Monitoring the abundance of a canopy-dwelling butterfly species
Methods
The method developed to record S. w-album reported here was based on the fixed time observation
point method. The method was adapted to take into account the behaviour and ecology of canopy
dwelling lepidoptera.
An initial site survey was undertaken to identify potential sites of S. w-album colonisation. Sites
were identified on the basis of having conditions that could potentially support a S. w-album colony.
This was defined as presence of at least one mature Ulmus spp. (>10m high, >3m diameter) or a
localised cluster of at least 3 immature Ulmus spp (>3m high, any diameter). All sites used in this
study had higher numbers of trees than the minimum defined.
A detailed secondary vegetation survey of each site was made. Percentage ground cover of Ulmus
spp., other trees and shrubs as well as plants that are potential nectar sources of S. w-album were
estimated at the beginning of S. w-album's flight period. Potential nectar sources were noted as
Rubus fruticosus, Cirsium arvense, Ligustrum vulgare, Centaurea spp., Origanum vulgare, and
Senecio spp. according to Oates (1984) and Davies (1992). The number and species of Ulmus spp.
at each site were recorded, and the height of the tallest elm tree was measured using a clinometer.
The proportion of Ulmus spp. at each site suffering from Dutch Elm Disease was estimated. Site
boundaries was delineated by the canopy of Ulmus spp. together with other frees, shrubs and potential
nectar sources within or adjacent to the Ulmus canopy.
Seven monitoring sites at Langdon Nature Reserve were established, five of which had previously
been recorded as having 5. w-album present. Four out of the seven sites had incidences of Dutch
Elm Disease to varying degrees. Each of the seven designated sites (A to G) was monitored, by the
same observer, for S. w-album three times a week during the flight period (04th July to 15th August
1998) using a fixed time observation point method.
On arriving at each of the designated sites, the observer remained silent and stationary for a period
of at least a minute. A ten-minute observation of the site area, particularly the canopy layer, using
binoculars (8 x 50) was then made. Identified observations of S. w-album were recorded, and any
butterflies ncctaring or honeydewing were noted with associated nectar or honeydewing sources.
Whilst walking between sites on the S. w-album monitoring route potential nectar plants were scanned,
and any incidence of S. w-album were recorded, noting the distance from the nearest elm tree. Weather
conditions were recorded during each survey (Pollard et al. 1986). The entire survey was completed in
a period not exceeding 2.5 hours between 10.30 and 16.00 on all of the monitoring sessions.
The BMS transect method (Pollard 1977; Pollard et a!. 1986) was also used to record the abundance
of 5. w-album. The same route as that of the adapted method was followed, to enable a comparison
to be made between the number of S. w-album observations made using each of these methods. The
route was walked once a week during the flight period of S. w-album on the same day as and
immediately after one of the fixed time observation point surveys.
Statistical Analysis
The total number of S. w-album observations made per hour using the fixed time observation point
method and the transect method, on the dates when both survey methods were carried out, were
compared using the Mann-Whitney (7-Test (Neave & Worthington 1992).
Essex Naturalist (New Series) 18 (2001)
J 23