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We measured the density changes of 22 species of vertebrates during a snowshoe cycle in northern Canada. Hares were the dominant herbivore in the system and changes in their numbers were correlated with changes in numbers of arctic ground squirrel, spruce grouse, ptarmigan, lynx, coyote, great horned owl, goshawk, raven and hawk owl. Hare numbers were not correlated with numbers of red-backed vole which showed peaks during the low, increase, and early decline phases of the hare cycle. Hawk owls were the only predator whose numbers correlated with changes in red-backed voles while boreal owls and weasels were correlated with densities of Microtus. Red squirrel, American kestrel, red-tailed hawk, northern harrier, wolverine, magpie, and gray jay showed no correlation with hare or vole numbers. We conclude that species in the boreal forests of Canada do not exhibit the strong synchrony found between voles and other members of the vertebrate community in northern Fennoscandia. We discuss some of the possible reasons for these differences.

We examined preferences of snowshoe hares for twigs of 2 winter forage plants: chemically defended balsam poplar Populus balsamifera and comparatively less defended gray willow Salix glauca. We used an optimal foraging model that explicitly incorporates the effects of secondary plant chemicals to predict foraging preferences of hares. The model predicted that the foraging preferences should be conditional: preference should depend on the ratio of digestible nutrient content in the defended and comparativeIy less defended plants relative to the ratio of the wet mass/dry mass of defended and comparatively less defended plants

The response of different salt-marsh plant communities to grazing by lesser snow geese and isostatic uplift was examined at La Perouse Bay, Manitoba on the Hudson Bay coast. Results are based on direct experimental manipulation of swards and previously published floristic and biomass data. At the seaward end of the salt-marsh the vegetation is composed of Puccinellia phryganodes and Carex subspathacea. This assemblage of species is also present in the upper marsh, but only as a consequence of intense grazing by geese. Grazed swards of these species persist over a period of at least 10 years (...)

SUMMARY (1) The effects of the frequency and timing of multiple grazings (including faecal input) by captive goslings of the lesser snow goose on net above-ground primary production (NAPP) and shoot nitrogen content of swards of the forage grass, Puccinellia phryganodes were examined. The effects of clipping and addition of nitrogen on the growth in pots of individual tillers of this grass were also investigated in a factorial experiment designed to separate the effects of each treatment. The experimental design of both investigations was closely based on the foraging behaviour of wild geese on Puccinellia swards. (2) The clipping of leaves per se which mimicked the leaf demography of shoots grazed by wild birds had a detrimental effect on shoot growth and NAPP. Addition of nutrients ameliorated the adverse effects of clipping on shoot growth. Plants which received nutrients, but whose leaves were not clipped, produced the highest amount of above-ground biomass. (3) During the summer of 1987 plots of Puccinellia were either grazed once every 12 days (one to six occasions), or once every 24 days (two or three occasions), or on three occasions at intervals of 12 days late in the season after the wild geese had left the marsh. Plots grazed on three occasions at intervals of 12 days from late June to early August during the period of rapid growth of above-ground biomass most closely mimicked the foraging behaviour of the wild geese. (4) Seasonal cumulative NAPP in 1987 was significantly higher in plots grazed on three occasions than in ungrazed plots, plots grazed once, or plots grazed repeatedly throughout the season. In 1988 plots were not grazed and a similar trend in NAPP was obtained, except that the NAPP of plots grazed late in the season in 1987 was low. The departure of wild geese in early August may be necessary if swards are to recover from the effects of defoliation. (5) During the summer of 1987, forage quality (N content) and NAPP together were highest in plots grazed on three occasions at intervals of 12 days - the treatment closest to the foraging pattern of wild birds. In July of the following summer shoot nitrogen content did not differ among the exclosed plots, irrespective of the treatment in 1987, and all values were lower than that for shoots from adjacent swards which were grazed and fertilized by wild geese. (6) The results show that NAPP and the nutritional quality of forage are de

The effects of grazing by captive goslings of the lesser snow goose on the vegetation of the La Perouse Bay salt marsh were investigated. On seven separate occasions during the summers of 1986 and 1987 goslings fed on different swards of Puccinellia phryganodes for up to 150 min. Net above-ground primary production and forage quality (amounts of nitrogen and carbon in tissues) of vegetation were measured in grazed and ungrazed plots.