeks and the text in written in such a way to (hopefully) allow my non butterfly enthusiast bosses to be able to understand my train of thought. Please feel free to comment and discuss
As with previous years, the results broadly follow the wider local and national trends. The major impacting factor during the 2019 season is believed to be a continuation of the effects of the 2018 drought. The species which make up the bulk of the butterfly sightings over the two transects (Gatekeeper, Speckled Wood and Small White) are all late brooded butterflies, The Gatekeeper flies as a single brood in July and the Speckled Wood and Small White are both multi brooded with the last broods on the wing from August through to the first frosts. The Gatekeeper hibernates as a half-grown larva, the Speckled wood as either a half-grown larva or a pupa and the Small White as a pupa. In order to reach these stages in time for hibernation there needs to be a good source of larval foodplants (grasses in the case of the Gatekeeper and Speckled Wood and Crucifers for the Small White), but due to the drought it is likely that many more failed than normal simply due to a reduction in good quality food sources leading up to hibernation. This we believe resulted in fewer than average making it through hibernation leading to the population crashes we can see in two of these species and only a small increase in the third (the Gatekeeper).
By focusing on one species, the speckled Wood, we can get a clearer idea of how the drought influenced the population on the car park site.
Graph 1 shows monthly counts in the car park transect, showing a recurring curve supporting other evidence that this is a resident population. It also illustrates how badly they fared during the 2019 season. By looking in more detail in graph 2, the weekly counts show how the drought in 2018 is the probable cause of the 2019 population crash. In a normal year we see a 3rd spike in adult numbers who’s offspring hibernate and become the first brood of adults the following spring. However, the drought of 2018 desiccated the larval food source (grasses) (pers. obs) which caused a failure of the third spike in adult numbers. This in turn would have led to a reduction in offspring produced which would have been further compounded by the lack of suitable food. The result was a very poor first brood and as can be seen, the population struggled to recover over the course of the year. If this theory is correct, we should see a stronger first brood in 2020 due to a lack of a prolonged hot dry spell in 2019 meaning a higher proportion of offspring should successfully make it through hibernation.
What these results indicate is how fragile butterfly populations can be when they become fragmented. In their natural habitat these population crashes would be naturally buffered by existing in large, unbroken areas of habitat with varying micro habitats where the population could survive during periods of unsuitable weather. Cities with their small pockets of suitable habitat don’t allow for this to happen, and with the onset of climate change putting ever greater pressure on butterflies, the isolated populations simply have nowhere to hold out and wait for conditions to become more favourable, which ultimately leads to localised extinctions becoming more and more common place. The key is to ensure known populations aren’t isolated and are connected by some form of habitat corridors allowing metapopulations to exist which will help to buffer future population crashes.
Interesting comparisons there, BB. I must say the greatest casualty of 2018's drought round my way was the Small Tortoiseshell. I barely saw double figures of adult hibernators, although numbers recovered from mid-June onwards.
Speckled Woods were down in early 2019 (as well as late emerging) although I probably saw about half the normal numbers over the course of April & May as I usually do.
David M wrote:Interesting comparisons there, BB. I must say the greatest casualty of 2018's drought round my way was the Small Tortoiseshell. I barely saw double figures of adult hibernators, although numbers recovered from mid-June onwards.
in the 5 years I've been doing my transects I've seen a grand total of 12 Small Tortoiseshells, 10 of which were in 2015/16 seasons
That is an interesting theory. We haven't had Speckled Woods in East Lothian that long (with them first being recorded here in 2009). So since our first Speckled Wood recorded on a transect in 2012 the numbers have increased year on year. However, 2019 is a little lower than last year.
Our graphs also look a little different and we tend to see more adults in the later generations.
