Climate change poses great uncertainty about the future for many species, including butterflies.
Aside from more extreme and frequent weather events, the prospect of climate warming could present opportunities for some species (e.g. Brown Argus) and threats to others, as appropriate climatic envelopes are expected to generally shift upwards1. As highly climate-sensitive species with rapid generations and potential for dispersal, butterfly species restricted to the south have aroused increasing interest in Great Britain, and the prospect of expanding northward through the country, as previously too cool areas are becoming climatically suitable. Many of these southern butterflies are early succession specialists and are therefore disproportionately declining as a group2, and there is hope that climate change could “save” some of these species and slow / reverse downward trends.
One such species is the Grizzled Skipper (Pyrgus malvae), which declined in abundance by 55% between 1976 and 20183 and has shown no signs of moving north within the United Kingdom4 despite expectations of climate-related northern expansion within the United Kingdom4. As priority species of the UK Biodiversity Action Plan and priority species for butterfly conservation, there is considerable interest in the decline drivers of the Grizzled Skippers and the future prospects for conservation.
Grizzled Skipper, Copyright Thomas Hansen, from the Surfbirds Galleries
As a result, we conducted a study of the butterfly’s regional trends and the role of climate in those trends to shed light on the prospects for the Grizzled Skipper in the UK and to examine the predictions of spatial data-based extensions more generally. We examined the frequency trends because relatively positive trends in the north may be a precursor to the expansion of the north that has not yet been observed. Using a mixed modeling approach, we modeled the population size of Grizzled Skipper in the UK from 1976 to 2016 using time, north, east and 5 km2 resolution climate data. The population size data is taken from the UK Butterfly Monitoring System index values and the climate data is taken from the monthly Met Office UKCP09 climate readings, specifically mean temperature and total rainfall. Further details of our methods and results are published and are openly accessible5.
As in other studies in England, we have seen a long-term decline in the Grizzled Skipper population size. What was remarkable, however, was that the declines in the north and west were significantly steeper than in the south and east. While we observed that December temperature, like summer rainfall, had a negative impact on population size, the effect sizes were small. The long-term population size trends turned out to be independent of climate effects and therefore indicated another unidentified driver for declines that were not compensated for by the previously observed climate change.
Our results of a greater decline in the north at Grizzled Skipper contradict expectations of north expansion in a changing climate4 and suggest that climate change has not yet negated the effects of other drivers. Our results highlight some dangers of climate envelope models that are based solely on spatial data and may not identify factors that limit movement and settlement (e.g. host plants, vegetation structure, dispersal capacity), especially in a non-equilibrium system6.
Consensus from the literature shows that the likely drivers of decline are habitat loss and degradation7 resulting from a range of pressures including changes in agricultural intensification, loss of natural grazing pressure and land abandonment. Differences in regional declines could indicate that habitat in the north and west is more lost and deteriorated than in the south and east, or that northern and western populations are more sensitive to habitat changes.
The results of this study underscore the importance of habitat creation and conservation for the Grizzled Skipper and other habitat specialists who, based on climate associations, are expected to be the “winners” of climate change. Potential regional differences in the specificity of habitat requirements for the Grizzled Skipper are currently being investigated as part of a PhD thesis at the time of this writing to inform conservation planning and prioritization of butterfly conservation and further land management measures to the conservation community.
1. Parmesan, C., N. Ryrholm, C. Stefanescu, JK Hill, CD Thomas, H. Descimon, B. Huntley, L. Kaila, J. Kullberg, T. Tammaru, WJ Tennent, JA Thomas and M. Warren . (1999). “Poleward Shift in Geographic Areas 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 JB Isaac. (2015). “Recent trends in British insects inhabiting early successive stages of ecosystems.” Biological Journal of the Linnean Society 115 (3): 636-46. doi: 10.1111 / bij.12527.
3. Brereton, TM, MS Botham, I. Middlebrook, Z. Randle, D. Noble, S. Harris, EB Dennis, AE Robinson, K. Peck and DB Roy. (2019). “UK Butterfly Monitoring System Report for 2018.”
4. Settele, J., Otakar Kudrna, Alexander Harpke, Ingolf Kühn, Chris van Swaay, Rudi Verovnik, Martin Warren, Martin Wiemers, Jan Hanspach, Thomas Hickler, Elisabeth Kühn, Inge van Halder, Kars Veling, Albert Vliegenthart, Irma Wynhoff , and Oliver Schweiger. (2008). “Climate 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 are stuck in the south: reactions of an early successive specialist butterfly at the population level to the climate in its British area of distribution for over 40 years.” Diversity and diffusion, no. January: 1–11. doi: 10.1111 / ddi.13245.
6. Elith, Jane, and John R. Leathwick. (2009). “Species Distribution Models: Ecological Explanation and Prediction over 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 Great Britain and Ireland.” British wildlife. Oxford University Press, Oxford, UK. doi: 10.1111 / j.1096-3642.2007.00339.x.