Scotland's indicators - Terrestrial Insect Abundance – Butterflies

Published 2023

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 useful indicators of environmental change due to their rapid and sensitive responses to subtle habitat or climatic changes, and as representatives for broader biodiversity.

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.

Monitoring Scotland’s Butterflies

In Scotland, butterflies are monitored through the Butterfly Monitoring Scheme (UKBMS) and the Wider Countryside Butterfly Survey. In the UKBMS, volunteers walk fixed route transects from April to September each year.  The indicator describes trends for 20 of the 34 regularly occurring butterfly species in Scotland at 619 sample locations.

How Scotland’s butterfly populations are changing

Butterfly populations can vary markedly from year to year. To help understand the underlying trend, the data is smoothed. Doing this removes the fluctuations and shows that overall butterflies that use a range of habitats (generalist species) are increasing. Those that are more restricted (specialist species) are stable. There are differences in the trends for individual species that contribute to the indicator. Table 1 shows the trends for all of the species included in the indicator.

Table 1. Trends of butterfly species in Scotland. Significance of trends: * P < 0.05 (significant), ** P < 0.01 (highly significant), ***P < 0.001 (very highly significant)

SPECIES	Start Year	%CHANGE 2021-2022	Series trend (%)	10-yr trend (%)	5-yr trend (%)
Orange-tip	1999	17	375***	57	-2
Large White	1979	-11	106*	21	0
Small White	1979	41	72	18	17
Green-veined White	1979	-15	-5	-47	-55*
Speckled Wood	2001	87	168***	191**	71
Large Heath	2003	62	-26	-35	80
Small Heath	1979	-40	145***	46	38
Scotch Argus	1990	-53	-11	-48*	-40
Ringlet	1996	-2	280***	16	77
Meadow Brown	1979	0	1	36	254**
Grayling	1990	6	-90***	-38	150
Pearl-bordered Fritillary	2002	-48	225***	98	-15
Small Pearl-bordered Fritillary	1979	-57	98**	73	18
Dark Green Fritillary	1979	-26	17	-1	54
Red Admiral	1980	118	833***	272	-47
Peacock	1995	52	224***	56	11
Small Tortoiseshell	1979	-31	-61**	-40	93
Small Copper	1979	-38	-46*	-30	-41
Green Hairstreak	1990	3	29	20	9
Common Blue	1979	-50	39	-25	-9

Nine butterfly species have increased significantly. Amongst the largest increases were red admiral, orange-tip and peacock, with climate change and weather being the likely driver. Eight species are stable (i.e. showing no significant trend), and three species show significant declines, small copper, grayling and small tortoiseshell. Small tortoiseshell declines are not well understood: a study by Gripenberg (2011), looked into a parasitoid that arrived in the UK when declines where first observed in the mid-2000s. The analysis showed no clear link, which suggests that other drivers such as weather, climate and habitat may be causing declines. Grayling is a coastal specialist whose declines are due to changes in habitat. Its main food host plant is sea holly. Small copper declines have been apparent in other parts of the UK in previous years, this is the first time a significant decline has been observed in Scotland. Reasons for the decline include climate change and habitat loss. Sheep’s sorrel, one of its principal larval foodplants has declined since the mid-20^th Century (Stroh et al. 2023). Gardens may be an increasingly important habitat for small copper, and wildlife-friendly gardening may counteract a more general decline in urban butterflies (Plummer et al. 2023).

Drivers of change of butterfly populations

Broadly, we can separate the main drivers of change for butterfly populations into five categories: weather; climate change; habitat; pesticides and pollution; and conservation action. Full details of the drivers of change for butterflies are available online.

