Chalkhill blue

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Butterflies emerging earlier due to rising temperatures

More than 90% of British butterflies emerge earlier in years with a warm spring or summer, Museum researchers have found - potentially too early for the plants they eat.

Working with 83,500 digitised specimens from the Museum's butterfly collection, the scientists looked at the spread of collection dates from each year between 1880 and 1970.

This allowed the team to estimate when the butterflies were active in any given year.

By comparing this information to historical temperature records, the researchers found that 92% of the 51 species studied emerged earlier in years with higher spring temperatures.

'This relationship with temperature is highly significant,' says Steve Brooks, entomologist at the Museum and lead author on the Ecography paper. 'For every 1°C rise in spring temperatures, these butterfly species emerged between one and nine days earlier.'

'The warming climate is already causing butterflies to emerge earlier - and unless their food plants adapt at the same rate, the insects could emerge too early to survive.'

Chalkhill blue butterflies, Polyommatus coridon, in the Museum's collection. The butterflies have been imaged, given a barcode that links to their collection data and rehoused as part of the digitisation project.

Historical trends

For the research, Brooks and his team compared butterfly collection records - their date and location collected - to the Central England Temperature record, which has daily temperature readings dating back to 1772.

They found that between 1880 and 1970 all but one of the 51 butterfly species were affected by yearly changes in climate - and 47 of these species emerged earlier when spring temperatures were higher.

Some butterflies, such as the orange tip and the green hairstreak, emerged as much as nine days earlier for every 1°C increase in temperatures between March and May.

While looking into why some species were more sensitive to change than others, the team found that  the most affected were those that already emerged early in the year, such as the orange tip.

'This may be because species that emerge later in the year are influenced by both spring and summer temperatures,' explains Brooks, 'so there's a moderating effect. Early emerging species only experience the one season, so this could be why they are more sensitive to change.'

Side view of a male orange tip, Anthocharis cardamines. Photo by Wikimedia Commons user Quartl, licensed under CC BY-SA 3.0.

Adapt to survive

Other species whose emergence date was strongly dependent on climate were those found in southern Britain (such as the chalkhill blue) and those that eat a wide variety of plants while larvae (such as the green hairstreak).

'We think this is because these butterfly species are able to adapt to a wider variety of conditions,' says Brooks, 'so they are more able to survive if they emerge early.

'For example, the butterflies that inhabit southern Britain have more land to expand into, further north, if temperatures are warm enough.

'And the species that can feed on a variety of host plants are able to survive even when they emerge at a different time of year.'

In contrast, species with more restricted food requirements may struggle to adapt as climate changes.

Green hairstreak, Callophrys rubi. Photo by Wikimedia Commons user Charlesjsharp, licensed under CC BY-SA 4.0.

Life-changing effects

Scientists know that animals are affected by climate change in many different ways. For example, some move into regions they were previously unable to inhabit, while others become smaller as a result of environmental stress.

However, few studies have looked at how variations in climate over long periods (such as a century) affect an animal's phenology - the timing of their periodic life events, such as egg laying and maturation into adults.

'Most studies are based on short time series,' explains Brooks. 'Often they cover just the last few decades, which happens to coincide with a period of rapid climate warming.

'This means these studies lack the historical data to put the findings into perspective, so the long-term effects of climate change could be obscured by short-term variability.'

'Fortunately, the Museum's butterfly collection has detailed records spanning more than a century, so we were really able to get this long-term perspective.'