BUTTERFLY RECORDS FROM WINDMILL HILL 1982 TO 2002
Terry Knight
| Recording
In 1976 a method was developed by the Institute for Terrestrial Ecology for
the recording of butterflies on a selection of about a hundred sites
across the country to monitor national trends in populations. This is
known as the Butterfly Monitoring Scheme. The Institute subsequently
published a booklet of instructions for independent recorders so that
the same method could be used elsewhere (ITE 1981). Since its
publication the method has been widely adopted to record butterflies and
is now used at over 400 other sites in the country.The basis of the method is as follows. A route is fixed through the site and
divided into no more than fifteen sections. It is walked once a week from the
beginning of April (Week 1) to the end of September (Week 26). During the walk
the number of each species of butterfly seen in a section is noted. Pupae,
larvae and ova are ignored just the winged stage (imagines) being recorded. Only
individuals within five metres in front and about two and a half metres to each
side and above ground are counted. The booklet gives more detailed instructions
and limits that apply and also gives methods of analysing the data.
The site
The half mile long west facing escarpment comprising the site was purchased
by the Wildlife Trust as a nature reserve in 1979 and is a Site of Special
Scientific Interest. It is well suited for the monitoring of butterflies as the
route and habitats are well defined. The first nine sections lie along the
bridleway at the top of the bank and the remaining six are about two-thirds the
way up the bank through five paddocks of calcareous grassland with some scrub
(see plan). Apart from the half mile of escarpment to the south, which is in
private hands, the reserve is the only such habitat of any size within a
distance of about four miles. The surrounding countryside consists primarily of
arable with some market gardening and intensive grazing.
Species
Over the last thirty years, thirty two species have been seen on the
reserve. The status of some of these is unclear but the best guess is as given
in Table 1. Due to the difficulty of distinguishing between Small skipper and
Essex skipper when in flight these are recorded as Small skipper, Small/Essex ?
skipper, or Essex skipper as appropriate. The criteria used for assessing the
likelihood of residence are :- larval foodplants are present in significant
quantities on the reserve; imagines are seen practically every year.
Food plants
Two-thirds of the reserve is grassland comprising both fine and coarse
leaved species of grass. Of these the principal ones are Tor grass Brachypodium
pinnatum, fine leaved fescues Festuca rubra/ovina and Upright brome Bromopsis
erecta but Cocksfoot Dactylis glomerata and Yorkshire fog Holcus
lanatus , with others, are also present.
Other possible food plants include :- abundant Birds-foot trefoil Lotus
corniculatus, Ivy Hedera helix, Hairy violet Viola hirta; some
Bramble Rubus sp., Garlic mustard Alliaria petiolata, Nettle Urtica
dioica; a little Dock Rumex sanguineus; and six Buckthorn trees Rhamnus
cathartica. In recent years the amount of Bramble has declined as a result
of restoring scrub invaded grassland. Garlic mustard, too, has reduced as the
belt of mature scrub along the bottom of the reserve has become more open.
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Nectar plants
Imagines rely on nectar and this attracts individuals onto the reserve in
the summer, as well as supplying the resident butterflies. Sources on the
reserve include Bramble, Violets (Viola sp.), Birds-foot trefoil, Privet
Ligustrum vulgare, Wild thyme Thymus polytrichus, Pyramidal orchid Anacamptis
pyramidalis, Common knapweed Centaurea nigra, Field scabious Knautia
arvensis, Greater knapweed Centaurea scabiosa, Woolly thistle Cirsium
eriophorum and Ivy. These flower through the season from April to November
with the majority flowering from June to October. Birds-foot trefoil, Common
knapweed and Field scabious are abundant.
Factors affecting numbers
The imago stage, the "butterfly" as it is usually thought of, is
the one used for recording. However, this is the most mobile part of the life of
the insect and individuals can travel great distances. Some such as Clouded
yellow, Large white, Small white, Red admiral and Painted lady can fly here from
abroad. Even species considered as resident are able to move to and from other
sites. Consequently, the numbers on the reserve can be influenced by local,
regional, national and even international conditions.
Factors on the reserve which will affect numbers are - availability of food
plants and nectar flowers for residents, availability of nectar flowers for
non-residents, a balance between sunshine and shade areas, shelter from the wind
and lack of disturbance at critical times.
