Testing the effects of deer grazing on two woodland rodents, bankvoles and woodmice

Over the past 200 years, deer numbers have risen world-wide. The associated increase in browsing pressure and trampling often resulted in significant vegetation changes and associated conservation problems in other taxa. In this study we investigated the effects of increased deer browsing (0.4-1.5 deer/ha) on a temperate woodland ecosystem by comparing small mammal communities and vegetation characteristics in 4 deer-free exclosures with adjacent woodland transects subjected to deer grazing. Jolly-Seber analyses of Capture-Mark-Recapture (CMR) results revealed that the significant reduction of bushes and shrubs in deer-grazed woodland was associated with significantly lower rodent densities than in deer-free exclosures at all times. However, whereas bankvoles (Myodes glareolus) outnumbered woodmice (Apodemus sylvaticus) in exclosures by 4:1, on average there were 1.6 times as many woodmice in the open woodland than bankvoles. Population structure did not differ between animals caught in exclosures vs. open woodland in terms of body weight, reproductive status, age structure nor sex ratio in either species. Our results highlight the need of bankvoles for denser understorey as protection from predators, compared with the woodmouse's greater agility and resulting ability to escape predators in open woodland as well as the ability to exploit arboreal habitats. The study emphasises the necessity for effective and informed deer management in order to conserve forest ecosystems.