Small rodents, their habitats, and the effects of flooding at Wicken Fen, Cambridgeshire



The habitat preferences of small rodents were studied by live trapping methods at Wicken Fen, Cambridgeshire, a relict area of East Anglian fenland. The study sites were chosen to cover areas of managed fields, dominated by sedge (Cladium mwiscus) or by litter (grasses with sedges); the adjacent fen carr “bush growth”; and an area of unmanaged sedge. Clethrionomys glareolus is the commonest rodent species, occurring at relatively high densities, and closely associated with sedge growth. It is ubiquitous in unmanaged sedge but in managed sedge and litter field areas there is a marked preference for the field/fen carr transition where sedge growth is not cut. Thick sedge seems to afford the optimum habitat for this species probably because of the three-dimensional vegetation structure which provides dense ground cover with a natural thatch of dead leaves; this decays only slowly, even in succeeding fen carr. Sedge fields provide a less stable habitat and recolonization of a harvested field by Clethrionomys only began after eight months of sedge regrowth. Micromys minutus are caught only in sedge and litter. Apodemus sylvaticus seem to show no marked habitat preference but there are some data which suggest that high Clethrionomys densities can affect Apodemus distributions locally. Microtus agrestis prefers grassy patches within sedge fields; trap success points tending to be associated with the presence of Calamagrostis canescens. Microtus avoids fen carr and unmanaged sedge and is generally caught more often in litter fields than in sedge fields; however, this preference seems to disappear at the end of the Clethrionomys breeding season. During flooding, Clethrionomys found refuge in the dense sedge and sedge/carr transition habitats; prolonged flooding caused a decline in numbers of Clethrionomys and the elimination of Microtus, while the distribution of Apodemus was apparently unaffected.