Grazing
Normally, sheep are grazed in paddocks 1, 3, 4, and 5 (see plan) in the
autumn and spring. This therefore has an effect on most of the grassland during
the year and consequently on the butterflies, either directly or indirectly
through the food and nectar plants. The five most numerous species (Marbled
white, Meadow brown, Gatekeeper, Ringlet and Speckled wood) all feed on grasses
and pass the winter ((September) October to April (May)) as caterpillars usually
hidden in the base of grass tussocks during the day. As this stage in the life
cycle is somewhat mobile it has the ability to cope with changes due to grazing
provided this is not too severe or rapid. In order to prevent the food and
nectar plants being shaded out by coarse vegetation, controlled grazing is
required. This also provides areas of longer and shorter grass to suit the
requirements of different species of butterfly.
Scrub clearance
Until recently, the invading scrub (mostly hawthorn) has been cleared from
one paddock each year by cutting it off at ground level. As a result only about
a fifth of the site has been altered per year and the resulting re-growth is at
a different stage in each paddock. The effect on butterflies in the short term
is likely to be very limited. However, over a longer time span just cutting off
the scrub may well have had a detrimental effect as its resulting thickening and
increased encroachment has reduced the area of grassland.
During the last two years more resources have been put into scrub clearance
in order to halt its spread and to try to get the grassland back to its
approximate 1979 boundary. This is being done by uprooting almost all the scrub
back to its 1979 limit and only leaving small isolated blocks in the centre and
top of the paddocks. So far nearly half of the four grazed paddocks have been
done. The result, it is hoped, will be an increase in the food and nectar plants
while still retaining some scrub as shelter.
Habitats
At the top of the reserve is the bridleway which can be taken as one habitat
(Habitat A). It consists of a strip of bare ground (mud when wet!) about one
metre wide created by passing horses. Each side is bordered by short grass and
herbage which increases in height away from the track. 75 metres of it has
grazed paddock on one side and corn on the other, 240 metres has hedge and/or
scrub both sides of it and is shaded, the remaining 485 metres has hedge and/or
scrub on one side and grazed paddocks on the other giving shelter but limited
shade. Some bramble and a little privet provide nectar in addition to herbs
bordering the track. Over the last four years the grass and bits of the scrub
along the edges of the track have been strimmed back a time or two during the
summer by the County Council. Previous to this some scrub was cut back in the
early 1980's but very little in the intervening years.
The second paddock from the north has not normally been grazed and consists
of a mixture of long grass, discrete regenerating hawthorn bushes, permanent
blocks of blackthorn scrub and some hawthorn trees. This has been taken as
Habitat B. Nectar sources include taller herbs such as knapweeds, scabious,
ragwort and thistles plus a small amount of bramble. The transect route through
the paddock is only short being 165 metres.
The remaining four paddocks have a cumulative length of 555 metres and have
been taken as Habitat C. They have normally been grazed by sheep but the
southern only since 1993. As they are very over-run with Tor grass, grazing has
been concentrated on the autumn, (winter), spring periods to reduce litter and
to eat back the freshly emerging grass in order to allow the many herbs to
flourish and flower. For practical reasons this has not always been possible and
sometimes no grazing was done. Grazing has also occasionally been done in the
summer thereby reducing the nectar sources available at that time.
Details of work done along the bridleway and grazing in the paddocks are
given in Figure 1. The three habitat types represented can therefore be
considered as in Table 2.
Butterfly densities
The maximum number of imagines recorded on one weekly count was 669 on 25th
June, 2000 when 272 Gatekeepers, 150 Meadow browns, 137 Marbled whites plus
others were seen. This calculates out as one butterfly seen on average every 2.3
metres (7.4 feet) of the transect with one seen on average every 1.3 metres (4.4
feet) in the centre paddock. The lowest maximum weekly count was in 1987 when
only 208 were recorded.
Overall the average weekly count is calculated from 44,700 records in 26
weeks of 21 years giving 81.8 records or one butterfly every 18.6 metres (60.9
feet).
