influence du climat et de la prédation sur l'utilisation de l'habitat et la ...

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this model as a starting point for mode! selection (Lebreton et al. 1992). As survival rates
were the parameters of interest in our study, we first mode lied sighting probabilities to have
more statistical power when modeling survival (Lebreton et al. 1992). We carried out model
selection following the parsimony principle based on Akaike infonnation criterion corrected
for small sample sizes (AI Cc, Burnham & Anderson 2002). We considered models that were
different by less th an two units of AICc to be competitive to explain the data (Burnham &
Anderson 2002). As it was impossible to distinguish mortality from pennanent emigration,
we refer to apparent rather th an absolute survival. However, we believe apparent survival is
close to absolute survival in our population because we only observed five marked
individuals in the regularly surveyed areas adjacent to our study site.
We first tested for the effects of sex and age on survival. lndeed, North American
porcupines are sexually dimorphic (male/female mass ratio in late summer = 1.22) and
mammalian populations are commonly age-structured (Charlesworth 1994), so that we
expected age and sex-differences in survival. Following Klvana et al. (2004), we then tested
for an effect of winter precipitation, snowfall, and spring temperature on survival rates. We
used standardized weather data and tested for direct and delayed (lags: 1-2 years) effects on
survival.
The percentage ofyearly variations in survival that is explained by an environmental
covariate was calculated following Schemper (1990):
2 Deviance(covariate) - Deviance(constant)
r =--------~------~--------~------~
Deviance(year) - Deviance(constant)
(2)
where covariate, year and constant refer to the models with covariate-dependent, timedependent
and constant survival rates.
Causes of mortality
We located dead porcupines during surveys (n = 13) or using radio-telemetry (n = 47).
Since we did not perfonn telemetry in winter 2001 and years 2005 and 2006 (Table 3.1), we
considered that sem"ching effOlt was insufficient those years (n = 4 carcasses found) and
excluded these periods from analyses. Three porcupines died during anaesthesia and three
died when they got hung by their collar in a tree. We excluded these unnatural deaths from
analyses. We therefore determined probable cause of mortality for 50 animais that died in
2000, 2002, 2003 and 2004. We classified carcasses as dead from starvation (not injured and