JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology.Summary 1 Changes in demography and studies on physical condition of the Riviere George caribou Rangifer tarandus herd have suggested that its size may be primarily regulated by the amount of forage available on the summer range. 2 We therefore document the impact of grazing and trampling on composition and productivity of two plant communities, the shrub tundra and stands of dwarf birch, within this range. Ungrazed sites were rare, but four previously located small areas were used as control sites. 3 For the shrub tundra, the lichen mat was absent in grazed sites and ground previously occupied by lichens was either bare, covered by fragments of dead lichens and mosses or recolonized by early succession lichen species. Ground cover of shrubs not eaten by caribou was lower in grazed sites than in ungrazed sites, and coverage of graminoids, forage shrubs and forbs did not differ significantly between grazed and ungrazed sites. 4 In stands of dwarf birch grazed by caribou, ground cover and leaf biomass of Betula glandulosa was significantly lower than in ungrazed sites. 5 Productivity of forage plant species over the summer range was estimated at 22.5 g m-2 year-l in an ungrazed condition compared to 10.3 g m-2 year' when grazed. 6 At the landscape level, caribou have fragmented the distribution of their food resource by reducing biomass of shrub tundra and stands of dwarf birch to a very low level. 7 The serious negative impact of migratory ungulates on plant productivity of their summer range may be explained by characteristics of the vegetation and the high carrying capacity of winter compared to summer ranges. Significant factors related to the vegetation are its low resilience and productivity and the absence of a response of vascular plants following removal of lichens.

The range used for calving and for the first month of lactation by the Rivière George Caribou Herd (RGH), which peaked at over 600 000 individuals in the mid-eighties, showed signs of overgrazing, in contrast to that used by the adjacent Rivière aux Feuilles Herd. Density of females in the tundra habitat below 600 m asl averaged 11.2 animals/km2 on the overgrazed range in 1988, in comparison with ≈0.5/km2 on the other range. Inadequate foraging conditions during the first month of lactation caused complete exhaustion of fat reserves in females on the poor range; milk production was seemingly insufficient, as calves were lighter and grew at a slower rate than calves born on the better summer range or born in captivity from dams fed ad libitum. Moreover, females on the poor range were shorter and lighter than their counterparts on the good range, illustrating that priority goes to reproduction rather than to growth under suboptimal foraging conditions. Accretion of body fat and protein in RGH females was rapid in autumn, but individuals with a calf at yield accumulated smaller fat reserves than nonreproductive adult females. Condition did not affect ovulation. However, interruption of gestation was suspected in females that were unable to accumulate enough body reserves in autumn or early winter, judging from the progressive decline in the pregnancy rate and in the autumn cow:calf ratio observed for the RGH since 1984, concomitant with the levelling off of the herd. Summer nutrition seemingly regulated the RGH through a combination of decreased fecundity and survival.

The ranges of two large caribou herds, the Rivière aux Feuilles and Rivière George herds, were sampled in June and July 1988 to compare vegetation composition. Lichens occupied more than 50% of the ground cover at Rivière aux Feuilles, whereas mosses, bare soil, and graminoids prevailed at Rivière George. Shrubs were more abundant on Rivière aux Feuilles, but the difference was not significant. Plant cover was similar whether sampling sites were selected at random or based on the presence of caribou. Within a season, diets of lactating caribou determined fromanalysesof rumen contents reflected thedifferencesinvegetationcover. In June, lactatingfemales from theRivièreGeorge herd ate fewer lichens than those of the Rivière aux Feuilles, whereas in July their rumen contained fewer leaves of deciduous shrubs. In Rivière George caribou, graminoids compensated for the low lichen and leaf content in both collection periods; these animals exerted a high selection, particularly for lichens, which were 25 times less available but only 1.5-2 times less abundant in rumina. In July, fragments of CIadina sp. and leaves of Betula glundulosu were especially scarce in Rivière George samples. Protein contents of washed rumen samples were lower for both periods at Rivière George. This pattern of food selection by Rivière George caribou may be due both to long-term grazing of the summer range and to eco-climatic differences.

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ABSTRACT. Twenty females from the Rivikre George caribou herd were captured in April 1987 in northern Quebec and were held in a zoo in Quebec City. Until November 1989, they were kept in an enclosure with a male from the same herd and they were fed ad libitum with pelleted concentrates and hay, supplemented with fresh deciduous leaves in summer. Daily food consumption exhibited an annual cycle, peaking at over 100 g-kg-0.75 in summer and decreasing to CU. 70 g-kg-0.75 by late winter. Food consumption decreased at the end of the last summer, due perhaps to lower hay quality. Mean body mass of adult females increased from 90 kg upon arrival at the zoo to = 115 kg in the autumn of 1987, = 125 kg in September 1988, and then decreased to 113 kg in November 1989. Pregnancy rate increased from 65% in 1987 to 82% in 1989 for animals captured in the wild. Two females born in captivity in 1987 became pregnant as yearlings, while 1 of 3 yearling females ovulated in 1989. Mass of calves at birth was higher in 1988 and 1989 than in 1987, while the calving period advanced by two weeks in the last two years. Growth of calves in summer was unrelated to birth mass and was higher in 1987 and 1988 (450-490 g-d-1) than in 1989 (365 g-d-1). Male calves grew at a faster rate than females. Carcass composition, in terms of lipids, protein and water, did not differ much between calves and yearlings born in captivity and free-ranging animals collected in 1983-84. However, the mass of each component was much lower in free-ranging lactating females than in captive ones. All captive females that had ovulated before necropsy, including one yearling, had at least 7.2 kg of stored fat.