Relative contribution of decreased productivity and survival to recent changes in the demographic trend of the Rivière George Caribou Herd

Abstract

The Rivière George caribou herd (RGCH) was numerically low during the middle part of the century but apparently erupted in the sixties and the seventies. Puberty was early and pregnancy rate was high among RGCH females from 1973 until the mid-eighties when fecundity decreased significantly, in particular among sub-adults; productivity remained low in 1992. Autumn calf:female ratios reflected this trend in pregnancy, exceeding 50 calves: 100 cows between 1973 and 1983, but dropping thereafter progressively to a low of 24 in 1992. In 1993, this ratio rebounded back to 42. Annual adult survival rate of radio-collared females was high (0.95) at the beginning of the monitoring in 1984, but exhibited a variable but declining trend until 1992. Simulations were conducted to estimate the demographic trend of the RGCH between 1984 and 1992, using annual survival rates of radio-collared animals and annual autumn calffemale ratios to estimate calf production. Age structure played a minor role in estimating the finite rate of increase (Lamda). According to the simulations, the RGCH increased in size until 1987, and showed a slight decrease thereafter. The herd should have decreased by 12-15% between 1988 and 1993, according to the simulations. Productivity first caused a decline in Lamda, but in recent years decreased survival contributed slightly more than productivity to the reduction in Lamda. Estimation of the herd size by means of aerial censuses in 1976, 1984, 1988 and 1993 suggested a similar pattern in demographic trend, differences being statistically meaningless. We speculated on the future of the RGCH, that could have erupted after many decades of unfavourable weather. The herd will exhibit a rapid descent to low numbers if wolves show a numerical response to current caribou abundance, or if lichen availability on the winter range decreases due to competition with the adjacent and increasing Rivière aux Feuilles herd; otherwise it will exhibit dampened oscillations, whose amplitude will depend on the time-lag of vegetation reaction to decrease grazing pressure on the summer range

Historical Changes and Current Distribution of Caribou, <em>Rangifer tarandus</em>, in Quebec

“…Changes in abundance suggested by Morneau and Payette (1998) corroborate population increases noted during aerial surveys conducted during the 1970s and the 1980s . Telemetry data and computer simulations realized by Crête et al (1996) suggest a slight decline in populations after 1986 which seems to concur with the observations of Morneau and Payette (1998) but to contradict the 1993 aerial survey and demographic data which suggest a reduction in the rate of increase rather than a decline Messier et al 1988). Considering the limits of simulations models, we cannot exclude that the scars left on tree roots could have diminished due to a change in migration routes.…”

“…Many studies have examined the abundance, population dynamics and habitat of the Barren-Ground Caribou in northern Quebec and of the Gaspésie Mountain Caribou. The spectacular increase in numbers of the first ecotype and the precarious status of the second have made them favourite subjects for limitation and regulation mechanism studies (Messier et al 1988;Crête and Desrosiers 1995;Couturier et al 1996;Crête et al 1996). Some studies have also been carried out on northern populations affected by hydroelectric developments (Brown et al 1986;Paré 1987) and on the isolated populations in southern Quebec (Vandal 1985;Jolicoeur 1993 * ;Paré and Brassard 1994*;Crête and Desrosiers 1995;Ouellet et al 1996).…”

Winter range drift in the George River Caribou Herd: a response to summer forage limitation?

“…The demographic consequences of sub¬ optimal summer foraging were falling pregnancy rates and decreased survival . Collectively, these traits indicate that summer nutri¬ tion is an important component of population regu¬ lation in the GRCH (Messier et al, 1988, Hearn et al, 1990Crête & Huot, 1993;Crête et al, 1996). Nonetheless, this study shows that George River caribou shift winter range use between years, and that range drift is associated with use intensity (below a use threshold), where more use presumably results in added forage depletion, and leads to subse¬ quent avoidance during the following winter.…”

What Happened to the Beverly Caribou Herd after 1994?

“…Previously, agespecific Beverly survival rates for females more than three years old were estimated as 83% from life table analysis (Thomas and Barry, 1990b) when the herd was considered stable. Similarly, adult cow survival rates exceeding 80% were required for stability in the Bathurst and George River herds (Crête et al, 1996).…”