Population Biology of Snowshoe Hares. Ii. Interactions with Winter Food Plants

i (1) We investigated interactions between snowshoe hares (Lepus americanus) and their food plants in winter during a cyclic fluctuation in numbers at Kluane, Yukon, between 1977 and 1985. • (2) The winter diet of hares at Kluane was dominated by four species of shrubs and trees. Betula glandulosa, the most preferred species, was common on only four of nine study plots. Salix glauca was eaten most often in mid-winter, when Betula was covered by snow. Where Betula was absent, Salix was eaten at or above the level expected from its relative abundance. Picea glauca was generally not preferred, but was eaten, especially where Betula was absent. Shepherdia canadensis was eaten fourth most often. (3) By the peak of the cycle in the winter of 1981-82, hares had removed much of the biomass of small twigs of Betula glandulosa and Salix glauca from three study plots. Hares had little effect on the biomass of Pice a glauca twigs. The biomass of these three species on two control plots at the end of the 1981-82 winter was, however, still sufficient to support the hares residing there. A larger population of hares on a third plot with added food depleted their supply of natural foods more severely. ( 4) An analysis of the fates of tagged twigs showed that hares browsed an average of 63% of Betula glandulosa twigs, 26% of Salix glauca twigs, 20% of Shepherdia twigs, and 14% of Picea glauca twigs in the winter during the 3 years of peak density. At the peak in hare numbers in 1981-82, over 80% of Betula twigs were eaten. Twigs within 50 em of the ground were browsed most often. (5) Heavily-browsed Betula glandulosa bushes grew new twigs rapidly after hare numbers had declined. Salix glauca showed less terminal regrowth, but produced stump sprouts, which we did not measure. Picea glauca and Shepherdia were not heavily browsed, nor did they grow faster after browsing by hares. (6) Adult hares became heavier in spring as numbers rose, and then lighter as their numbers declined. The latter effect was absent on plots with added food. Growth rates of juvenile hares in summer and autumn declined as hare numbers rose, but did so more slowly on two plots where food was added. Juveniles lost mass in winters of peak density, but did not do so on two plots with added food. The growth rates of juvenile hares recovered within 2 years of the decline. (7) Our results suggest that snowshoe hares at Kluane did not experience an absolute food shortage in winter at peak densities. Also, radio telemetry studies at Kluane showed that most hares died of predation, not starvation. We suggest that a relative food shortage