Grizzled Skippers stuck in the south: Population level responses of an early successional specialist butterfly to climate across its UK range over 40 years

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Fiona A. Bell, Department of Evolution, Ecology and Behaviour, University of Liverpool, Liverpool, United Kingdom, Marc S. Botham, UK Centre for Ecology and Hydrology, Oxfordshire, United Kingdom, Tom M. Brereton, Butterfly Conservation, Wareham, United Kingdom; Marine Life, Hampshire, United Kingdom, Andy Fenton, School of Biological Sciences, University of Liverpool, Liverpool, United Kingdom and Jenny A. Hodgson, Department of Evolution, Ecology, and Behaviour, University of Liverpool, Liverpool, United Kingdom

Background/Question/Methods
Climate change has been predicted to facilitate poleward expansion of many early successional specialist invertebrates. The Grizzled Skipper, Pyrgus malvae, is a threatened butterfly in long-term decline that has been expected to expand beyond its current northern limit in Britain under warming conditions based on spatial-data only analysis.
However, P. malvae is an early successional specialist occupying a fragmented UK landscape, and has not met expectations of northern expansion in Britain, possibly indicating that climate change has not improved northern habitat suitability, or that another driver (e.g. land-use change) is masking its effects. The long-term count data available, including at a cool range-edge, allows exploration of regional differences in abundance trends that could emerge before colonisations are detected.
Here we explore the effect of climate on population size trends over four decades, and whether any regions show an improving population trend that may be a precursor to northern expansion. Examining detailed spatio-temporal abundance data can reveal unexpected limitations to population growth that would not be detectable in widely-used climate envelope models.
Mixed models were used to investigate P. malvae population size in relation to time and monthly climate measures across its UK range since 1976, based on repeated transect walks.
Results/Conclusions
We found that P. malvae population size declined more over time in the north and west of its UK range than in the south and east, and was negatively related to high December temperature and summer rainfall. However, the effect sizes of temperature and rainfall were minimal, and the modelled trajectories of long-term trends in population size were largely unchanged when climate values were kept fixed at 1976-1980 levels.
The last 40 years of climate change have not ameliorated climate suitability for P. malvae at its range edge, contrary to expectations from spatial-only climate envelope models. The clear long-term downward trends in population size are independent of climate change and we propose probably due to habitat deterioration.
Our findings highlight potential hazards in predicting species range expansions from spatial models alone, particularly in a non-equilibrium ecological system. Although some climate variables may be associated with a species’ distribution, other factors may be more dominant drivers of trends and therefore more useful predictors of range changes.