It has now been one year since butterfly banks were built on Wildlife Trust nature reserves in Pegsdon and Totternhoe, Bedfordshire. In September 2021, eight banks, each 15 metres long and two metres high, were constructed with diggers, taking scrapings of chalk earth to transform areas of previously flat habitat. Four banks were put in place on each site in the shape of giant letter E’s, with one E facing each direction of the compass, providing a wide range of microclimates across their many different slopes.

Although butterfly banks have been used before in many locations, this project is the first time that they have been surveyed and analysed in this way, to see how they can help support whole communities of species during periods of climate change.  

Data collection before and after the banks

A key part of the project was to collect baseline data on what was in the flat fields before the banks were built, which could then be followed up with repeat data collection each year, to see how this management technique impacts the temperatures and species observed.

Starting my PhD in January 2022, I continued the work of Wildlife Trust staff and volunteers, as well as other researchers from the University of Cambridge, who as a team have been collaborating on the project, establishing and monitoring the banks.

One part of the data collection includes photographing the banks from set angles and positions over time, so that we have a visual catalogue of how they have changed. Comparing the images below, you can see that in just one year, quite an impressive amount of vegetation has sprung up on the banks and transformed them form blank white canvasses, when they were first built.

In addition to the photographs, I have carried out many different types of survey, repeating data collection that was carried out before the banks were put in, enabling before and after comparisons. The next step will be to analyse this data and continue adding to it over the coming years. Although the project is titled Banking on Butterflies, we are also interested in how all kinds of other flora and fauna might use the banks.

• Data loggers – these are tiny devices that record temperature every hour in 88 locations across each butterfly bank. They will give us a really good idea of how the banks alter the temperatures around them and how this differs from the flat fields that used to be in their place.

• Butterfly surveys – systematically searching the butterfly banks and their surrounding fields will show us how butterflies of different species are using them.

• Vegetation surveys - gridded quadrats placed around each of the 88 data loggers will help us record how quickly the bare chalk is recolonised by plants and which species get established.

• Terrestrial invertebrate surveys – pitfall traps at each of the 88 data loggers will show us how a whole community of invertebrates use the banks in addition to the butterflies flying above.

Extreme Heat Surveys

Aside from repeating surveys from previous years, the record-breaking heatwave during the summer of 2022 offered the chance for us to collect some completely new and extremely important data.

Although vegetation has been growing on the banks, they are still quite bare and therefore, under ‘normal’ conditions, we might expect butterflies to spend most of their time in the surrounding fields, full of caterpillar food-plants and nectar sources. In time, these plants will populate the banks, but we are really interested in the ability of the banks to help heat up or cool down butterflies in extreme conditions.

On July 19th 2022 the hottest day in the UK was recorded, with highs exceeding 40°C. This gave a snapshot of what future climates in this country could look like, how this could impact local wildlife, and whether conservation management on reserves could help buffer against some of the negative impacts. During these extreme temperatures, I surveyed the butterfly banks at Pegsdon Hills and Hoo Bit nature reserve and the surrounding field, recording all the butterflies I observed.

The findings turned out to be a mix of being both extremely worrying and exciting.

In the midday heat, absolutely no insects were flying. On every other survey this past year, butterflies were recorded throughout the open parts of the field but on July 19^th, just 2 were seen hunkered down in the long grass. The only place the butterflies were reliably seen in the field was in the shaded slopes of the banks, with each slope harbouring multiple, inactive individuals hiding from the sun.

To the eastern edge of the field there is a boundary of woody scrub. When extending surveys to include this area, we found that within just a few meters of shade, multi-species groups of butterflies and other insects, including hoverflies, were all resting together amongst the cool vegetation. It seemed that the 40°C temperatures were simply too warm for insects to be active, and they all huddled together, making use of the only shade available.

Although this is a worrying peek into what the future might hold for local wildlife, with increasing bouts of inactivity and associated fitness implications of not being able to search for food or mates, it does show evidence of the banks working. During extreme weather events the banks offer a refuge, where species like butterflies can survive until conditions become suitable again. As climates continue to change, it will become increasingly important to keep nature reserves varied, managing them for a mix of habitats and temperatures, providing as many options as possible for species struggling to survive in a rapidly changing world.

This project has been supported by The Evolution Education Trust through the Cambridge Conservation Initiative's Knowledge-Exchange Studentship programme, and the People's Postcode Lottery's Nature-based Solutions Fund.