Scotland's Indicators - Terrestrial Insect Abundance - Butterflies

Published 2024

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 632 sample locations.

Scottish butterfly population trends (1979-2023)

How Scotland’s butterfly populations are changing

Butterfly populations can vary markedly from year to year. To help understand the underlying trend, the data are smoothed. 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 2022-2023	Series trend (%)	10-yr trend (%)	5-yr trend (%)
Orange-tip	1999	-4	354***	21	-28
Large White	1979	44	94*	126	-27
Small White	1979	-5	97*	64	-35
Green-veined White	1979	-19	-12	-46	-64**
Speckled Wood	2001	109	283***	440**	236
Large Heath	2003	124	-11	33	109
Small Heath	1979	-14	131**	9	-33
Scotch Argus	1990	43	-16	-41	-42
Ringlet	1996	-26	254***	1	-10
Meadow Brown	1979	26	15	88	154**
Grayling	1990	-37	-91***	2	23
Pearl-bordered Fritillary	2002	35	201***	53	-31
Small Pearl-bordered Fritillary	1979	2	110***	17	-17
Dark Green Fritillary	1979	-21	16	33	-21
Red Admiral	1980	224	998***	166	121
Peacock	1995	23	232***	60	1
Small Tortoiseshell	1979	0	-60**	-21	-11
Small Copper	1979	14	-45	-34	-69*
Green Hairstreak	1990	79	23	32	16
Common Blue	1979	8	31	-18	-47

Ten butterfly species have increased significantly. Amongst the largest increases were red admiral, orange-tip and speckled wood, with climate change and weather being the likely drivers. Eight species are stable (i.e. showing no significant trend), and two species show significant declines, small tortoiseshell and grayling.

Small tortoiseshell declines are not well understood: a study by Gripenberg (2011), looked into a parasitoid that arrived in the UK when declines were 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.

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 30 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 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.