UK butterflies suffer in heatwave


Last summer’s heat and drought led to a sharp decline in British butterfly populations later in the year, according to a new study by the UK Butterfly Monitoring Scheme (UKBMS), which is led by Butterfly Conservation and the UK Centre for Ecology and Hydrology.

The study found that common butterfly species, such as the brimstone, small tortoiseshell, peacock, green-veined white, and small white, appeared in good or average numbers during the spring and early summer of 2022. However, numbers in subsequent late-summer generations were greatly reduced, as the food plants that the caterpillars of the next generation feed upon died during the drought, leaving fewer caterpillars to survive and transform into the next generation of butterflies on the wing.

The impact of this mass caterpillar die-off last summer will only be visible this year for some butterfly species that have only one generation each year. Grass-feeding species such as the marbled white, meadow brown, and small skipper may be particularly vulnerable after last summer’s grasslands were parched in the drought.

Dr Richard Fox, head of science for Butterfly Conservation, stated that warm, sunny weather is generally good for butterflies as they can be active, finding food, mating, and laying eggs. However, drought is a major problem as plants wither and die, meaning female butterflies may struggle to find anywhere to lay their eggs, or there is not enough food for the caterpillars when they hatch.

The data revealed that 2022 was an average year for butterflies, although Fox described it as “a year of two halves” with good numbers of butterflies in early summer but then greatly reduced abundance after the heatwave and drought. However, concerns remain about the longer-term impact of the drought, as the UKBMS has previously revealed the negative impacts of droughts on butterflies in 1976 and 1995. Some species have never recovered their former abundance levels after the 1976 drought, although scientists say that habitat destruction is a major factor in their failure to bounce back.

Today, most British butterfly species are in decline, and therefore the negative effect of seasonal droughts could be more long-lasting. According to the 2022 State of the UK’s Butterflies report by Butterfly Conservation, 80% of species have declined in abundance or distribution or both since the 1970s. The UKBMS records collected from more than 3,000 sites by volunteers was an invaluable long-term dataset that enables scientists to measure how butterflies are faring as well as assessing the health of the countryside generally. The UKBMS data are vital in assessing the effectiveness of government policies and progress towards the UK’s biodiversity targets, said Dr Marc Botham, butterfly ecologist at the UK Centre for Ecology and Hydrology.

Despite concerns about the longer-term impact of the drought, 2022 was a good year for rare butterfly species including the purple emperor, large blue, chequered skipper, and dark-green fritillary, all of which have been the focus of targeted conservation work in recent years. The purple emperor and the large blue – both stars of David Attenborough’s Wild Isles series – recorded their second-best ever years since scientific monitoring began in 1976. Purple emperor caterpillars feed on leaves of sallow – a tree – and so may be less affected by drought.



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