Habitats and abundance of butterflies on agricultural land at Writtle College
Discussion
Twenty-four species were recorded on the Writtle College estate over the five year mon itoring period.
Some species of note were recorded including A. agestis and L. megera which are Priority species
listed in The Regional Action Plan for Anglia. A. agestis is a species of chalk and limestone downland,
coastal dunes and cliffs with the larval foodplants Common Rock-rose Helianthemum nummularium
and Common Stork's-bill Erodium cicutarium typically present. A range expansion was experienced
by this species in the early to mid 1990s (Joy & Bourn 2000), being recorded in a wide variety of
habitats such as farmland where the larval foodplant could be crane's-bill species Geranium spp.
However, only a small colony is probably present on the College estate which maybe susceptible to
fluctuations in numbers.
A rapid decline was noted for L. megera in the early 1990s in Essex, being especially severe at
inland sites (Joy & Bourn 2000). The College estate had a small colony but none have been recorded
on the transect for two years, probably due to the lack of suitable dry, sparsely vegetated areas of
open grassland habitat.
The number of butterflies declined over the five year survey period, with these declines most noticeable
in the conservation areas on the estate, particularly in green lane and meadow habitats with other
more unfavourable butterfly habitats such as hedgerows and arable fields retaining stable populations.
Common species such as A. urticae, T. lineola, T. sylvestris and O. venata displayed declines in
abundance over the monitoring period. The declines of Thymelicus species were most severe in
their preferred meadow habitats. The possible causes of these declines could be wet and dull weather
in recent years or the loss of suitable nectar sources from the meadows especially for Thymelicus
species and O. venata.
One possible reason for the decrease in butterfly abundance may be the wetter weather in the later
years of monitoring stimulating the rapid growth of coarse grasses such as Cock's-foot Dactylis
glomerata and False Oat-grass Arrhenatherum elatius. Taller, ranker swards were particularly
evident in the College meadows with fine-leaved grass species such as Red Fescue Festuca rubra
and Common Bent Agrostis capillaris being rare. C. pamphilus may have been most affected by the
changing vegetation structure as its preferred sward height is 10 to 50 millimetres, with fine-leaved
grasses as major larval foodplants (Pollard 1991). Pollard (1988) states that rainfall, in addition to
the indirect effects on plant growth, may have direct harmful effects on butterflies especially to
larvae and pupae. Thus, for many species such as O. venata, heavy rain in April may be harmful to
the caterpillars, and a wet June may affect the chrysalis and emergence of adults.
However, some species such as P aegeria and /. io are becoming increasingly common. The gradual
recovery of P. aegeria in Britain (after declining in the 1800s) may have started in the 1920s (Thomas
& Lewington 1991) with the decline of coppicing stated as a possible cause for this range expansion.
On the College estate, this species increased in abundance in woodland and hedgerow habitats,
particularly in non-intervention areas.
The major habitats for butterflies on the College estate were the conservation areas which included
meadows, woodlands and green lanes. Green lanes can be refuges for butterflies in very unfavourable
arable habitats (Sparks et al. 1999) because of greater nectar and foodplant abundance, reduced
agrochemical impact and increased shelter. One section of green lane on the estate, however, suffers
from increasing recreational usage and inappropriate management. This section was mown in June,
2000 which significantly reduced butterfly abundance especially of M. jurtina.
Essex Naturalist (New Series) 18 (2001)
J37