Monitoring the abundance of a canopy-dwelling butterfly species
There were two observations of S. w-album made between sites (i.e. where Ulmus spp. were not
present); both classified as nectaring behaviour on areas of high density R. fruticosus. The ranges of
flight from the nearest elm tree for these observations were 60 - 100m.
Discussion
The results obtained from this study illustrate that S. w-album is under-recorded when using the
transect method of the Butterfly Monitoring Scheme (Figure 2). The fixed time observation point
method used in this study provided a more comprehensive set of data concerning the abundance of
S. w-album at specific sites on a transect, and also provided further details of the habitat requirements
of this species.
Other canopy dwelling species are also probably under-recorded when using the transect method,
and therefore the fixed time observation point method could be adapted to use with other species. It
may be possible to monitor N. quercus by identifying sites with Quercus spp. and observing the
canopy of the trees using binoculars. Limitations to the fixed time observation point method do
exist. It is easier to identify S. w-album in canopies of smaller trees with sparse foliage than in taller
trees with dense canopies. It is also easier to sight S. w-album when it is nectaring, resulting in sites
with more nectar resources appearing to support a greater abundance of S. w-album. It has not yet
been established to what extent the species and structure of vegetation present influences the frequency
of S. w-album observation.
Davies (1986) used the mark recapture technique to obtain an indication of size of population of S.
w-album, sex ratios, data on behaviour, and an estimation of the life expectancy of individuals.
However, even though the mark recapture technique has the potential to acquire the most
comprehensive results, the technique is difficult to implement with canopy dwelling species. This is
mainly due to the difficulty in re-capture of butterflies flying in the tree canopy and therefore a
reliable indication of population size may not be calculated. Using this technique it would only be
possible to monitor annual fluctuations in the number of S. w-album that come down from the tree
canopy to nectar. It has also been suggested that monitoring the egg, larval and pupal stages, rather
than the adult, may be a more appropriate way of monitoring this butterfly (Oates 1984; Davies
1986). However, S. w-album eggs and larvae arc probably found on the lower branches of trees in
some years and in the upper levels in others, making it difficult to obtain quantitative data (Oates
1984). New et al. (1995) states that larval counts can only be accurately performed for caterpillars
on low vegetation, and consequently most estimates of abundance are generated from counts of
adult butterflies.
The fixed time observation point method used in this study takes into account S. w-album's sedentary
behaviour by monitoring only on sites consisting of Ulmus spp. The method used also included a
vegetation survey at each of the sites monitored. This provided valuable information on the
relationship between the vegetation present and the number of S. w-album observations made. The
results from this study show that the number of S. w-album observations made at Site B was far
greater than those made at any of the other sites in the study (Figure 1). The reasons for this could be
that there was no incidence of Dutch Elm Disease, a relatively high population of mature U.
carpinifolia, a high percentage cover of R. fruticosus, and it experiences sunny conditions for the
majority of the day (Table 1). However, the fact that Sites C and D were characterised by similar
conditions to Site B, except that they were in the shade for the majority of the day, suggests that
aspect is an important factor in determining the location of S. w-album colonies. Also, the stand of
R. fruticosus at Site B, on which many S. w-album were observed nectaring, faced a southerly
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Essex Naturalist (New Series) 18 (2001)