Monitoring the abundance of a canopy-dwelling butterfly species
Site B was characterised by mature Ulmus carpinifolia and no incidences of Dutch Elm Disease
were observed. The site faced a southerly direction and was in the sunshine for the majority of the
day. The site was also characterised by a large (approximately 20m x 3m x 2m) stand of Rubus
fruticosus that was covered with blossom during July. Rubus fruticosus was the only plant on which
S. w-album was observed nectaring; however, honeydewing was observed on Crataegus monogyna,
Quercus robur, Sambucus nigra, Ulmus carpinifolia and Ulmus procera. Sites C and D were
characterised by very similar conditions to those of Site B. However, a major difference between
these sites was that Sites C and D were both in the shade for the majority of the day (Table 1). Sites
A, E, F and G consisted of U. procera, which had Dutch Elm Disease to varying degrees and tallest
Ulmus trees that measured between 10.7m and 13.8m in height (Table 1). The Ulmus trees on Sites
B, C and D were taller, with the height of the tallest tree ranging from 18.3m to 21.5m (Table 1).
The total number of S. w-album observations recorded during the flight period on the twenty surveys
using the fixed time observation point method was one-hundred and seventy-two. A total of five S.
w-album observations were made during the seven surveys carried out using the transect method.
The number of observations of 5. w-album made per hour using the fixed time observation point
method (mean — 6.52, s.e. - 2.54, range 2.5 - 20.6), on the seven surveys earned out on the same
day as the transect method surveys (mean = 0.66, s.e. = 0.26, range 0 - 1.8), was significantly
greater (PO.01) than those recorded using the transect method.
The flight period of S. w-album was from 5th July 1998 to 13th August 1998 using the fixed time
observation point method and from IO11' July 1998 to 2nd August 1998 using the transect method
(Figure 2). The fixed time observation point method results show that the flight period was
characterised by a unimodal population structure (Figure 2). The highest number of S. w-album
observations using both methods were made on 25th July 1998: on that date there were 20.6 sightings
made per hour using the fixed time observation point method compared with only 1.8 sightings per
hour using the transect method (Figure 2). Satyrium w-album was observed on all of the fixed time
observation point surveys, but was observed only on 0.6 of the transect surveys.
Fig. 2. The total number (all sites) of Satyrium w-album observations made per hour using the
transect method and the fixed time observation point (FTOP) monitoring method
Essex Naturalist (New Series) 18 (2001)
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