Wednesday, 9 April 2014

UK butterfly statistics 2013 – the story behind the headlines

The annual UK Butterfly Monitoring Scheme (UKBMS) data tables have been released over the last few days, the 38th time of such a release since the scheme began in 1976. Each year the main focus of media interest tends to be on the increases and decreases for each species compared to the previous year. This year is no different with the numbers showing that in 2013 there was an increase in abundance for the majority of species (46 out of 56) as a result of last year's warm summer. The 'good' summer of 2013 followed the "worst year on record" (in butterfly terms) in 2012. Media reporting, see the Guardian here and the Daily Mail here, has reflected this change in fortunes over the annual cycle.

The common Green-veined white butterfly had its worst year on record in 2012
but bounced back in 2013. Photo: Ross Newham

Significant decline of some species

However the annual changes don’t tell the full story. As CEH butterfly ecologist Dr Marc Botham said in the press release accompanying the 2013 statistical release:

“Annual changes are largely associated with the weather. However, the data show that a number of species have been significantly declining over the last 38 years.”

In fact over the full UKBMS series, stretching back nearly four decades, 33 species have declined, 1 has remained relatively stable, and just 22 have increased, compared to the 46 that increased in abundance between 2012 and 2013. There can also be differences in the results between the full series and the last decade. Over the last 10 years 40 species have decreased in abundance.

The migrant Clouded Yellow was seen in large numbers in 2013.
Photo: Tim Heoflich

A good example of these differences is the Clouded Yellow. The species had a fantastic year in 2013 as an influx of migrants resulted in a 4373% increase over the 2012 figures, and has also had a fantastic run over the long term (back to 1979 when the species was first recorded). However its record over the last 10 years is relatively poor when compared to other species. The species list on the Butterfly Conservation website offers the following explanation:

“Clouded Yellow - Numbers on UKBMS sites vary considerably from year to year, the huge fluctuations in the collated index plot demonstrating the good migrant years where some sites record double, sometimes triple, figure numbers. In 2000 Clouded yellow produced a three figure index on four different UKBMS sites. The data show no significant trend. However, as the collated index plot shows, there has been a large increase since monitoring began and the lack of significance is likely a result of the recent run of poor migrant years.”

Another example is the Small Tortoiseshell. Across 1755 recording sites the species had a great year in 2013 compared to 2012 but it has done much less well over the last decade, and very badly over the full series of record (back to 1976). The UKBMS factsheet - see the graph under Log Collated index - shows this long term decline. The factsheet offers some explanation:

"The data show a significant decline in the Small Tortoiseshell since 1976. Although this species does generally show annual fluctuations in abundance, the recent pattern of underlying decline has seen it regularly produce some of its lowest indices of the series."

The garden favourite Small Tortoiseshell, photo by Ross Newham

New analytical method

The 2013 numbers are also the first to incorporate trend estimates using a new analytical method. As the 2012 UKBMS report states:

“A paper describing the method has been published in a widely respected peer-reviewed scientific journal, Methods in Ecology and Evolution (DOI: 10.1111/2041- 210X.12053). Briefly, the method uses all butterfly counts in a season to estimate the seasonal pattern of butterfly counts for that year, and uses this to weight observed counts when calculating annual population indices and trends over time. Compared to the current analysis approach, the new method has several advantages. Firstly, it provides a more precise method for measuring butterfly trends. Secondly, it enables us to include all transect counts within the assessment of trends, thereby making more efficient use of recorders’ efforts and covering more sites within the geographic range of each species. Finally, for reporting trends in the future, we intend to combine data from traditional transects and data from the Wider Countryside Butterfly Survey, enabling butterfly trends to be more representative.”

The observant among you will notice that the 2013 results from the Wider Countryside Butterfly Survey (WCBS), incorporated in today’s 2013 UKBMS results, were separately released in February  and showed that Farmland butterflies thrived during last years hot summer. One of my previous blogs explains the difference between the WCBS and the UKBMS.

In summary

All of the above highlights the vital importance of maintaining long-term monitoring, which is reliant on the immense dedication of thousands of volunteers, mixed with high quality analysis from the supporting scientific team at the Centre for Ecology & Hydrology and Butterfly Conservation. It is only by continuing to do this that we can ensure the best available evidence is collected to best inform the UK’s conservation policy.

Barnaby Smith, CEH Media Relations Manager

Additional information

Data table - summary of species abundance 2013

Data table - significant difference between England, Scotland and Wales

United Kingdom Butterfly Monitoring scheme

UKBMS is run by CEH and our partners in the scheme Butterfly Conservation (BC) and the Joint Nature Conservation Committee, and has collated records since 1976. The scheme now involves thousands of volunteers collecting data every week throughout the summer from more than 1,000 sites across the UK. CEH and BC are very grateful to all the volunteer recorders for their hard work and tenacity in walking their butterfly transects during 2013.

Slideshow of butterflies featured in 2013 butterfly statistics

In 2012 I blogged about how the UKBMS annual figures are used as indicators by Government to monitor long term changes in the UK’s biodiversity (See: Butterflies, biodiversity indicators and long-term change).

Links to CEH news stories about the Wider Countryside Butterfly Survey

2013 results: Farmland butterflies thrived in 2013
2012 results: Grass-feeding butterflies defy 2012 deluge
2011 results: Small Tortoiseshell decline highlighted in new study

Links to CEH news stories about the United Kingdom Butterfly Monitoring scheme

2012 results: Washout 2012 was the worst year for UK butterflies on record
2011 results: Record-breaking 2011 Spring helped some of the UK's rarer butterflies
2010 results: Rare butterflies show signs of recovery
2009 results: Fears grow for some of Britain's rarest butterflies
2008 results: UK butterfly numbers fall to new low
2007 results: Butterflies at record low after wet summer of 2007

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