Natasha Allen, recent graduate from the University of Stirling and aspiring Ecologist, is our latest guest blogger. For her MSc Dissertation, she set out to determine if a reduced mowing regime and wildflower seed mix sowing were helping to produce more favourable habitat for butterflies in Stirling green spaces.
“Meadows and other species-rich grasslands are key habitats for UK pollinators. Consequently, the destruction of over 97% of UK meadows since the 1930s has brought about hard times for our pollinating invertebrates. Butterfly Conservation’s recent publication of the State of UK’s Butterflies 2022 indicates an eye-watering 80% of UK butterfly species have declined since the 1970s.
There is no denying the cultural significance of the charismatic butterfly; their beauty has forever inspired us. But there is so much more to butterflies to get excited about: these deceptively sturdy invertebrates are useful indicators of environmental health and the impacts of climatic change, due to their sensitivity to temperature, relatively high mobility, and short lifecycles. Not to mention they play vital ecological roles across different ecosystems!
With encouragement and guidance from organisations such as Butterfly Conservation Scotland and local community project On the Verge, Stirling Council has set aside sections of amenity grassland (grass frequently cut short for recreational use) in towns across the county to trial run a more pollinator-sympathetic management regime. Since 2021, these areas have been mown once annually with cuttings collected (Regular Pollinator or RP sites). But prior to this trial run’s establishment (between 2011-2015), the composition of vegetation at 3 different amenity grasslands in Stirling were directly altered using wildflower seed mixes and have since been treated with the same annual cut-and-collect method (Wildflower Pollinator or WP sites).
For my dissertation project with the University of Stirling, we carried out survey work across all three management types to compare the impacts of each regime on plant and butterfly diversity.
Twelve sites were surveyed in total: 3 WP sites, 3 RP sites and 6 amenity grassland sites situated adjacent to each of the 6 pollinator-sympathetic regime sites.
- Observations of butterflies were recorded as the surveyor walked along fixed zigzag transects within each of the 12 sites. These surveys were carried out at least once a week (when the weather permitted it) from mid-June to late July 2022.
- To assess the plant diversity at each site, two vegetation surveys were conducted during this period, to assess the frequency of all plant species within each butterfly walk transect.
- Over 7 weeks, 10 replications of butterfly surveys were performed, providing data from 460 butterfly transect walks.
In total, 8 butterfly species were recorded over the course of the study: Meadow Brown, Ringlet, Small White, Green-veined White, Common Blue, Small Copper, Small Tortoiseshell and Peacock. Rather unsurprisingly, Meadow Brown and Ringlet butterflies were the most abundant, together making up 81.4% of all the butterflies we observed.
During fieldwork, it was evident that both pollinator-sympathetic regimes supported much greater butterfly diversity than the amenity grasslands could. Our results indicated a 36% chance of seeing a butterfly at both RP and WP sites, compared to 1% on amenity grassland sites. Out of the 86 individuals observed during the survey work, only 3 butterflies (2 Meadow Browns and a Ringlet) were ever recorded on the amenity grassland sites.
As expected, the WP sites had the highest count of plant species. Yellow Rattle, Creeping Thistle, Ox-eye Daisy, Meadow Vetchling, Common Bird’s-Foot Trefoil and Meadow Buttercup were among the many forb species sprinkling colour amongst the grasses of the WP sites. It will take longer for plant diversity to improve on RP sites, as the simple cut-and-collect regime relies solely on the long-term removal of cuttings to decrease nutrient levels in the soil, so that less competitive species can thrive.
Despite the relatively similar vegetation composition between the RP and amenity grassland sites, butterflies clearly favoured one over the other. The simple act of reducing mowing frequency down to once annually (allowing vegetation to grow tall) clearly increased the probability of butterflies being present. Not only does letting the vegetation grow taller provide more floral food resources for adult butterflies, it also provides vital shelter and temperature regulation for butterflies. For species such as the Meadow Brown or Ringlet, long grasses play an integral role in their lifecycle, providing food during the larval stage of their development. Other animals such as birds, bats and hedgehogs, feed on butterfly caterpillars. So, by letting the grass grow tall, these green spaces can better support insect larvae and, in turn, the wider ecosystem.
Both management regimes have their place in pollinator conservation. Simply limiting cutting to once annually is easily implemented and the removal of cuttings year after year will gradually promote further plant diversity. Sowing wildflowers can be utilised to provide wonderful opportunities for public engagement and education, which is necessary if we want to shift public attitudes towards biodiversity-favourable green space management. It can also be harnessed to support target pollinator species. In the case of Newton Park in Dunblane where Common Blue butterflies were observed, the population of Common Bird’s-Foot Trefoil (the butterfly’s primary larval foodplant) is currently minimal. On the Verge are in the midst of establishing a plot of wildflower sown grassland at Newton Park. With the inclusion of this Trefoil in the native wildflower seed-mix used, On the Verge’s important community work could help sustain the Common Blue at Newton Park for years to come.
Diversity of habitat is key if we are to encompass the needs of a broad range of butterfly species and other pollinator taxa in our management of town and city green spaces. Alongside our efforts to increase the pollinator-sympathetic management of our agricultural land, mosaics of long grass and wildflower meadows within our urban parks and roadside verges should be maintained to provide further refuge for butterflies.
With thanks to Anthony McCluskey of Butterfly Conservation Scotland (Engagement Officer), Leigh Biagi of On the Verge (Stirling), Guy Harewood (Biodiversity Officer for Stirling Council) and Dr Joanne Clarke (Lecturer in Biological and Environmental Sciences at the University of Stirling).“
Images all (c) and courtesy of Natasha Allen