Provide shady spots to protect butterflies from climate change

In a new study published today in the Journal of Animal Ecology, researchers have discovered significant variations in the ability of different UK butterfly species to maintain a suitable body temperature.

Species that mostly rely on finding shade to keep cool are at greatest risk of population decline, due to climate change and habitat loss.

Dr Andrew Bladon, first author of the report, is a Postdoctoral Research Associate in the University of Cambridge’s Department of Zoology. He says the results will inform conservation strategies to protect butterflies.

The study shows that larger and paler butterflies including the Large White (Pieris brassicae) and Brimstone (Gonepteryx rhamni) are best able to buffer themselves against environmental temperature swings. They angle their large, reflective wings in relation to the sun, and use them to direct the sun’s heat either away from, or onto their bodies. These species have either stable or growing populations.

More colourful larger species such as the Peacock (Aglais io) and Red Admiral (Vanessa atalanta) have greater difficulty controlling their body temperature, but even they are better than their smaller relatives like the Small Heath (Coenonympha pamphilus).

Some butterfly species rely on finding a spot at a specific temperature within a landscape - termed a ‘microclimate’ - to control their body temperature. Air temperatures vary on a fine scale: a shaded patch of ground is cooler than one in full sun, for example. These ‘thermal specialists’, including Brown Argus (Aricia agestis) and Small Copper (Lycaena phlaeas), have suffered larger population declines over the last 40 years.

All butterflies are ectotherms: they can’t generate their own body heat. The populations of two thirds of UK butterfly species are in decline: habitat loss and fragmentation, and more monotonous landscapes have removed many of the microclimates butterflies need to survive. Climate change is compounding the problem by causing more extreme weather events and greater fluctuations in temperature.

Insects, including butterflies, pollinate around 85% of our food crops - providing a vital service worth billions of pounds globally. Protecting a diverse range of species will provide long-term resilience: if numbers of one species fall there are others to fill the gaps. Insects are also an important food source for many other species, including birds.

"We need to make landscapes more diverse to help conserve many of our butterfly species," said Bladon. "Even within a garden lawn, patches of grass can be left to grow longer - these areas will provide cooler, shady places for many species of butterfly. In nature reserves, some areas could be grazed or cut and others left standing. We also need to protect features that break up the monotony of farm landscapes, like hedgerows, ditches, and patches of woodland."

Landscapes with a diversity of heights and features have a greater range of temperatures than flat, monotonous ones. This applies on scales from kilometres to centimetres: from hillsides to flower patches. Such structural diversity creates different microclimates that many butterflies use to regulate their temperature.

In total, 29 different butterfly species were recorded. The work reveals that butterflies are either thermal generalists or thermal specialists, and this does not always correspond with their current categorisations as either habitat generalists or specialists.

Over the past thirty years, many species of butterfly have expanded their range northwards, as more northerly places have become warmer due to climate change. The ranges of species adapted to cooler environments are shrinking. These trends have been tracked for butterfly populations as a whole, but no previous study has investigated how the individual butterflies that make up these populations are able to respond to small scale temperature changes.

The work was carried out at grassland sites in Bedfordshire, UK, on reserves owned and managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire. Data was also collected at two Royal Society for the Protection of Birds (RSPB) reserves in Wiltshire and Cumbria, and a National Trust for Scotland site in Perthshire. The study was conducted in collaboration with The Wildlife Trust for Bedfordshire, Cambridgeshire & Northamptonshire; the RSPB Centre for Conservation Science; Lancaster University; and the Czech Academy of Sciences.

It was funded by the Isaac Newton Trust/ Wellcome Trust ISSF/University of Cambridge Joint Research Grants Scheme.

Reference: Bladon, A.J. et al. ‘How butterflies keep their cool: physical and ecological traits influence thermoregulatory ability and population trends.’ Journal of Animal Ecology, 24 Sept 2020. DOI: 10.1111/1365-2656.13319

Image credits: Film clips by Eleanor Bladon; photographs by Andrew and Eleanor Bladon

Researcher contacts: Dr Andrew Bladon; Dr Ed Turner.

Media enquiries: Jacqueline Garget

^{First published September 2020, updated September 2021}