THÈSE - Université de Franche-Comté

over a relatively long-term (four years) period. Indeed, Jacob et al., observed a direct negative
impact of the mowing by assessing the number of voles one month prior to and after mowing.
Indeed, mowing can generate temporary disturbances for voles (disturbance and mortality by
farming machinery, decrease of plant shelter and food supply) (Jacob 2003). However, the
high reproductive potential of the vole species enables populations to recover to pre-mowing
levels rapidly (Taitt & Krebs 1983; Edge, Wolff & Carey 1995). Furthermore, some authors
suggested that voles may relocate from disturbed areas to adjacent non-crop refuge (hedges,
tree rows, road verges, fence lines) (Rogers & Gorman 1995). In our study area, farmers
provide forage to cattle during winter months (October to March) and thus store hay. As a
result, they use grassland fertilisation to increase hay production. The number of annual
cuttings was positively correlated with the annual forage yield, which was also positively
correlated with organic nitrogen fertilisation (Fig. 3). In the latter case mowing could be an
indicator of higher grass production, and, thus, the real cause of vole higher relative
population abundance. Specifically, our study also revealed a significant positive correlation
between the amount of organic nitrogen fertiliser used annually and the early increase in A.
terrestris population in the parcels. On the one hand, this result could be explained by the
favourable effect of organic nitrogen on primary production, which could provide a better food
supply and shelter from predators to the vole population. Furthermore, repeated mowing is
favourable to plant biodiversity as it increases nutrient cycling (Fedoroff et al. 2005). On the
other hand, this result suggests some indirect positive effect of organic nitrogen on A.
terrestris relative abundance via T. europaea relative abundance during the low density phase
of A. terrestris. Indeed, higher organic fertilisation increases the abundance of earthworms
(Lopez-Hernandez et al. 2004), the principal food source for T. europaea (Edwards et al.
1999). Our results reflected the influence of T. europaea. These results are in agreement with
those of Delattre et al. (submitted), who found a significant positive correlation between T.
europaea relative abundance during A. terrestris low density phase and the abundance level
reached by A. terrestris populations during the increase phase.
THE HIGH DENSITY PHASE OF THE A. TERRESTRIS POPULATION CYCLE
In this study, we showed that A. terrestris relative abundance declined with increasing
intensity of grazing (Fig. 5a) or soil work (Fig. 5b). These two specific farming practices may
have similar consequences on population dynamics. The mere presence of cattle may affect
Thèse C. Morilhat 2005 107