Moths have declined faster in British woods over the last half-century than on farmland or in cities, despite woodlands having increased and moths being shielded from chemical and light pollution by the trees.
Forest populations of moths halved between 1968 and 2016 compared with average national losses of a third, according to a study.
Moths are a well-studied indicator of wider insect declines that recent research has often linked to pesticide use by industrial farming, habitat loss and urban light pollution – none of which directly affect woodlands.
Species experiencing particularly drastic declines include the brindled beauty (-78%), the canary-shouldered thorn (-68%) and the scalloped oak (-69%).
The loss of moths from woodland could be partly because of climate change or less woodland management such as coppicing, leading to more shading or increased deer density causing more intensive grazing. However, the team of scientists found moths whose caterpillars fed on shade-intolerant plants or shrubs and woodland flowers grazed by deer did not decline any more than species relying on unaffected grasses, trees and lichens.
Dr Dan Blumgart, from Rothamsted Research and lead author of the study in Insect Conservation and Diversity, said: “Climate change is known to have contributed to the national decline in moths and it is likely that this has driven at least part of the decline observed in woodlands.
“But this can’t explain why the declines have been worse in broadleaf woodland compared to other habitats. We might expect the shade provided by woodlands to help buffer against the effects of climate change but that is clearly not the case.”
Based on data from the Rothamsted Insect Survey, which has counted more than 8 million individual moths from more than 400 larger species since 1968, the researchers examined four indicators of moth population health in woodland and six other habitats over the period 1968-2016.
Broadleaf woodland was the only habitat in which all four measures declined severely with abundance down 51%, biomass down 52%, species richness down 14% and species diversity down 15%, compared with smaller nationwide declines in abundance (34%) and biomass (39%) with richness unchanged and diversity increasing by 10%.
The woodland declines are particularly striking because they are so recent with further analysis revealing that the abundance in broadleaf woodland was stable up to the late-1980s, after which it plummeted.
There were greater losses from woodlands in the south of Britain than the north which matches the general pattern of moth decline. A similar decline in woodland moths has been observed in Germany but not in a Hungarian study.
According to Blumgart, other possible causes of woodland moth decline could include the spread of invasive shrubs such as rhododendron or laurel and habitat fragmentation, with woodland moths still affected by agricultural intensification beyond the woods.
He said: “Although the amount of woodland in the UK hasn’t declined since the 1960s there might be more fragmentation due to the loss of hedgerows and trees outside woodlands so that could be something for future studies to look at.”
