The success of an introduction operation can be measured by the increase of the number of places where the animal is settled. The graph on the right shows the number of new sites occupated by year and the cumulated number of sites. After a stage of acclimatization which extended up to 1990, it was noted that each year new sites were observed.
The representation of the cumulative total shows a regular increasing, leading to 86 referenced sites at the end of 2007 .
Descriptive Statistics of site number increasing
Period
Total Nb Sites
Mediane by year
Mean by year
Theoretic Number
increasing rate
Doubling Time
Before 1989
16
1
1.6 ± 2.08
10
1.5
0.46
From 1990 to 1999
28
3
2.8 ± 2.04
30
0.18
3.85
Since 2000
42
4.5
5.3 ± 4.95
36
0.12
5.78
2010 Forecasts
50
xxx
45
0.114
6.08
2020 Forecasts
70
xxx
65
0.074
9.36
Total Number of Sites : it represents the total number of new sites observed during the related period. Median by year : it represents the median number m of new sites observed by year during the related period. Mean by year : it represents the mean number of new sites by year for the related period. Mean is completed by ± the standard-error. Theoretic Number : it represents the theoretic number of new sites for the period, calculated by multiplying the median m by the number of years n : NT = m * n Increasing Rate : the increasing rate by period is calculated by substracting the number of sites S(t) of the starting year from the number of the last year S(t+n). The increase [S(t+n) - S(t)] is related to the whole number of known sites during the periode S(t) * n (with n = number of yearss) : TA = [S(t+n) - S(t)] / S(t) * n Doubling Time: is the number of years necessary to have twice more sites, in an hypothesis of linear increase. It is approximated by the formula TD = ln(2) / TA.
