bitte übersetzen:       Chapter 3 / From butterfly counts to indicators
Introduction
Butterflies can be found all over Europe and are one of the best-known groups of insects. Although
popular, until recently little was known about their density and trends. In this chapter we will
illustrate how counts are made and how they can be used to detect trends and to build indicators.
Fieldwork
The butterfly indicators are based on the fieldwork of
thousands of trained professional and volunteer
recorders, counting butterflies on more than 10,000
transects scattered widely across Europe, with almost
5,000 visited in 2020. These counts are made under
standardised conditions, providing high-quality data
that are suitable to assess species status and trends.
National co-ordinators collect the data and perform
the first quality control.
All schemes apply the method initially developed for
the UK Butterfly Monitoring Scheme (Pollard & Yates,
1993). The counts are conducted along fixed transects
of 0.5 to 3 kilometres in length, divided into smaller
sections for recording. The fieldworkers record all
butterflies that are observed 2.5 metres to their right,
2.5 metres to their left, 5 metres ahead of them and 5
metres above them (Van Swaay et al. 2008). Butterfly
counts are conducted between March-April to
September-October, depending on the region. In some
places (e.g., Andalucia, Canary Islands) there are
places where monitoring takes place all year round,
sometimes stopping in July-August during the hot and
dry summer. Visits are only conducted when weather conditions meet specific criteria. The
recommended number of visits varies from every week, e.g., in the UK, Catalonia and the
Netherlands, to 3-5 visits annually in France. Austria and Switzerland BMS use a stratified sampling
design with a subset of transects monitored every year. This protocol enables to maintain high
frequency monitoring in areas where access present specific challenges (alpine sites). In this
protocol, all sites are monitored at least once every four years.
European Butterfly Monitoring Scheme database
The European Butterfly Monitoring Scheme (eBMS) database collates standardised butterfly counts
recorded along Pollard walks repeated in time (since 1976 in the UK). Since 1990, volunteers and
professionals have recorded more than 5.5 million butterfly count events documented at the species
level in the eBMS database. These counts have been recorded over more than 991,000 monitoring
events (e.g., BMS transect visits). In terms of sampling effort, these visits took place in over 12,000
transects (Figure 1), distributed across 25 monitoring schemes and 23 countries (Figure 2b), with
nearly 7,000 monitoring sites located in EU27 Member States.
Recording butterflies on a transect.
Butterfly Conservation Europe 2022 | EUROPEAN GRASSLAND BUTTERFLY INDICATOR 1990-2020 14
The eBMS database is updated on an annual cycle, each update being released as a major version
(e.g., v4.0), with subsequent corrections, additions and bug fix being identified and released as minor
version updates (e.g., v4.2). Source data files are sent by national Butterfly Monitoring Schemes and
processed programmatically to ensure adequate standardisation and formatting of the data before
being integrated into the eBMS database.
Transect selection
To be able to draw proper inferences on the temporal population trends at the national or regional
level, transects should ideally best be selected in a grid, random or stratified random manner
(Sutherland, 2006). Several recent schemes, e.g., in Switzerland, France and in parts of Austria, have
been designed in this manner (Henry et al. 2008). If a scheme aims to monitor rare species, scheme
co-ordinators preferentially locate transects in areas where rare species occur, leading to an overrepresentation of special and protected areas. In most schemes transects were selected by free
choice of observers, which in some cases has led to the overrepresentation of protected sites in
natural areas and the under-sampling of the wider countryside and urban areas (Pollard & Yates,
1993). However, this is not the case in all countries (e.g., Germany, Kühn et al. 2008).