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

52
increase in winter precipitation led to a decrease in survival of porcupines. How winter
precipitation influenced winter survival of porcupines is examined below.
Mechanism linking winter precipitation to win ter survival
North American porcupines are short-limbed animais and presence of snow covering the
ground greatly reduces their mobility (Roze 1984, 1987). Snow increases probability of
starvation of porcupines (Sweitzer et al. 1997), possibly by increasing energy expenditure
(through elevated locomotion costs) and/or decreasing energy acquisition (through a limited
access to food resources). Snow may also increase porcupines' probability of death by
predation either directly by reducing their escape ability (Huggard 1993) or indirectly
because starved animais may be more susceptible to predators (Sweitzer 1996). Furthennore,
predators may show variable hunting efficiency (Gese et al. 1996b, Thibault & Ouellet 2005)
and/or may shift to more vulnerable prey (Patterson et al. 1998, Patterson & Messier 2000)
according to snow conditions.
We observed higher predation rates in years with high levels ofwinter precipitation. We
also found that predators killed adult porcupines almost exclusively when snow was covering
the ground. Either because adult porcupines were more vulnerable in presence of snow coyer,
because predators were more efficient, or because they shifted to hunt porcupines when snow
deepened, predation rates appeared strongly related to snow conditions in our system.
Fishers were the main predators of porcupines in our study area. Fishers are efficient
predators of the NOith American porcupine (Earle & Kramm 1982, Powell 1993) and their
number has been rapidly increasing in eastern Québec since the middle of the 1990s (Poulin
et al. 2006). They feed primarily on snowshoe hares wh en available (Powell 1993, Maltin
1994), but may weil switch to porcupines when snow deepens. lndeed, snowshoe hares are
light animais that are characterized by a low foot-load (Murray & Boutin 1991) so their
sinking depth in fresh snow should increase more slowly compared to fishers' and
porcupines'. ln deep snow, fishers may then be unable to chase hares and therefore switch to
preying on porcupines, which are a more risky (because of quills) but less energetically costly
prey to run after.
Because we did not monitor predator density, we are not sure wh ether higher predation
rates were due to an increase in predator density, to a functional response of predators to
increasing snow coyer, or both. However, because we found that winter precipitation was