Variation by week
As might be expected, the numbers of butterflies recorded each week are much
less in the colder, darker days of spring and autumn than in mid-summer. Figure
2 shows the total count of all species each week over the period from Week 1 to
Week 26 for three cases. These are :- the minimum weekly count for any year, the
average weekly count for all years and the maximum weekly count for any year.
Each case is similar with significant numbers being recorded from week 13 (24th
June) to week 20 (18th August) only.
Variation by year
Table 3 lists the fifteen most abundant species and shows their variation in
numbers between 1982 (Season 1)and 2002 (Season 21). It is based on a best-fit
log line and gives the back-calculated trend number in both years and the
calculated compound rate of percentage increase or decrease per year. The trend
numbers have been calculated by using a straight best-fit line on the graph of
log(butterfly numbers) against Season and then converting the log intercepts at
Season 1 and Season 21 back to actual numbers for 1982 and 2002. Calculated
numbers are also given for the sum of all species and the percentage of resident
species more or less confined to the grassland (Common blue, Marbled white,
Meadow brown and Small heath).
Figures 3 to 5 show the detailed variation each year in numbers of the main
species, all species and percentage of grassland species. Overlaid on these are
the calculated best-fit log trend lines.
Variation by habitat
Over the recording period there does seem to have been a slight increase in
the proportion of butterflies recorded in Habitat C (grassland) at the expense
of Habitat A (bridleway). This can be seen in Figure 1 where five of the eight
species appear to show this trend. It has been caused by a decline in numbers on
the bridleway, an increase in the paddocks or a combination of both. The three
other species (Speckled wood, Ringlet and Gatekeeper) are more associated with
woodland or hedgerows and their distribution appears to have remained more
stable.
National variation
National trend data are taken from The Millennium Atlas of Butterflies in
Britain and Ireland (Jim Asher et al. 2001). It should be noted that these
trends are from the transects recorded for the Butterfly Monitoring Scheme. As
the majority of the sites monitored are nature reserves or other sites known to
be important for butterflies they are comparable to Windmill Hill but not
necessarily the wider countryside.
Species trends
(Figures for the variation in numbers of the most abundant species are given
in Table 3 and the year by year variations are shown in Figures 3 to 5).
1 - WALL
The largest variation over the recording period is for this species which
suffered a severe decline from 1983 to extinction in 1997. The cause of the
decline remains a mystery but is not confined to Windmill Hill. Large areas of
south-east and midland England have also been affected although since the late
1990's a slow recovery may have started.
2 - SMALL HEATH
Nationally, since 1976, this species has shown a steady decline its numbers
being reduced to about a quarter by 1999. Some sites over the country, including
Windmill Hill, appear to have suffered a much greater decline than others, the
reason for this is not known.
3 - LARGE WHITE
The main food plants of the Large white are cultivated Brassicas (sprouts,
cabbages, cauliflowers etc.). These have been, and to some extent still are,
grown on an industrial scale in the surrounding Vale of Evesham. Hence, numbers
on the reserve are more likely to be an indication of the quantity of local
crops and effectiveness of pesticides than of conditions on the reserve itself.
The significant decline probably is a local phenomenon reflecting a contraction
of the industry.
4 - COMMON BLUE
There has been a significant decline on the reserve in contrast to the
country as a whole where numbers have remained fairly constant. However, year by
year numbers show a similar pattern of peaks and troughs as the national
numbers. Hence, the reason for the decline may be due to a constant slow change
on the reserve, possibly the increase in hawthorn scrub and taller summer
vegetation.
5 - LARGE SKIPPER
The national figures indicate a drop in numbers by about a quarter which is
less than that at the reserve. The reduction at Windmill Hill seems concentrated
on the bridleway (Habitat A). Strimming along the edges during the summer in
recent years has severely reduced the coincidence of long and short grass which
is thought to be the favoured habitat of this species.
6 - PEACOCK
At the reserve the peaks and troughs and general increase in numbers mirror
the national data. As this butterfly is likely to be a casual visitor to the
reserve this is to be expected.
7 - MEADOW BROWN
The year by year variation in numbers nationally and on the reserve are
similar but a significant increase has occurred at Windmill Hill. This may be a
result of a change in management of the grassland from sporadic burnings prior
to 1979 to sheep grazing since. (It should be noted that worn specimens in
flight of Meadow brown and Ringlet are difficult to distinguish and consequently
some under/over recording may have occurred.)
