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Climate change raises a great deal of uncertainty for the future of many species, including Lepidoptera.
Aside from more extreme and frequent weather events, the prospect of warming climates could bring opportunities for some species (e.g. Brown Argus), and threats for others, as suitable climate envelopes are expected to generally shift polewards1. As highly climate-sensitive species with rapid generations and potential for dispersal, southern restricted butterfly species in the UK have garnered increasing interest, and the prospect of northward expansion through the country as previously too-cool areas become climatically suitable. Many of these southern butterflies are early successional specialists and therefore as a group disproportionately in decline2, and there are hopes that climate change could ‘rescue’ some of these species and slow/reverse the downward trends.
One such species is the Grizzled Skipper (Pyrgus malvae), which has declined in abundance by 55% between 1976 and 20183, and despite expectations of climate-driven northern expansion within the UK4, hasn’t shown signs of moving northward. As a UK Biodiversity Action Plan Priority Species, and a high priority species for Butterfly Conservation, there is considerable interest in the Grizzled Skippers’ drivers of decline, and future prospects for conservation.
Consequently, we undertook an investigation into regional trends of the butterfly, and the role of climate in those trends, with hopes of shedding light on the prospects of the Grizzled Skipper in the UK, and more generally exploring the predictions of spatial data-based expansions. We studied abundance trends as relatively positive trends in the north could be a precursor to northern expansion which has not yet been observed. Using a mixed model approach, we modelled Grizzled Skipper population size in the UK from 1976-2016, against time, northing, easting, and 5 km2 resolution climate data. Population size data came from UK Butterfly Monitoring Scheme index values, and climate data were taken from Met Office UKCP09 monthly climate values, specifically mean temperature and total rainfall. Further details of our methods and findings are published and are open access5.
As in other studies, we found long term declines in the population size of the Grizzled Skipper in England. Of note, however, were our findings that declines were significantly steeper in the north and the west, than the south and the east. Furthermore, while we observed that December temperature had a negative impact on population size, as did summer rainfall, the effect sizes were small. The long term population size trends were found to be independent of climate effects, and therefore suggested another unidentified driver of declines, which were not being compensated for by climate change observed so far.
Our findings of greater northern declines in Grizzled Skipper run contrary to expectations of northern expansion under a changing climate4, and suggest that climate change so far has not negated the impacts of other drivers. Our results highlight some hazards of climate envelope models based on spatial data alone, which may not identify factors that limit movement and colonisation (e.g. host-plants, vegetation structure, dispersal capacity), particularly in a non-equilibrium system6.
Consensus from the literature indicates that the likely drivers of decline are habitat loss and degradation7, resulting from a number of pressures including changes in agricultural intensification, loss of natural grazing pressure, and land abandonment. Differences in regional declines could indicate greater rates of habitat loss and degradation in the north and west, compared to the south and east, or that northern and western populations are more sensitive to changes in habitat.
The outcomes of this study further reinforce the importance of habitat creation and maintenance for the Grizzled Skipper, and other habitat specialists that may be expected to be climate change ‘winners’ based on climate associations. Potential regional differences in habitat requirement specificity for the Grizzled Skipper are currently under investigation at the time of the writing of this article as part of a PhD study, with the aim of informing conservation planning and action prioritisation for Butterfly Conservation and the wider land management and conservation community.
1. Parmesan, C, N Ryrholm, C Stefanescu, J K Hill, C D Thomas, H Descimon, B Huntley, L Kaila, J Kullberg, T Tammaru, W J Tennent, J A Thomas, and M Warren. (1999). “Poleward Shift in Georgaphical Ranges of Butterfly Species Associated with Regional Warming.” Nature 399 (June): 579–83. http://dx.doi.org/10.1038/21190.
2. Thomas, Jeremy A., Mike Edwards, David J. Simcox, Gary D. Powney, Tom A. August, and Nick J.B. Isaac. (2015). “Recent Trends in UK Insects That Inhabit Early Successional Stages of Ecosystems.” Biological Journal of the Linnean Society 115 (3): 636–46. doi:10.1111/bij.12527.
3. Brereton, T.M., M.S. Botham, I. Middlebrook, Z. Randle, D. Noble, S. Harris, E.B. Dennis, A.E. Robinson, K. Peck, and D.B. Roy. (2019). “United Kingdom Butterfly Monitoring Scheme Report for 2018.”
4. Settele, J, Otakar Kudrna, Alexander Harpke, Ingolf Kuehn, Chris van Swaay, Rudi Verovnik, Martin Warren, Martin Wiemers, Jan Hanspach, Thomas Hickler, Elisabeth Kuehn, Inge van Halder, Kars Veling, Albert Vliegenthart, Irma Wynhoff, and Oliver Schweiger. (2008). “Climatic Risk Atlas of European Butterflies.” BioRisk 1: 1–618. doi:10.3897/biorisk.1.
5. Bell, Fiona, Marc Botham, Tom M. Brereton, Andy Fenton, and Jenny Hodgson. (2021). “Grizzled Skippers Stuck in the South: Population‐level Responses of an Early‐successional Specialist Butterfly to Climate across Its UK Range over 40 Years.” Diversity and Distributions, no. January: 1–11. doi:10.1111/ddi.13245.
6. Elith, Jane, and John R. Leathwick. (2009). “Species Distribution Models: Ecological Explanation and Prediction Across Space and Time.” Annual Review of Ecology, Evolution, and Systematics 40 (1): 677–97. doi:10.1146/annurev.ecolsys.110308.120159.
7. Asher, J., Richard Fox, M. Warren, P. Harding, G. Jeffcoate, and S. Jeffcoate. (2001). “The Millennium Atlas of Butterflies in Britain and Ireland.” British Wildlife. Oxford University Press, Oxford, UK. doi:10.1111/j.1096-3642.2007.00339.x.