Demographic processes in colonizing populations of isard (Rupicapra pyrenaica) and ibex (Capra ibex)

Abstract

Rates of increase, survival and reproduction were studied in two re-introduced populations of mountain ungulates, the Pyrenean chamois (also named isard, Rupicapra pyrenaica) and the ibex (Capra ibex) in the French Alps. The populations were monitored from 1983 to 1998 (ibex) or 1984 to 1999 (isard) using capture, marking and resighting of individuals. Population rates of increase were estimated during the colonizing phase from: (1) a time series of estimates of population size; (2) projection matrices based on rates of survival and fecundity. Both populations had exponential rates of increase, with annual multiplication rates of 1.28 for isard and 1.30 for ibex. Age of first reproduction was 2 years in both species, with 77% of females breeding at this age in isard and 43% in ibex. From 3 years of age onwards, 90% of isard and 87% of ibex females bred each year. Survival rates of adult females, estimated using capture–recapture models, equalled 0.94 for isard and 0.98 for ibex, whatever the age and year. The projection matrix led to a slightly lower asymptotic multiplication rate than the time series of population size. Despite adult body size of the ibex being about twice that of the isard, no difference was detected in the intrinsic rate of increase between the two species. This supported the hypotheses of homogeneous life-history traits among ungulates, varying little with body size.