THÈSE - Université de Franche-Comté

wildlife by trampling, thus disturbing voles by soil vibrations. Ploughing and harrowing turn
over the first centimetres of soil at a depth of 30 and 5cm, respectively. These actions could
destroy vole galleries (Kopp 1993) and plant supply (Jacob 2003). The destruction of
vegetation shelter by grass browsing (Schmidt & Olsen 2003) or by soil work (White, Wilson
& Horskins 1998) exposes voles to predation and their ground habitat to dehydration by
increasing the surrounding temperature (Kopp 1993).
More specifically, trampling by cattle may also slow down soil prospecting by voles and plant
roots (vole principal food source) by compacting the soil. However, only intensive pastures
presented significant lower relative abundance of A. terrestris. This suggests that the intensity
and duration of this farming practice had a disturbing effect on voles (Jacob 2003). It may also
explain why we did not find a negative impact of grazing in the study at the low density phase
and the increase phase of the A. terrestris cycle. Indeed, the sample sites did not include
intensive pastures but only typical parcels under the forage production system.
Concerning soil work, machinery could have a direct lethal effect (Jacob & Hempel 2003). A
significant difference of A. terrestris relative abundance was detected between ploughed and
harrowed parcels. This suggests that the negative effects of soil work increased with the depth
of the practice. However, this study was conducted in the same year as soil work application
(Table 2) and soil turn over constituted a temporary disturbance. Indeed, it is important to
keep in mind that the negative effect of soil work could become positive after germination and
growth of the sown vegetation because the soil may become more prospectable for voles and
plants (Meylan & Höhn 1991). Jacob & Halle (2001) and Jacob (2003) thought that annual
elimination of voles by ploughing, must be followed by immigration from adjacent refuges to
recolonise the devastated area, thus suggesting the importance of long-term studies on the
impact of farming practices. This was also previously illustrated by the difference between the
short-term negative effects and the long-term positive effects of mowing. Further research is
now required to investigate the long-term effects of soil work, although initial work by
Giraudoux et al. (1997) showed that, at a regional scale, areas, which are composed of an
appreciable ratio of ploughed parcels to agricultural land (>15%), were less susceptible to
population outbreaks.
Thèse C. Morilhat 2005 108