The comparative ecology of the two British species of the genus Apodemus (Rodentia, Muridae)
Apodemus sylvaticus and A. flavicollis are distinct but sympatric species in Britain and parts of north-western Europe. Much data exists on the ecology of A. sylvaticus in areas where A. flavicollis is absent. This has been summarised and compared with a new study of a mixed A. sylvaticus / A. flavicollis population in central Essex. In the presence of A. flavicollis the winter average population densities of A. sylvaticus were significantly lower than in single species populations. In other respects investigated (survival, breeding season, movements, habitat selection within woods, annual population cycle) A. sylvaticus ecology did not appear to be affected by the presence of A. flavicollis . A. flavicollis ecology differed from that of A. sylvaticus in the following respects: a. It was rarer---representing about 1/4 to 1/5 of the Apodemus population. b. A. flavicoilis was less selective of habitat within woods but less likely to be resident outside woodland habitats. c. Individuals were more mobile. d. The breeding season may be shorter. In Essex both species of Apodemus are widespread but A. flavicollis does not spread so far into urban areas as does A. sylvaticus nor is it equally common in all woodland areas. There is some evidence that the most favourable woods are those in close proximity to arable land. On a national basis the restricted range of A. flavicollis is difficult to explain but the species seems to show some tendency to favour lowland areas with a mixture of woodland and arable land. If this relationship is genuine then large edge effects must occur. It is suggested that A. flavicollis can avoid competitive exclusion by A. sylvaticus when habitat conditions allow a partial separation of breeding habitat by the two species, and that the two species were brought into competition by man's fragmentation and intermixing of woodland and more open habitats.