Rodents-Predators relations - Abundance - from 1989 to 2018

(1) Predator data collection were performed using night and day roadside counts. Each sampling event consisted of driving a car with 4 people (the driver, a data recorder, and two observers) along a fixed track at less than 20 km/h. The length of the track was 18.6 km from 1999 to 2009 and then 19.6 km due to a slight variation in the itinerary (trail blocked by mud, see Figure 1). Observations were performed using 100-W spotlights at night and binoculars for species identification.
(2) Small mammals relative abundance was assessed using a transect method; a 5-m-wide transect across the study area was divided into 10-m-long intervals and the proportion of intervals positive for fresh indices (tumuli, molehill, runway, feces, cut grass in holes) was considered an index of abundance. The total transect length was 11.6 km. Sampling was carried out once a year in April 2007 and then in August from 2008 to 2010, followed by at least twice a year generally in spring and autumn from 2011 to 2018.
(3) Prey assessement at the commune-scale was done by a local organism, in 7 communes crossed by the roadside count itinerary; using a ranking system that ranges from 0 to 5. 0—no A. amphibius sign in any parcel within the commune; 1—low or no A. amphibius tumuli, voles and moles (T. europea) cohabiting the same tunnel systems; 2—A. amphibius tumuli present in some parcels within the commune and mole burrow systems still present in some parcels; 3—A. amphibius tumuli present in some parcels within the commune, few, or no mole burrow systems present in the commune; 4—A. amphibius colonies established in the majority of meadows and within pastures; 5—all of the commune colonized by A. amphibius.
The index not directly translates to transect-based indices, partly because it is applied at the commune-scale and not the parcel scale, but Giraudoux et al. (1995) found that levels 0–1 correspond to densities < 100 voles.ha−1, level 2 to 100–200 voles.ha−1, and levels 3–5 to >200 voles.ha−1. For a given year, the median score of the 7 communes was taken as a score of abundance.