Patrons de reproduction des femelles d’isard (Rupicapra pyrenaica pyrenaica) dans une population non chassée et conséquences démographiques

Abstract

From the long-term monitoring of isard females ( Rupicapra pyrenaica pyrenaica Bonaparte, 1845) marked in two areas of the Parc National des Pyrénées (France) with a quite constant population size for 20 years, we have conducted an analysis of age variation in recruitment (estimated as the proportion of offspring surviving through the winter per female). As predicted for a well-established high-density population, recruitment rates were low for young females (0.156 at 3 years of age and 0.221 at 4 years of age), but also for prime-aged females (0.414 per year for 5–16 year old females). No female older than 16 that we monitored recruited in any year. Recruitment rates were highly variable among females, among years (0.183 in 2002 vs. 0.635 in 1996 for prime-age females), and among areas (0.562 kid at Mayouret vs. 0.359 at Péguère). The use of an age-structured demographic model allowed us then to compare population dynamics between the well-established isard population in the Park and the colonizing population of isards at Bazès, in the eastern part of the Pyrénées. As predicted, the natural rate of increase of the isard population was much higher at Bazès (1.262) than in the Park (1.037). Likewise, the age structure of isard in the Park markedly differed from that obtained at Bazès (50% of females were younger than 5 years of age, and 19% were older than 10 years of age in the Park versus 67% and only 3.6%, respectively, at Bazès). Such differences in both population growth and age structure between isard populations with contrasted demographic regimes were well illustrated by marked differences in generation time (4.97 years at Bazès vs. 8.25 years in the Park). Our comparative study therefore demonstrates that generation time can vary markedly in relation to the demographic status within a given species.