Decline of Wall Brown (Lasiommata megera) in UK

[Nick Greatorex-Davies]

More food for thought on the recent decline of the Wall Brown in the UK
(Lasiommata megera) from the results of the UK Butterfly Monitoring
Scheme (BMS). The BMS started in 1976 with just 36 sites nationwide.
Today we receive data from approximately 125 sites. Unfortunately we
do not have the resources to handle any more sites, though there are
probably in excess of 500 sites throughout the UK that are being (or
have been in the recent past) monitored for butterflies independently of
the BMS but using the same methodology. These are being done mostly
by/for conservation organisations by volunteer recorders. This is now -
by our standards at least - a long-term data set and is producing all kinds
of useful information on trends in butterfly numbers, butterfly ecology,
changes in range, info. possibly relating to climate change, info. to help
site managers etc.

With reference to the Wall Brown, the Collated Annual Index (too
complex to explain how we calculate this here) shows a big drop in
numbers in 1977 after the 1976 drought. Numbers rose again to a high in
1982 and 1983. Numbers dropped sharply again after the hot dry
summer of 1984 with numbers remaining low in the years 1985-88 (nadir
in 1987). Numbers were high again in 1989 and 1990. 1989-1991 were
generally hot dry summers, with July and August being particularly hot in
1990. There was a substantial drop in the Collated Annual Index from
1990 to 1991, and numbers have dropped almost annually since, with the
Collated Annual Index in 1996 being the lowest for the Wall Brown since
the BMS began, though not that much lower than the previous nadir of
1987. Data is only just coming in for 1997, but at least locally here in
Cambridgeshire numbers may be increasing again.

Apparent extinctions at BMS sites (ie not recorded for at least four years
on a transect, where formally it was recorded regularly) are nearly all on
inland sites. At coastal sites the Wall Brown seems to be faring rather
better.

Weather has been shown to have a major effect on the fluctuations of
butterfly numbers from year to year and there is often a high degree of
synchrony in fluctuations across sites and within some species groups.
My colleague David Roy has recently carried out extensive analysis of
BMS data with some weather variables  (previous and current years
temperature and rainfall)(not yet published). For the Wall Brown these
analyses are inconclusive, but there is some indication that a wet July
may have a negative effect on Wall Brown numbers and that a wet
autumn and spring may have a positive effect on numbers of this
species.

It is interesting to note that the biggest drops in numbers were after the
long hot dry summers of 1976, 1984 and 1990. There was also a drop
after the 1995 long hot dry summer however this was less pronounced,
but numbers had been dropping almost annually since 1990.

It will be very interesting to continue to follow the fortunes of the Wall
Brown over the next few years and hopefully to be in a better position to
understand what are the most important factors controlling numbers.

Kind regards

Nick Greatorex-Davies
(UK Butterfly  Monitoring Scheme co-ordinator)
Institute of Terrestrial Ecology
Monks Wood
Abbots Ripton
Huntingdon  PE17 2LS
UK

Email: n.greatorex-davies at ite.ac.uk