Small Tortoiseshell Butterfly Population Decline In UK Linked To Climate Breakdown

The small tortoiseshell butterfly had its worst year on record in England and has fallen by 82% throughout the United Kingdom since 1976.

Linked To Climate Breakdown

(Photo : Getty Images/JACK GUEZ)

The significant drop in numbers of the once-common garden butterfly has perplexed biologists, but it is thought to be linked to climate change.

In 2023, it had its worst year on record in England, the second worst in Wales, and the joint-fifth worst in Scotland, while it performed well in Northern Ireland, posting its second-best year.

The yearly UK Butterfly Monitoring Scheme (UKBMS), the world's largest and longest-running scientific butterfly dataset, yielded conflicting results for the UK's populations. Butterfly Conservation, the UK Centre for Ecology and Hydrology (UKCEH), the British Trust for Ornithology, and the Joint Nature Conservation Committee are the organizations behind the program.

The drought in 2022 may have impacted butterfly numbers in 2023, though not as significantly as previously anticipated.

Caterpillars are especially vulnerable to the impacts of drought because if their feeding plants wither, they will starve. Ringlet and green-veined white butterflies, two species that were severely affected by drought, had a terrible year in 2023, which could have been a result of the preceding dry summer.

"Butterflies are an indicator species, meaning they can tell us about the health of the wider environment, which makes the UKBMS data invaluable in assessing the health of our countryside and natural world in general," Dr. Marc Botham, a butterfly ecologist at the UKCEH.

58 Species Observed

Half of the 58 species observed had a better-than-average year, while the other half performed below-average at monitored sites.

Some species, such as the green-veined white and the ringlet, are still suffering from the drought of 2022. This is because the food plants that the caterpillars of the next generation eat died during the drought, leaving fewer caterpillars to survive and develop into the next generation of butterflies.

Brown argus, comma, holly blue, marbled white, and black hairstreak were among the best-performing species last year. Many of these butterflies are currently expanding their range in the UK.

This suggests that they are expanding across a larger geographical area as the climate becomes more favorable to them.

In the 1970s, for example, the comma was restricted to the southern half of Great Britain, but it has swiftly expanded north and west in recent decades, colonizing northern England, southern Scotland, and south-east Ireland.

The chequered skipper, which was reintroduced to England in 2018 after going extinct in the 1970s, also had its best year ever. Last year's success is a testament to the diligent conservation efforts that have enabled its reintroduction.

The red admiral was arguably the star species of 2023, with its numbers reaching a record high. This migratory species arrives in the UK in varying numbers each summer from southern Europe and North Africa, with 2023 being an especially good migration year for this butterfly.

Since the monitoring program began nearly 50 years ago, the population of this species in the UK has more than tripled. As our winters become milder, it is more likely to overwinter here rather than migrate south.

The UKBMS now collects data on over 3,000 sites per year, which is used to analyze changes in insect populations and provide signs of climate breakdown and its impact on species.

Dr. Richard Fox, the head of science at the charity Butterfly Conservation, stressed that butterfly counts fluctuate naturally from year to year owing largely to weather, but long-term trends in UK butterfly populations are primarily driven by human activity, such as habitat damage and destruction, pesticide usage, pollution, and climate change.

"By monitoring long-term butterfly trends we can learn about the impact of climate change and other factors on our native wildlife,' he added.