Scotland's indicators - Terrestrial Insect Abundance – Butterflies

Butterflies are a familiar sight in the summer months across Scotland. Some live in a range of habitats (they are generalists) – these include species such as meadow brown and small tortoiseshell, commonly found throughout Scotland. Others are limited to specific habitats such as large heath, which is typically a wetland species.

Butterflies are relatively well-recorded, which enables their population trends to be assessed. As well as being important in their own right, changes in their numbers over the years can provide an indication of habitat loss and fragmentation and the impacts of climate change.

A peacock butterfly (Aglais io) feeding on the SNH wildflower meadow at Battleby, Perthshire. ©Lorne Gill/SNH. For information on reproduction rights contact the Scottish Natural Heritage Image Library on Tel. 01738 444177 or www.nature.scot

Monitoring Scotland’s Butterflies

In Scotland, butterflies are monitored through the Butterfly Monitoring Scheme (UKBMS) Volunteers walk fixed-route transects weekly from April to September each year. The indicator describes trends for 20 of the 34 regularly occurring butterfly species in Scotland at 533 sample locations.

Scottish butterfly population trends (1979-2019)

UKBMS for Scotland

 

Graph - Scottish butterfly population trends (1979-2019)

Since the start of the time series in 1979 to the most recent assessment in 2019:
The all-species and generalist species groups increased. The specialist species group shows a stable trend.
Of the 20 species looked at, 8 have increased significantly, two have decreased. The remaining 10 show no significant change.

 

How Scotland’s butterfly populations are changing

Butterfly populations typically show large natural fluctuations mainly due to environmental features, especially the weather. To find out what is happening we need to gather long-term data.  Long-term changes in abundance and distribution of many insects have been linked to a range of factors including habitat loss and fragmentation, land-use changes (especially agricultural intensification) and climate change (Asher et al. 2001; Burns et al. 2016, Habel et al. 2019). 

Overall, butterflies in Scotland show a moderate increase, probably in large part as a consequence of warmer summers.  There are differences between the groups though, with generalist species showing a moderate increase whilst specialist species are stable.  Within the groups, there are both increasing and decreasing species, highlighting a complex picture.

Some butterfly populations in the UK continue to shift northwards as a response to climate change (Mason et al. 2019),  Butterfly species expanding their range northwards into southern Scotland; include small skipper, Essex skipper (Fox et al., 2007; 2015,) and most recently white-letter hairstreak. Three range expanding generalist butterflies show significant long-term population increases:  ringlet, peacock, and orange-tip. The small heath has also increased significantly more in Scotland than across the UK as a whole. Speckled woods have, since 2019, also expanded their range from their strongholds in Highland and south-east Scotland into new areas.

Regular migrant butterflies such as the red admiral are also increasing over the long-term as a response to warming.

Orange tip butterfly. ©SNH. For information on reproduction rights contact the Scottish Natural Heritage Image Library on Tel. 01738 444177 or www.nature.scot

Generalist butterflies are faring better in Scotland than in England, where the trend is stable. This could be the result of better environmental conditions or greater impact of climate change in Scotland (Fox et al., 2015). Generalist species in long-term decline include the small tortoiseshell, which may be due to poorer overwinter survival in warmer and wetter winters.

Scotland’s specialist butterflies show a stable trend. Grayling have declined significantly. However, the small pearl-bordered fritillary and pearl-bordered fritillary increased significantly.  These two species may be benefitting from native woodland planting and targeted management at some sites.

pearl-bordered fritillary

Scotland’s Pollinator Strategy

In 2017, NatureScot published the first Pollinator Strategy, this aims to improve the habitats across Scotland for pollinators. Butterflies part pf the suite of pollinators. Butterflies are not classed as vital pollinators, but improving habitat for butterflies, particularly providing sources of nectar rich native flowers benefits other pollinators.

Source data and updates

This indicator is a multi-species index compiled by Butterfly Conservation and the Centre for Ecology &Hydrology, using data primarily from the UK Butterfly Monitoring Scheme (UKBMS).

Since 2018 an improved Generalised Abundance Index method (GAI) method has been used to calculate species indices (Dennis et al., 2016), whilst trend smoothing methods were unchanged. The GAI uses all butterfly counts collected at UKBMS sites (303) and randomly selected 1km squares of the Wider Countryside Butterfly Survey (213).

By using more data, the GAI produces more representative trends for Scottish butterflies, which may lead to differing results from previous assessments.  In 2020, further improvements were made to better model trends for species that have expanded in range and colonised new UKBMS sites.

Want to help Scotland’s butterflies?

There are a number of things that you can do to help butterflies. Some species require specialist habitat management which can involve volunteers to help maintain sites. Joining the Butterfly Monitoring Scheme and helping with surveys also helps us improve our knowledge of what is happening.

Find out more by contacting Butterfly Conservation Scotland.

References

  • Asher, J., Warren, M., Fox, R. et al. 2001. The Millennium Atlas of Butterflies in Britain and Ireland. Oxford University Press, Oxford.
  • Burns, F., Eaton, M.A., Barlow, K.E. et al. 2016. Agricultural management and climatic change are the major drivers of biodiversity change in the UK. PLoS ONE, 11 (3), e0151595. 18, pp. 10.1371/journal.pone.0151595.
  • Dennis, E.B., Morgan, B.J., Freeman, S.N., Brereton, T.M. & Roy, D.B. 2016. A generalized abundance index for seasonal invertebrates. Biometrics, 72(4), pp.1305-1314.
  • Fox, R., Warren, M.S., Asher, J. et al. 2007. The state of Britain’s butterflies 2007. Butterfly Conservation and the Centre for Ecology and Hydrology, Wareham, Dorset.
  • Fox, R., Brereton, T.M., Asher, J., et al. 2015. The State of the UK’s Butterflies 2015. Butterfly Conservation and the Centre for Ecology & Hydrology, Wareham, Dorset.
  • Habel JC, Ulrich W, Biburger N, Seibold S, Schmitt T 2019 Agricultural intensification drives butterfly decline. Insect Conservation and Diversity. 12(3), 289-295.

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