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The two grids each consist of fifteen trapping points about 20 m apart with five traps at each point, bringing the total to 150 traps. Trapping takes place twice a year, in the first week of June and the last week of November.
On each trapping session the traps are pre-baited with oats for 48 hours, then set. They are examined on the evening of the same day and on the following morning. In this manner, the diurnal bank voles and the nocturnal wood mice may be monitored successfully.
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A further day is included if time permits to test the recapture rate. Captured animals are identified, weighed, sexed, and their breeding condition and point of capture noted, then they are marked by a fur clip or permanent marker and released. The total number of individuals caught is used to give estimates of the numbers of each species. In summer the number of adults is used for analysis of population dynamics as juvenile numbers vary with the timing the onset of breeding and the date of trapping.
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Additionally a grassland site was included to assess field vole numbers and traps were also set around woodland buildings to catch small mammals for teaching purposes. In forest sites, where principally woodmice and bankvoles were anticipated, a 25 point square grid was laid out with 20m between points and 2 longworth traps at each. A similar trap grid was used on grassland, but with 8m between trap points to account for the smaller territory sizes of field voles. Traps were checked twice daily, in the early morning and evening, for 3 consecutive days. Upon first capture each animal was given a unique clip, thus it could be identified as a recapture at future encounters. All animals were identified to species, sexed, weighed and their status as adult or juvenile determined. Summer and autumn trapping was undertaken in each habitat type.
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Numbers of each species caught were used in a capture-mark-recapture analysis in order to estimate population abundance. |
The most striking finding from our trapping was the relatively low number of bank voles (Clethrionomys glareolus) seen in the ancient woodland habitat. Bank voles thrive best where the ground cover is thick, typically in mature ancient woodland, but due to excessive deer grazing in Wytham over the last few years this component of the ancient woodland habitat was lacking. Indeed, bank voles reached their highest abundance in the secondary woodland which included areas of more open canopy with nettles, brambles and bracken at ground level. Coniferous woodland also lacked bank voles, though this is more typical of the open groundcover found in conifer plantation. Woodmice, however, were found in larger numbers, though not as high as in ancient woodland, especially in October.
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