Methods for recording butterfly transects

The methodology and development of transect monitoring for butterflies has been reviewed in detail elsewhere (Pollard and Yates, 1993). In brief, a fixed-route walk (transect) is established at a site and butterflies are recorded along the route on a regular (weekly) basis under reasonable weather conditions for a number of years. Transect routes are chosen to sample evenly the habitat types and management activity on sites. Care is taken in choosing a transect route as it must then remain fixed to enable butterfly sightings to be compared from year to year. Transects are typically about 2-4km long, taking between 45 minutes and two hours to walk, and are divided into sections corresponding to different habitat or management units.

Butterflies are recorded in a fixed width band (typically 5m wide) along the transect each week from the beginning of April until the end of September yielding, ideally, 26 counts per year. Transect walks are undertaken between 10.45am and 3.45pm and only when weather conditions are suitable for butterfly activity: dry conditions, wind speed less than Beaufort scale 5, and temperature 13°C or greater if there is at least 60% sunshine, or more than 17°C if overcast. Due to the vagaries of the British and Irish weather, it is rare in practice to achieve a full set of 26 weekly counts. However, a small number of missing values can be estimated using other counts during the season.

Single species (as opposed to normal 'all species') transects have been increasingly established in recent years. Whilst such transects must follow the standard methodology and must record populations at least once a week throughout the flight period, the focus on a single (or small number of) species reduces both the time required to walk each transect and, more significantly, the number of weekly counts. With many demands on the time of site management staff and volunteer recorders, this reduced method has enabled population monitoring of particular threatened butterflies to be undertaken when otherwise it would not have been possible. By regularly recording a fixed route in standardised conditions, the number of butterflies seen on a transect can be compared from year to year.

Detailed instructions for setting up and recording a transect are available on our Resources page.

Both transect counts and timed counts provide an annual estimate of the abundance of a butterfly species at site. This site index is not an absolute measure of the size of a population, but has been shown to relate closely to other, more intensive, measures of population size such as mark, release, recapture (MRR) methods (Pollard . The site index can be thought of as a relative measure of the actual population size, being a more or less constant proportion of the number of butterflies actually present. The proportion seen is likely to vary according to species; some butterfly species, such as Marbled White are conspicuous, whereas others such as Dingy Skipper are much less easy to see.

Although a relative measure, site indices can be combined to derive regional and national collated indices and be used to estimate trends over time. However, this collation is not a straightforward calculation because not all of the 1000 or more transect sites in the UKBMS dataset have been recorded each year; some transect sites have operated for twenty years or more but the great majority have not and some have only been recorded for a few years. A statistical model is therefore needed to produce a regional or national index of how butterfly populations have changed each year.

A number of techniques have been suggested to calculate national and regional collated indices of abundance from wildlife monitoring data (ter Braak et al. 1994). In common with most butterfly and bird monitoring schemes in Europe, a log-linear Poisson regression model as performed by the statistical software TRIM (Pannekoek & van Strien, 1996) has been used to analyse the UKBMS data for this publication. In this approach, the expected count at a particular site in a given year is assumed to be a product of a site and a year effect. Put more simply, the model attempts to take account of the fact that some years are generally better than others for numbers of a particular butterfly species (the year effect), e.g. if weather is generally favourable. Similarly, the model accounts for some sites supporting higher numbers of a particular species than other locations (the site effect), e.g. if habitat conditions are highly suitable. In this way, for years where a transect site has not been recorded, the model imputes an estimated site index that allows for the general conditions of the year in question and the how favourable the site is. The national collated index is then calculated as the mean (on a log scale) of the imputed and recorded site indices for each year.

» See technical details of how the index is calculated

Collated indices have been calculated for butterfly species that have been recorded from five or more sites per year. For most species, this allows a plot of the collated index to be provided from 1976 to 2004, showing how the overall population abundance of each species has changed over this time. The regression slope of log collated index on years was used to measure the trend over time and the significance of this trend was determind by the correlation coefficient between the log collated index and years (Pollard et al. 1995) .

Colonisation is defined as 'absence' from a site followed by 'presence';

Extinction is the reverse with 'presence' followed by 'absence'.

The identification of population extinction and foundation can rarely be certain however. In particular, the absence of a species from a site can not be proven by the absence of records and the presence of records does not prove the existence of a breeding population, as butterflies may fly through areas in which they do not breed. In spite of these difficulties, definitions have been adopted to identify extinction and colonisations of populations from butterfly monitoring data. Full details of the method are given in Pollard and Yates (1992).

We assume that there was a breeding population at a site if a species was seen in four successive flight-periods and assume that there was no breeding population if it was not seen in four successive flight-periods. Population extinction was therefore assumed if 'presence' was followed by 'absence' at some later period; population foundation or colonisation was assumed when the converse occurred. Thus, a run of data of eight flight periods (note: 4 years for a bivoltine species; 8 years for a univoltine species) was required for either extinction or foundation to be identified.

Trends are calculated using indices of abundance at individual sites.

The regression slope of log index on years was used as the measure of trend.

The significance of the trends was also assessed to classify trends as:

• Increasing where the regression slope is positive and p < 0.05;
• Declining where the regression slope is negative and p < 0.05;
• Stable where p >= 0.05, e.g. not significant.

A minimum of eight index values was adopted for the inclusion of the data set for a site in the calculation of trends and tests of significance.

Years with zero index values were omitted from the individual site data for the calculation of trends.

Pollard, E. & Yates, T.J. (1992) The extinction and foundation of local butterfly populations in relation to population variability and other factors. Ecological Entomology, 17, 249-254.

Rothery, P. & Roy, D.B. (2001) Application of generalized additive models to butterfly transect count data. Journal of Applied Statistics , 28 , 897-909.

ter Braak, C.J.F., van Strien, A.J., Meijer, R., & Verstrael, T.J. (1994). Analysis of monitoring data with many missing values: which method? In Bird Numbers 1992: Distribution, monitoring and ecological aspects. (eds W. Hagemeijer & T. Verstrael), pp. 663-673. SOVON, Beek-Ubbergen, Netherlands.

Pannekoek, J. & van Strien, A. (1996) TRIM (TRends & Indices for Monitoring data) Statistics Netherlands, Voorburg, The Netherlands.

Pollard, E., Moss, D., & Yates, T.J. (1995) Population trends of common British butterflies at monitored sites. Journal of Applied Ecology , 32 , 9-16.