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Development of Butterfly Monitoring in the UK

The Butterfly Monitoring Scheme

Detailed population monitoring of butterflies commenced at a national scale in the UK in 1976 with the launch of the Butterfly Monitoring Scheme, co-ordinated by the Centre for Ecology and Hydrology and Joint Nature Conservation Committee.

Under the scheme, observers make counts of butterfly numbers at specific sites by weekly recording along a fixed route (transect) under favourable weather conditions throughout the main butterfly flight period (Pollard and Yates 1993).

Transect monitoring required a significant commitment: ideally 26 transect walks per year, each of which might take 1-2 hours to complete, over many years. For this reason (and because of the limited capacity to co-ordinate the scheme centrally) the number of BMS sites was constrained.

Initially, the scheme involved 34 sites across the UK, but this increased steadily to 134 sites in 2004. Most of the BMS transects were carried out at protected sites, such as nature reserves, with semi-natural habitats (biotopes). All of this effort has proved extremely valuable. It provides a standardised annual measure of the changing status of butterfly species, which can be used to generate short-term trends; something that cannot be derived from distribution recording.

Furthermore, BMS data have played a key role in many of the advances in knowledge of butterfly ecology in the UK over the past three decades (Pollard and Yates, 1993). Through the data, scientists have unravelled the dependence of butterfly populations on the climate (e.g. Pollard 1988, Pollard and Yates 1993, Roy et al. 2001). Not only has this paved the way for assessments of the impact of climate change on our biodiversity, but has greatly helped our understanding of how landscape, land-use and habitat changes affect butterflies.

The analysis of BMS data can allow for the over-riding influence of the weather, thus enabling other influences on particular butterfly populations to be detected. For example, site managers can assess the impact of small-scale habitat management and policy makers can monitor the effectiveness of national-scale agri-environment schemes (Brereton et al. 2006)

With an output of over a hundred scientific research papers and improvements in analysis of the data, the BMS went from strength to strength. Nevertheless, improvement were needed in two main areas.

Firstly, because of the limited number of sites in the scheme, insufficient data was available to generate population trends for some of the rarer species.

Secondly, most BMS sites were protected areas that are managed, at least in part, with biodiversity conservation objectives.

As a consequence, the national population trends generated by the scheme may not have been representative of the landscape as a whole. Both of these limitations have now been addressed under the UKBMS project and the future of butterfly monitoring is brighter than ever.

Butterfly Conservation's Transect project

Although the number of sites contributing to the BMS has remained limited, the scheme's transect methodology developed by Dr Ernie Pollard has been taken up by many conservation organisations, landowners and amateur naturalists.

The number of transects operating outside the BMS grew steadily at first, but increased rapidly after 1990. By 2003, over 500 transects were being recorded by more than 1000 recorders, with 80 new ones established in that year alone (Brereton et al. 2006). Although some local co-ordination of results was already being undertaken by pioneering Branches of Butterfly Conservation, national collation and analysis of all these independent transects only commenced in 1998.

Since then, support from the Department of the Environment, Food and Rural Affairs (Defra) and its predecessor, the Ministry of Agriculture Fisheries and Food (MAFF) enabled Butterfly Conservation to co-ordinate independent transects (and other standardised survey data e.g. timed counts).

This not only allowed the collation and analysis of an important new data set, but has encouraged and trained the people who walk transects (most of whom are volunteers). It has also streamlined the flow of data, instigated new transects in under-recorded regions and led to the development of new analysis techniques and software.

Although most transects co-ordinated by Butterfly Conservation were started relatively recently and do not have the benefit of long time-series of data, the large number of monitored sites enabled the calculation for the first time of reliable population trends for many of the rare and threatened species.

In addition, although like BMS transects, much of the independent monitoring is carried out on protected sites, there is a much greater range and representation of different habitat and land-use types in the Butterfly Conservation data set. Indeed, the initial impetus to collate the data was to enable an assessment of the impact on butterfly populations of agri-environment schemes in the farmed landscape

UK Butterfly Monitoring Scheme

BC, CEH and JNCC had been collaborating over butterfly monitoring since the inception of BCs independent transect network in 1998. Over the following years, the many advantages of combining data and joining forces to a greater degree became more apparent, though need for funding and a suitable framework needed to be identified. In 2005, a consortium of Government agencies provided funding to substantially develop butterfly monitoring in the UK and to develop butterflies as Governmental Biodiversity Indicators through the UKBMS project. The UKBMS had two key objectives: (1) To develop an integrated UK-wide monitoring scheme by merging CEH/BC transect datasets and extending more effective, targeted coverage across the UK and (2) To develop an improved method for monitoring the status of butterflies across the wider countryside.

Wider Countryside Butterfly Survey

Despite the massive growth in butterfly transect monitoring through the 1990s and early 2000s, unbiased information on population trends of wider countryside butterflies was lacking, given that current transect locations were biased towards semi-natural and protected habitats. This knowledge gap was important because research over the period indicated that many wider countryside species underwent substantial declines in local and regional abundance during the 20th century; declines that were largely undetected by current monitoring and mapping schemes (Cowley et al. 1999). Intensive farmland and upland habitats were particularly under-represented in current butterfly monitoring. It was determined that a new scheme based upon stratified random sampling was required to monitor widespread species that would provide annual abundance indices representative of the UK landscape as a whole.

The Wider Countryside Butterfly Survey (WCBS) was developed and tested over the period 2006-2008. The design brief for the WCBS was to design a monitoring method that was efficient (requiring relatively few visits), scientifically robust (to pick up target species at a suitable level to produce trends) and appealing to a wide volunteer audience (including bird recorders who could potentially help deliver much of the recording). Pilot studies were carried out to:

  • Model when and how often to sample, through analysis of transect data.
  • Design of the sampling method, e.g. when and where to sample butterflies in the wider countryside.
  • Identify the most efficient and robust method through workshop consultation.
  • Field test a range of potential methods initially with a field researcher (2006) and then with Butterfly Conservation and British Trust for Ornithology (BTO) volunteers (2007) in upland and lowland areas.
  • Develop web-based online data entry system for use by volunteers.
  • Obtain feedback from recorders.
  • Assess likely uptake in a proposed scheme.

The WCBS method and sampling strategy closely followed that developed for the BTO’s Breeding Bird Survey (the ‘BBS’), with counting along two parallel 1-km long transects subdivided into 10 sections, located within randomly selected 1-km squares, but adapted for butterflies.

The scheme was rolled out in 2009 with 763 squares sampled and has run each year since.


Brereton, T., Roy, D., Greatorex-Davies, N., 2006. Thirty years and counting. The contribution to conservation and ecology of butterflymonitoring in the UK. British Wildlife 17, 162-170.

Pollard, E., 1988. Temperature, rainfall and butterfly numbers. Journal of Applied Ecology 25, 819-828.

Pollard, E., Yates, T.J., 1993. Monitoring butterflies for ecology and conservation. Chapman & Hall, London.

Roy, D.B., Rothery, P., Moss, D., Pollard, E., Thomas, J.A., 2001. Butterfly numbers and weather: predicting historical trends in abundance and the future effects of climate change. Journal of Animal Ecology 70, 201-217.

The UKBMS is run by  Butterfly Conservation (BC), the Centre for Ecology and Hydrology (CEH) and the British Trust for Ornithology (BTO), in partnership with the Joint Nature Conservation Committee (JNCC), and supported and steered by Forestry Commission (FC),  Natural England (NE), Natural Resources Wales (NRW), Northern Ireland Environment Agency (NIEA), and Scottish Natural Heritage (SNH). The UKBMS is indebted to all volunteers who contribute data to the scheme.