The population ecology of small rodents in Pennine woodlands

Abstract

Apodemus sylvaticus and Clethrionomys glareolus populations were studed in two mixed woodlands in the Pennine Hills; one was at the altitude of 400 m (Malham) and the other at 160 m (Coniston Cold). The former was studied for three years (1977–80) and the latter for one year (1979–80). At Malham 703 rodents were caught in the live trapping programme representing 4219 captures in 37,108 trap nights; the corresponding figures for Coniston were 259 animals, 1165 captures and 5400 trap nights. At Malham, both species were relatively numerous through spring and summer of 1977 (Apodemus 10–30/ha, Clethrionomys 6–23/ha). In later years, Apodemus had troughs from March to June (0.5–8.0/ha) and peaks from September to November (13–24/ha); Clethrionomys remained at low densities throughout this period with only, in two months, populations attaining more than 6/ha. At Coniston, Apodemus attained a density of 38.6/ha in October and Cleihrionomys 32.7/ha in August. Monthly sex ratios did not significantly differ from unity. Length of residence of Apodemus first caught (at Malham) in March-June and October-December did not exceed six and 16 months, respectively. Clethrionomys resided up to one year although over 80% disappeared by three months. Adult mice and voles increased their mean weight from late winter to reach maxima between April and June. The lightest animals were found from October to December. Most Juvenile mice were born between April and October. Testes were small from October to December, increased in size in January and were descended from March to October. At Malham, the months when females were pregnant varied annually, viz. 1977, April-November; 1978, March-September; 1979, May-October. At Coniston breeding started earlier. Clethrionomys at Malham had a short breeding season with no pregnancies observed before June, and the first juveniles typically occurring in July (the exception being when breeding probably occurred in the winter of 1976–1977). Breeding terminated in September-October. At Coniston it extended from April to October. Annual net production of the two species combined ranged from 9.62 MJ/ha/year to 5.02 MJ/ha/year at Malham. The Coniston figure was 17.62 MJ/ha/year. The results are discussed in relation to other woodland populations where climate is less harsh and the growing season appreciably longer.