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Other relevant issues are:
Some rarities are found in just one or a few sites in the area. Examples include numerous plants (e.g. some orchids), and many species of invertebrate (other types of insect, spiders, snails etc).
Good butterfly habitat supports our local biodiversity, and conservation measures need to take other wildlife interest into account.
Other changes in land use can also destroy butterfly habitat, but indirect destruction through lack of management is probably a greater threat.
Conversion of surrounding fields from pasture to arable crops has removed
the grazing from some of the remaining grassland sites. Formerly short turf
supporting several scarce butterfly species is soon replaced by long turf
supporting less diversity. Hawthorn or gorse scrub gradually spreads over
the site, and eventually the open grassland is completely lost.
The spread of scrub is illustrated by these two photographs, where both sides of a small valley used to be grazed in the 1970s and the grassland was rich in flowers and butterflies. At some point the grazing of one side was stopped or very much reduced, and the later photograph shows the increased extent of scrub cover as a result. The upper photograph was taken in 1976, the lower one in 1994.
|This hawthorn thicket has covered an area of grassland which used to support a Duke of Burgundy colony. The colony had survived for many years in a small clearing with plenty of cowslips, but eventually it died out.|
The Pearl-bordered Fritillary used to occur in many Cotswold woodlands. It has now gone from most of them, and whether or not lack of management was the original reason for this, many former sites are no longer suitable for the species.
One strong Small Blue colony was centred on part of a grassland field, and its eggs could be found on several much smaller patches of kidney vetch in the surrounding area, including two roadside embankments. Most of the field was ploughed up and although the main area of kidney vetch remained, it was then grazed much more heavily than in the past. The main Small Blue colony seems to have died out, and it has also not been recorded on most of the smaller sites since then.
One large area of unimproved grassland is managed mainly for its botanical interest, and grazed heavily. It does have quite a good range of grassland butterfly species, but populations are much lower than on smaller lightly-grazed sites. It too has lost its former colony of Small Blue.
Heavy grazing in woodland can have a similar effect, and parts of the Forest of Dean are not as good for butterflies as one might expect.
It is known that as the Holly Blue population builds up to a peak over several years, so too does the population of a parasite which eventually causes a crash in the populations of both. The cycle then repeats.
Particularly wet or cold summers can reduce breeding success, leading to population declines and local extinctions.
Particularly dry summers can also be very damaging, with larval foodplants withering and nectar sources drying up quickly. What was perhaps the best Duke of Burgundy colony in the area became just another small colony after a drought, although the introduction of grazing to the site probably also made its cowslip foodplants more vulnerable to drying out during the summer.