Changes in macro-moth abundance on the Writtle College estate, 1968-1993
small areas of woodland and scrub in conservation areas, possesses a reasonable list of species.
The Rothamsted light trap is designed to be inefficient and Waring (1994) states that average catches
of moths are perhaps 8% of those from Robinson traps, with the possibility that many of the rarer
species on the College estate may have been missed. The position of a trap also has a significant
effect on the catch, with vegetation cover surrounding the nap an important factor due to the influence
upon the visibility of the light. The Rothamsted trap was positioned to the north of the main College
building in a predominantly short grassland habitat with scattered amenity trees to the north. The
trap was in an open area where visibility of the light would not be restricted, but favourable habitats
such as woodland and scrub were predominantly absent. The unsuitable position of the trap may
have restricted the moth fauna captured.
Numbers of moths did not decline significantly, whilst the number of species increased slightly,
especially towards the end of the monitoring period. This increase may be due to changing land-use
on the College estate since the 1970s (Figure 4). Since the mid 1970s many conservation areas have
been established, increasing the diversity of habitats and area of uncropped habitat.
Areas ofwoodland were created in the late 1970s, approximately 150 metres to the east of the light
trap, with a small copse being planted with various native trees including Ash Fraxinus excelsior,
Oak Quercus robur and White Willow Salix alba. The amenity trees planted to the north of the trap
in the 1970s were fairly mature by the late 1980s, providing similar conditions to woodland, with a
large variety of non-native and native species providing ideal moth habitat.
Moths numbers increase rapidly when conditions are favourable and experience severe declines
when conditions arc poor (Young 1997). Animals with this type of strategy arc called 'r-stratcgists'
and the 'boom and bust' population cycles of moths are evident from the 23 years of data collected
at Writtle.
Sixty-four species were only recorded once, with several uncommon species identified. Several
possible reasons could explain the large number of singular records;
● Species are day-flying such as Cinnabar Tyria jacobaeae.
● Species are not attracted to light
● Inefficiency of Rothamsted trap means many species may be infrequently recorded
● Fixed position of trap means sedentary species could be missed
● Species are wanderers from surrounding area
● Moths may be rare immigrants
● Species are rare or uncommon
Many of the singular records were of uncommon moths with 56% of species recorded once being
described as local, scarce or rare (Firmin et al. 1975). Several species which are now considered
priority species in the Regional Action Plan for Anglia were identified. Some have been recorded
relatively recently and may still be present on the College estate as suitable habitats remain (Table
3). Trapping will be conducted in suitable localities during 2001, complemented by extensive searches
for larvae.
Essex Naturalist (New Series) 18 (2001)
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