The statistics behind the urban decline of butterflies

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Small Heath Butterfly
Flickr : Small Heath butterfly by Ferran Pestaña } CC BY-SA 2.0

Butterflies are declining more rapidly in urban areas than in the countryside, new research led by a University statistics expert has revealed.

Dr Emily Dennis, of the School of Mathematics, Statistics and Actuarial Science, worked with the charity Butterfly Conservation and the Centre for Ecology and Hydrology on the study, which has been published in the journal Ecological Indicators.

The research, entitled Urban indicators for UK butterflies, found that the majority of butterflies living in towns and cities are emerging earlier and are on the wing for longer than the same species living in rural areas.

Urban parks, gardens, brownfield sites and farms act as important refuges for butterflies and other wildlife, but in recent years these areas have come under increasing pressure from development, habitat loss and climate change.

The study compared trends for 28 species in urban and countryside environments. Over a 20-year period, urban butterfly abundance fell by 69% compared to a 45% decline for butterflies in rural areas.

The Small Copper and Small Heath declined much more dramatically in towns and cities than in the countryside. From 1995-2014 Small Copper abundance fell by 75% in urban areas compared to a 23% decline in rural areas.

The causes of these changes require further study, say the researchers, but it is likely to be due to the combined effects of habitat loss, intensification of land use and climate change. Butterflies are sensitive to environmental change. Declines in abundance in urban areas follow ongoing butterfly declines in the wider countryside.

The study found that butterflies in urban areas emerged on average two days earlier than their countryside counterparts with urban Brimstones emerging five days earlier than those found in rural locations. Flight periods for many of the species studied were also found to be slightly longer for urban butterflies than their rural counterparts.

The probable cause behind the earlier emergence and longer flight periods of urban butterflies is the ‘urban heat island’ effect – conditions in which towns and cities are slightly warmer than the surrounding countryside due to human activities

Dr Dennis, whose other collaborators were Professor Byron Morgan (University of Kent), Professor Tom Brereton (Butterfly Conservation) and Dr David Roy (Centre for Ecology and Hydrology), used sophisticated statistical techniques to reveal that practically all butterflies assessed were found to be struggling in urban areas.