Small Rodent Populations and Food Supply in a Derbyshire Ashwood

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Abstract
The numbers of bank voles [Clethrionomys glareolus] and wood mice [Apodemus sylvaticus] on a 0.45 ha area of ash woodland in Lathkilldale, Derbyshire, England [UK] were studied by live-trapping methods from 1971-1977. Estimates of the annual mean numbers of ash fruit and seed and of the wt of edible seed falling were obtained. Estimates of the relative volumes of the food items eaten by the 2 rodent spp. in the wood were made during 1971. Analysis of bank vole diet showed that ash seed was eaten throughout the year, reaching a peak (84%) in Jan. and a trough (3%) in July. The diet of the only sample of wood mice (in Jan.) included ash seed but was artifially supplemented by barley. The annual fluctuations in the ash seed crop after 1971 showed further evidence of the biennial fruiting pattern described by Gardner (1977). The heavy seed production in 1973-1974 reached only 15.7% of the highest recorded value prior to 1971. The numbers of bank voles on the study area in Dec. varied between 39 and 43 except in 1974. Numbers of adult bank voles caught in June varied from 7-35. The numbers of wood mice in winter were lower and less stable than those of the bank vole and summer numbers were usually lower than those of the previous winter. In both species an increase in numbers from winter to summer occurred only in 1973-1974, probably as a result of immigration. The overwintering minimum survival (Dec.-June) of bank voles show significant positive correlations with mean ash seed-fall from Oct. to April. Overwintering success of males is better than that of females but survival is similar. Similar correlations for the total wood mouse population are not evident. There are positive correlations between both overwintering success and survival of female wood mice and mean ash seed-fall. The failure of the male wood mice to show the correlation with ash seed-fall is probably the result of the supplementary food attracting them from further afield than females. The reasons for suggesting a causal relationship betweenrodent numbers and ash seed-fall are discussed and consideration is given to the potential food (energy) supply provided by ash seed. A density-dependent mechanism apparently is responsible for the relatively constant numbers of bank voles caught each winter.