Natural limitation of the abundance of the high Arctic Svalbard reindeer
A field study of growth, body composition and the demography of Svalbard reindeer Rangifer tarandus platyrhynchus was carried out between April 1979 and Sept. 1984 in Adventdalen (750 km2) in Svalbard, 78oN lat., to determine the potential significance of (i) the food supply and (ii) the weather in determining the rate of increase and abundance of reindeer. The study was based on measuring (i) total numbers, annual rates of birth and mortality and (ii) the total dissectible fat content (TDF) of reindeer shot throughout the year. All reindeer and carcasses were counted on foot once each summer. The disarticulation and disappearance of 78 carcasses was followed for five years. Carcasses usually remained in situ, with at least 50% of bones present, for not less than two years after death. Radio-tracking showed that the reindeer were sedentary and used small (ca. 5 km2), traditional, seasonal home ranges. The population was stable but not constant: numbers fluctuated betwen 401 and 771 reindeer (2.7-5.5 reindeer per km2 productive ground). There was pronounced variation in annual rates of fecundity (9.0-73.3%), mortality (1.5-25.7% of the population) and dispersal (0-25.1% of the population). The annual rate of increase (r) fluctuated between +43 and -47%. Mortality (of calves and adults) accounted for 46&37 of total annual losses and was a major factor limiting numbers: the principal single cause of variation in r, however, was variation in the annual birth rate. It is suggested that reproductive failure was caused by resorption of foetuses induced by acute starvation. Ninety percent of all natural mortality occurred in the second half of winter; 83% of all deaths were due to starvation. Calves suffered higher mean rates of mortality than both males and females aged ≥1 yr (35.8, 15.8 and 9.3% per annum, respectively). Reindeer were fat in autumn (TDF = 17% total body weight in adult females) and lean in late winter but survival was probably influenced principally by the supply of food in winter rather than the extent of fattening in summer. A model of a reindeer's energy balance showed that despite large autumn reserves of fat and muscle, a non-pregnant, adult female would normally have to meet not less than 75&37 of her daily energy requirements in winter by feeding. Survival also appeared to be influenced by reindeers' ability to process food efficiently; the molariform teeth of reindeer which starved to death were more worn than those of animals of the same age which were shot. Winters with heavy mortality were invariably followed by low rates of calving in spring and neither the birth rate nor the rate of mortality was significantly density-dependent across years. Evidently effects of grazing on plant biomass in winter were overridden by the effects of random variation in the weather. It seems that food shortage in winter can arise in several ways: through low production of forage in summer, through reduced availability of forage due to snow and ice and through increased competition. There was no evidence that weather conditions were exceptional in years when numbers declined. It is suggested that periodic die-offs and subsequent low calving are an integral part of the ecology of Svalbard reindeer. The population may be susceptible to the effects of weather because it has reached equilibrium with the winterfood supply. This contrasts with a previous suggestion that severe winter weather might keep populations of Svalbard reindeer below ecological carrying capacity.