Butterflies can be considered biological indicators due to their high sensitivity to environmental change.

Their relatively short life span resulting in a quick ‘turn-over’ of butterfly generations means we can observe natural selection and other evolutionary changes in response to changes in their environment. Variation in butterfly numbers and species composition can therefore be reflective of factors like climate and land-use change. Additionally, butterflies can be found in most terrestrial habitat types in the UK, thus can be used as indicators in many different areas.

On farmland, carrying out butterfly surveys can show us how valuable different areas of the farm are for butterflies, shedding light on where some habitat improvement may be needed.

My method.

In the early springtime of 2021, I set up butterfly transects as part of the UK Butterfly Monitoring Scheme (UKBMS) on Awnells Farm and Turnastone Court Farm, Herefordshire. These transects are routes which take me through a range of different habitats for 2.3km and 2.5km respectively. The full route is divided up into 6 sections based on the habitat composition in each. For instance, on Awnells Farm, section one goes through the garden and farmyard, section two takes me through a traditional cider and perry orchard, and section three takes me through permanent pasture for cattle livestock.

On these 26 weekly walks between April and September inclusive, I equip myself with close-focussing binoculars, a UK butterfly field guide (the field studies council are light and handy for quick identification), and a notebook. The aim of each survey is to record all the butterflies which come within an imaginary ‘recording box’ of 2 meters either side and 4 meters in front of the recorder. The number of each species is tallied up along with any other useful information, for instance the gender of butterfly (if distinctive) and the name of the flower, if visited.

Being ectothermic organisms, butterflies rely predominantly on gaining heat from their surroundings rather than producing it themselves. They are therefore most active when it is warm, dry and mild in minimum temperatures of 13°C with the sun beating down on their wings. Wind is another factor which influences how active butterflies are as, being light creatures, wind can whisk them away for great distances, and knock them about meaning butterflies tend not to fly when winds are strong. For these reasons, there are strict weather criteria (outlined below) in which surveys can be carried out.

• Minimum of 13°C with at least 60% of the survey having sunshine.
• If the temperature is 17°C or more, surveys can be done with less than 60% sunshine.
• No rain.
• No strong winds.
• Between 10am-17pm (ideally between 11am and 3pm).

To measure the percentage of sunshine, after I reach the end of each section, I note down the amount of time that it was sunny for in that section. For example, if the sun shines for half of the time it takes to survey section one, I would wright 50% sun for section one and repeat for all the sections then calculate an average for the survey as a whole once completed.

Findings for each farm

In 2021, at Awnells Farm, 566 butterflies from 17 species were identified and recorded during UKBMS data collection between April and September. The species with the most records was the Meadow Brown with 295 individuals spotted during surveys at Awnells and 69 at Turnastone Court Farm. Butterflies were most abundant in sections containing long-grass, standard orchard trees or scrub.

At Turnastone Court Farm, 16 different species were recorded overall with the highest diversity being found in section five of the transect which comprises habitat of small woodland, bracken glades and anthills in amongst the meadow grassland. In spring, flowers including bluebells, cuckooflower, dog violet, and primrose flourish without disruption from grazing livestock as the farmers take sheep off the area to protect bluebells which cannot photosynthesize if their leaves are trampled.

What does it all mean?

Butterfly numbers are suffering nationally, and the numbers here reflect the broader picture of butterfly populations in the UK. Data from UKBMS in 2021 show that a third (34%) of butterfly species in the UK are experiencing a significant long-term decline in abundance, and although when looking at the last decade, no species are showing a significant decline, the abundance of many species remains at a lower level than the high numbers typically seen in the 1970s. Overall on both farms, the area’s most diverse and abundant in butterflies were in areas where the land had been rested from grazing where flowering plants were more able to flourish. One way livestock farmers can give butterflies better access to this habitat is to take out awkward field corners to allow wildflowers to grow or implement rotational grazing which gives each field time to sufficiently recover and support more wildlife.

By Ruth Moss, CRT Wildlife Monitor