Red Lists are rankings of species according to their risk of extinction (globally or in a particular area such as a country) and form an important input into biodiversity conservation. Most Red Lists are produced using the International Union for Conservation of Nature (IUCN) criteria, which offer a global standard of scientifically-backed principles and quantitative thresholds against which species can be objectively assessed1. Originally, Red List categories were defined principally on the basis of rarity, but for the last two decades, the focus has been on recent rates of decline and factors that might contribute to decline.
Globally, the vast majority of vertebrate species have been assigned a Red List category, but only a tiny proportion of insects have been assessed due to the enormity of the task and the paucity of requisite data2. The situation is better in many European countries, particularly for certain, popular well-studied insect groups such as butterflies. Thus, it is possible to compare the extinction risk of butterflies – Britain ranks seventh worst out of 33 European countries in terms of mean Red List value for butterflies, a measure of the average level of threat faced by the butterfly fauna of each nation3.
Nevertheless, even in countries with a long tradition of biodiversity recording, it is extremely challenging to quantify accurately the population and range size of most insect species, and to measure trends over time4, in order to apply the IUCN criteria. Britain’s butterflies are probably the most comprehensively monitored insects on the planet, with spatially extensive data on species distribution and population abundance dating back to the 1970s. Thus, they provide a useful, best-case scenario for Red Listing of insects. Butterflies are also iconic insects that are important as umbrella species for biodiversity conservation, flagship species for public engagement, and as indicators of environmental change and wider invertebrate declines. Therefore, we need to maintain up-to-date assessments of the regional extinction risk faced by British butterflies.
A recent paper5 details the process and results of the latest Red List of Britain’s butterflies, published by Butterfly Conservation earlier this year. We used population monitoring data from the UK Butterfly Monitoring Scheme (UKBMS) and citizen-science derived occurrence records from the Butterflies for the New Millennium (BNM) recording scheme to assess all 62 resident, former resident and regularly-breeding migrant butterfly species against the IUCN criteria. We do not know the absolute population size of any butterfly species in Britain, but the butterfly records, contributed by tens of thousands of participants and verified by expert volunteers, were analysed to calculate range sizes, trends in abundance and distribution and other relevant metrics. IUCN criteria require trends from the last 10 years, but calculating these from just 10 years of measurements yields highly unreliable Red List assessments due to the boom and bust nature of butterfly (and other insect) population dynamics6. We mitigated for this problem by modelling the full time series available for each species and then estimating the trend over the last 10 years from the modelled values, as is recommended by IUCN.
The new Red List shows that four species remain Regionally Extinct in Britain and that, of the remaining 58 extant species, 24 species (41%) were classified as threatened (eight as Endangered and 16 as Vulnerable), five (9%) as Near Threatened and 29 (50%) as Least Concern. Half of the butterfly species that currently occur in Britain are therefore listed as either threatened with extinction or Near Threatened. Compared to the previous butterfly Red List, published in 2010, the new assessment represents a further deterioration in the status of Britain’s butterflies, with the number of threatened species up by 26% (from 19 to 24 species).
We are very grateful to Natural England for funding the new Red List of Britain’s butterflies and particularly to Andy Brown and Jon Curson for their input.
Head of Science, Butterfly Conservation
- Mace, G.M., Collar, N.J., Gaston, K.J., Hilton-Taylor, C., Akçakaya, H.R., Leader-Williams, N., Milner‐Gulland, E.J. & Stuart, S.N. (2008) Quantification of extinction risk: International Union for the Conservation of Nature’s (IUCN) systemfor classifying threatened species. Conservation Biology 22:1424–1442. doi:10.1111/j.1523-1739.2008.01044.x
- Eisenhauer, N., Bonn, A. & A Guerra, C. (2019) Recognizing the quiet extinction of invertebrates. Nature Communications 10:50. doi:10.1038/s41467-018-07916-1
- Maes, D., et al. (2019) Integrating national Red Lists for prioritising conservation actions for European butterflies. Journal of Insect Conservation 23:301–330. doi:10.1007/s10841-019-00127-z
- Didham, R.K., Basset, Y., Collins, C.M., Leather, S.R., Littlewood, N.A., Menz, M.H., Müller, J., Packer, L., Saunders, M.E., Schönrogge, K., Stewart, A.J., Yanoviak, S.P. & Hassall, C. (2020) Interpreting insect declines: seven challenges and a way forward. Insect Conservation and Diversity 13:103-114. doi:10.1111/icad.12408
- Fox, R., Dennis, E.B., Brown, A. & Curson, J. (2022) A revised Red List of British butterflies. Insect Conservation and Diversity 15:485–495. doi:10.1111/icad.12582
- Fox, R., Harrower C.A., Bell, J.R., Shortall, C.R., Middlebrook, I. & Wilson, R.J. (2019) Insect population trends and the IUCN Red List process. Journal of Insect Conservation 23:269–278. doi:10.1007/s10841-018-0117-1