Weather

Short term changes in butterfly populations are often driven by the weather. Cold and wet weather in spring or summer tends to reduce butterfly activity and can affect caterpillar growth and survival (Kamata & Igarashi 1994). Warm but damp weather in the spring can negatively influence the development of caterpillars. Temperature and the amount of rain and sunshine affects the availability of butterfly food sources – both larval food plants and nectar sources for adults. Studies have shown that nectar production in many flowers is significantly higher in sunny and in dry weather (Lack 1982, Benedek et al 2000, Pleasants 1983). However, while butterflies generally benefit from warm and dry spring/summers, drought conditions can reduce caterpillar food availability by reducing plant growth or shortening the flowering season.

Climate change

Climate change effects (e.g. an increase in extreme events such as very hot, dry summers) are already being observed (Pearce-Higgins et al 2015, McDermott Long et al 2016, Oliver et al 2015). Spring-flying species have been emerging progressively earlier in recent decades. However, any benefits of an earlier season for butterflies may be negated if it results in plants and flowers dying earlier than they used to, ending the flight season.

Over the past 20 years, species traditionally found in the UK’s warmer southerly climes, for example orange-tip and peacock, have become more abundant in Scotland. However, the potential for northward range expansion for butterfly species is likely to have more limited scope than generally thought because of interplay between a number of factors such as habitat availability and local weather conditions that affect butterfly survival and dispersal (Pelini et al 2009). A recent study indicated that climate envelope models overestimate the positive effects of climate change on north-west European butterflies. Overall, habitat specialist species are likely to be more vulnerable to climate change (WallisDeVries et al 2011).

Habitat related drivers

Habitat creation, loss, degradation, management and connectivity are major drivers of change in Scotland’s butterfly populations. For example targeted action has helped improve habitat for pearl-bordered fritillary. While agricultural intensification has adversely affected wildlife, farmers across Scotland are implementing practices such as conservation headlands and unsprayed field margins (Dover et al 1990, Longley & Sotherton 1997, Cole 2007).  Practices such as these can help butterflies and other beneficial insects. Many semi-natural habitats depend on management to maintain the distinctive features that their component species rely on, thus changes in traditional management activity can affect habitat quality. Urban greenspaces are also an important habitat for butterflies and work such as the Central Scotland Green Network are helping provide areas for pollinators including butterflies.

Habitat connectivity is key to a species being able to disperse to new sites, their ability to recover after population crashes (e.g. caused by extreme events such as drought) and their ability to respond to conservation management (Oliver et al 2012, Oliver et al 2015). Colonisation can take several years after an area of habitat has been restored, especially by species that are less mobile or are dependent on localised larval host plants (Woodcock et al 2012).

Pesticides

Alongside the habitat related drivers, the use of pesticides may have direct toxicity impacts on butterflies. Ongoing and future research into the effects of pesticides is an area where monitoring data will help determine the extent to which butterflies are affected by farmland chemicals. While much of the research into the impact of pesticides on beneficial insects has concentrated on bees, recent studies in the UK and in Europe have shown a correlation between butterfly declines and application of neonicotinoids (Gilburn et al 2015, Forister et al 2016, Basley & Goulson 2018).

Conservation action

Landscape scale conservation efforts play an important role in improving the fortunes of butterfly species. As our understanding of butterfly ecology increases, conservationists have been able to restore suitable habitat to help many declining species. This is especially the case for species with specific habitat requirements or poor powers of dispersal, and where the appropriate conditions may often rely on active habitat management. Such species can respond well to targeted conservation habitat management. Projects such as Species on the Edge, which includes projects focussed on northern brown argus and small blue, are helping through targeted action involving volunteers and conservation organisations.   

Scotland’s Pollinator Strategy

In 2017, Scottish Government published the first Pollinator Strategy, which aims to improve the habitats across Scotland for 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 UK 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 (365) and randomly selected 1km squares of the Wider Countryside Butterfly Survey (232).

By using more data, the GAI produces more representative trends for Scottish butterflies, which may lead to differing results from previous assessments.  In 2021, further improvements were made to better model trends for species that have expanded in range and colonised new UKBMS sites.

