THÈSE - Université de Franche-Comté

terrestris and T. europaea relative abundance changes were monitored on these parcels
between 2001 and 2004, inclusive. Relative abundance was estimated in early spring.
A questionnaire on ploughing practices in each parcel over the ten previous years (from 1992
to 2002) and for the last three years for the other variables, was given to farmers in 2003. A
multivariate analysis of the raw data led to the selection of 13 synthetic variables describing
farming practices. The value kept for each variable was the data mean over the last three years
(ten years for ploughing) (Table 1).
High density phase
For communes characterised by a population density score of 3, the effect of grazing and of
soil disturbance was specifically investigated. Groups of three parcels were selected to
investigate either grass or soil disturbances (Table 2). Information was recorded from farmers
for the year of study (2003). For each group, two levels of disturbances and a control parcel
were selected in the same area and in the same landscape context. Thirty groups were selected
for grazing types and 18 for soil work. The area of each parcel was 10 ha on average.
Relative abundance of A. terrestris and T. europaea was recorded as before in autumn 2003.
DATA ANALYSIS
At the beginning of the cycle, the population dynamic of A. terrestris was characterised in
each parcel based on the spring data for the relative abundance over time. Relative abundance
from 2001 to the year of maximal value was the only ones considered in this first step of the
analysis in order to study the increase phase of the A. terrestris cycle. Two variables
(maximum and delay) were then kept for further analysis. The maximum being the maximum
abundance value reached for each parcel and the delay being the time period (number of
months) between the start of the population increase in each parcel and in the corresponding
commune.
A principal component analysis on centred and normalised variables was used to represent the
correlation between farming practices variable and to select variables characterising the
relationship between parcels and their respective population dynamics (Legendre & Legendre
1998).
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