In a 12-year study on Clethrionomys glareolus, Apodemus sylvaticus, and in less detail, on Microtus agrestis in an area of mixed woodland just south of Durham it has been found that a variety of mark-and-recapture line trapping has provided reliable indices of abundance, making it possible to follow the changes occurring in the populations of the three species. Parameters investigated included distribution, density, sexual condition, sex ratio, mortality, recruitment, growth and variation in pressure on food supplies.

The abundance of Clethrionomys has varied cyclically, the periodicity being slightly under four years as previously recorded in the microtine rodents of tundra and grassland habitats. There has been evidence of abnormalities of the generations living during a crash similar to those reported in Microtus agrestis during this phase of a population cycle. Apodemus in contrast has had only a well-defined annual fluctuation in abundance, the rapid decline observed in its population density each spring perhaps being caused by an intensification of territorial behaviour at the beginning of each breeding season. lntraspecific competition also appears to account for male Apodemus, which grow more quickly and to a larger size than females, being more abundant than the latter in most years. After exceptionally favourable seasons, females are as numerous as males, apparently as a result of a temporary reduction of such competition. Microtus, for which the area of study provided a marginal habitat, showed less regular fluctuations in abundance than have been found to occur in this species on moorlands.

It is confirmed that Clethrionomys and Apodemus are important in woodland ecosystems as agents transporting, burying and eating tree-seed, and damaging or killing the seedlings. Their influence on regeneration of forest is complex and calls for detailed study.

The pressure exerted by these two species on token supplies of seed which were offered, was greatest in winter and least in late summer. Appetite and stockpiling activity were increased relative to rodent abundance, under crowded conditions. The significance of food shortage and of social stimulation respectively as possible causes of these increases are discussed.