References

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Benedek P., Kocsisne Molnar G. & Nyeki J. 2000. Nectar production of pear cultivars. International Journal of Horticultural Science 6, 67-75.

Cole L.J., McCracken D.I., Baker L. & Parish D. 2007. Grassland conservation headlands: their impact on invertebrate assemblages in intensively managed grassland. Agriculture, Ecosystems & Environment, 122, 252-258.

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.

Dover J., Sotherton N. & Gobbett K. 1990. Reduced pesticide inputs on cereal field margins: the effects on butterfly abundance. Ecological Entomology 15: 17-24.

Forister M.L., Cousens B., Harrison J.G., AndersonK., ThorneJ.H., Waetjen D., Nice C.C., De Parsia M., Hladik M.L., Meese R., van Vliet H. & Shapiro A.M. 2016. Increasing neonicotinoid use and the declining butterfly fauna of lowland California. Biology Letters 12: 20160475.

Gilburn A.S., Bunnefeld N., McVean Wilson J., Botham M.S., Brereton T., Fox R. & Goulson D. 2015. Are neonicotinoid insecticides driving declines of widespread butterflies? PeerJ 3:e1402.

GRIPENBERG, S., HAMER, N., BRERETON, T., ROY, D.B. and LEWIS, O.T. (2011), A novel parasitoid and a declining butterfly: cause or coincidence?. Ecological Entomology, 36: 271-281.

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Longley M. & Sotherton N.E. (1997): Factors determining the effects of pesticides upon butterflies inhabiting arable farmland. Agriculture, Ecosystems & Environment 61: 1–12.

McDermott Long, O., Warren, R., Price, J., Brereton, T.M., Botham, M.C. & Franco, A.M.A. (2017): Sensitivity of UK butterflies to local climatic extremes: which life stages are most at risk? Journal of Animal Ecology, 86, 108-116.

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Oliver, T.H., Marshall, H.H., Morecroft, M.D., Brereton, T., Prudhomme, C. & Huntingford, C. (2015): Interacting effects of climate change and habitat fragmentation on drought –sensitive butterflies. Nature Climate Change, 5, 941 –945.

Pearce-Higgins et al (2015): Final report to the Biological Impacts of Climate Change Observation Network Steering Group. Accessed online 16 June 2022.

Pelini S.L., Dzurisin J.D.K., Prior K.M., Williams C.M., Marsico T.D., Sinclair B.J. & Hellmann J.J. (2009): Translocation experiments with butterflies reveal limits to enhancement of poleward populations under climate change. PNAS (Proceedings of the National Academy of Sciences) 106:11160-11165.

Pleasants J.M. (1983) Nectar production patterns in Ipomopsis aggregate (Polemoniaceae). American Journal of Botany 70:1468-1475.

Plummer, K.E., Dadam, D., Brereton, T., Dennis, E.B., Massimino, D., Risely, K., Siriwardena, G.M. and Toms, M.P., 2023. Trends in butterfly populations in UK gardens—New evidence from citizen science monitoring. Insect Conservation and Diversity.

Stroh, P.A., Walker, K.J., Humphrey, T.A., Pescott, O.L. and Burkmar, R.J., 2023. Plant atlas 2020: mapping changes in the distribution of the British and Irish flora. Princeton University Press.

WallisDeVries, M.F., Baxter, W. & Van Vliet, A.J.H. (2011): Beyond climate envelopes: effects of weather on regional population trends in butterflies. Oecologia 167: 559-571.

WallisDeVries, M.F. & Swaay C.A.M. (2006): Global warming and excess nitrogen may induce butterfly decline by microclimatic cooling. Global Change Biology 12: 1620-1626.

Woodcock B.A., Bullock J.M., Mortimer S.R., Brereton T., Redhead J.W., Thomas J.A. & Pywell R.F. (2012): Identifying time lags in the restoration of grassland butterfly communities: A multi-site assessment. Biological Conservation 155: 50-58.