Bringing Back the Butterflies

When it comes to conserving nature, road building gets a bad press.  And while it can damage existing habitats and displace wildlife, it’s good to know that new roads can also produce spectacular biodiversity gains.

Recently published in the Journal of Insect Conservation and Diversity https://onlinelibrary.wiley.com/doi/10.1111/icad.12543, a study by myself, Dr Phil Sterling (Butterfly Conservation) and Dr Emma Coulthard (Manchester Metropolitan University) explains how Dorset’s Weymouth Relief Road has become a success story for butterflies.

Built to bypass a notorious local bottleneck, the Weymouth Relief Road has become an example of good practice in building new habitats. Creating a cutting through an existing chalk ridge provided an opportunity to apply ecological principles to road verge design and management. 

From a basic seed mix containing some 25 characteristic chalkland species, such as Kidney Vetch and Horseshoe Vetch – many of which were harvested from local sources – the road cutting has developed into a diverse plant community (above).   

Soil fertility, or rather the lack of it, has been the key.  By avoiding the usual recipe of topsoil with an amenity grassland seed mix, and instead hand-seeding carefully chosen wildflowers on a bed of little or no topsoil, the verges have seen an explosion of floral colour and diversity.

With the plants have come the butterflies.  Transects have been walked since 2012 by volunteers from Butterfly Conservation’s Dorset branch, and a total of 30 species have now been recorded on the cutting and verges. That’s just over half of the UK total – equivalent to many nature reserves. 

These include a number of chalk grassland specialists.  Small Blue was one of the first species to arrive, appearing in 2012.  It was followed in 2013 by the rarer Adonis Blue (below). Both depend solely on a single foodplant – Kidney Vetch and Horseshoe Vetch respectively.  Adonis Blue numbers have built up dramatically; in 2019 it was more abundant than Common Blue, although population numbers of the latter have been subject to large fluctuations over the recording period.  The most recent arrival has been Chalkhill Blue, another Horseshoe Vetch specialist, which was first recorded in 2018 – albeit in very small numbers.  Our study shows a dynamic pattern of species arrivals and local extinctions. Nevertheless, overall species richness has continued to increase during the study period.

Increasing species richness has been accompanied by a growth in overall butterfly abundance.  The average number of butterflies recorded per survey visit grew almost five-fold between 2012 and 2020. The study suggests that this increase is consistent with a model of butterfly populations establishing on the cutting and verges as the roadside vegetation has become more mature and diverse. 

We also looked at the factors affecting the speed of colonisation by butterfly species.  Some, like Small Blue, Common Blue and Large White were quick off the mark to arrive at the site, while others (such as Grizzled Skipper) took several years to turn up.  By mapping the presence of each species in the wider area, using the data set held by DERC, we found that only one factor had a significant effect on colonisation speed.  This was the number of existing populations in a 10km square centred on the new road cutting – what we have termed the “Index of Local Presence”. 

Interestingly, this factor appeared more important than the simple proximity of existing populations to the study site, suggesting that colonisation occurs as a result of more complex movements than a simple range expansion.  The study also showed that some butterfly species that are classed by some authors as ‘sedentary’, such as Small Blue and Essex Skipper, have perhaps a greater potential to disperse to new locations than has previously been suggested, being able to cross landscapes that are apparently devoid of suitable habitat and resources.

Looking forward, it is clear that the increase in species richness cannot be sustained indefinitely.  Nevertheless, the study tentatively suggests that there may yet be a few more arrivals.  Areas of as-yet unvegetated bare ground may assist the colonisation of the declining Grayling butterfly, while the arrival of Tor-grass in the road cutting raises the tantalising prospect that the site may be suitable for the locally restricted Lulworth Skipper – the caterpillars of which feed on this species.

In the meantime, the Weymouth Relief Road (below) has become a model for the design and maintenance of road verges and cuttings across the country.  Highways England have promoted this project as a way of improving biodiversity as well as reducing construction and management costs – surely a win-win!

Mike Hetherington

Acknowledgements – huge thanks to all of the people who have carried out butterfly surveys on the Weymouth Relief Road verges and a massive shout out to Phil Sterling who initiated and is championing the project.  More details on Butterfly Conservation’s Building Sites for Butterflies project can be found here.  Thanks also to co-author Emma Coulthard for her advice and guidance; Alison Stewart at DERC; Annabel King and Chris Cocker at Dorset Council; and Emily Dennis and Sam Ellis of Butterfly Conservation, as well as two anonymous reviewers, for comments on early drafts of the paper. Photos (c) M Hetherington.

More details on the flora of the site can be found in Bryan Edwards 2019 Resurvey of the Flora of the Weymouth Relief Road Cuttings published by the Dorset Environmental Records Centre (DERC) for Butterfly Conservation.