8 - SMALL SKIPPER
This butterfly only occurs in small numbers at Windmill Hill and so the
trend is only approximate. Figure 1 shows that the decrease is almost totally
from Habitat A (the bridleway) since 1987. This would suggest that scrub
invasion of the long grass along the edges is the cause. Nationally the species
has increased over the recording period.
9 - GATEKEEPER
Year by year variation is similar nationally and on the reserve. However, in
the former the numbers have remained fairly constant but at Windmill Hill they
have increased. As this is basically a hedgerow species the increase may be a
reflection of the thickening and spread of scrub on the site.
10 - SMALL TORTOISESHELL
The reserve has had a significant decrease in numbers of this butterfly over
the 21 years despite a small increase nationally. However, the calculated rate
of decrease would be a lot less if the extremely high count of 1982 was
discounted. The year by year variation is somewhat similar to the national one.
As the species is probably not resident on the site, conditions in the
surrounding Vale may have had an effect.
11 - SMALL WHITE
Both the year by year variation and the small decrease in numbers are
similar at Windmill Hill and nationally. This is probably not surprising as it
is migratory and one of the commonest species in the wider countryside.
12 - GREEN-VEINED WHITE
As with the Small white there has been a very similar small decrease in
numbers at the reserve. It too is one of the commonest species in the wider
countryside though it is not migratory.
13 - RINGLET
Since 1982 the national numbers of this butterfly have remained remarkably
constant but during the 1990's its range has spread significantly. Consequently,
this may hide a slight decrease in long established sites during that decade and
be more consistent with the trend at the reserve. There is a considerable
difference in year by year variation, with Windmill Hill showing large swings
which are not reflected in the national data. This may be due in part to the
difficulty of identification (see note for Meadow brown). However, as its
preferred habitat is tall grass and scrub, variations in grazing and scrub
clearance may have had a significant effect on its year by year numbers.
14 - SPECKLED WOOD
This is basically a shade loving woodland species and mostly occurs along
the bridleway (Habitat A) particularly where this is bounded on both sides by
hedge and/or scrub. The variation in numbers reflects the national ones although
the general increase is not quite as marked. This may be because the hedge and
scrub have matured over the period and become more open in the bottom, hence
providing slightly less shelter. However, in the West Midlands generally the
numbers do not reflect the recent national increase (Neil Gregory 2003), but
show a slight decrease.
15 - MARBLED WHITE
Marbled white holds the record for the highest annual transect count
amounting to 995 individuals which occurred in 2000. Nationally it has had a
significant increase in numbers but with a marked increase in its range.
Consequently, it is difficult to relate that increase with the more modest
increase at the reserve. However, the two patterns of year by year variations
are similar.
16 - OTHER SPECIES
These occur in small numbers and so make calculation of a trend line
unreliable.
Outlook
Over the 21 years the number of butterflies has remained more or less stable
despite extinction of the Wall and near extinction of the Small heath due to
some unknown cause affecting these two species nationally. The effect of the
introduction of more concentrated management of the paddocks over the last few
years appears to be beneficial as the highest numbers ever recorded occurred in
the year 2000. Provided suitable management can be maintained the prospects look
good for this site to remain a place to see an abundance of butterflies in
mid-summer.
ACKNOWLEDGEMENTS
From 1983 to 1993 the recording of this transect was shared with Bob
Woodroofe who has kindly allowed his data to be used in compiling this article.
He has also given helpful comments on the draft of the article as have Neil and
Corinna Gregory.
REFERENCES
INSTITUTE OF TERRESTRIAL ECOLOGY 1981. Butterfly Monitoring Scheme -
Instructions for independent recorders, NERC
ASHER,J et al 2001. The Millennium Atlas of Butterflies in Britain and
Ireland, OUP
GREGORY N 2003. VC 37 (Worcestershire) Butterfly Transects Annual Review 2002, Worcestershire
Record 14 : 20.
Figure 1 giving details of work done along the bridleway and grazing in the
paddocks. information relating to the three habitat types are given in Table 2.
