Population Biology of Snowshoe Hares. III. Nutrition, Plant Secondary Compounds and Food Limitation

Sinclair, A; Krebs, C; Smith, J; Boutin, S

(1) We studied the population dynamics and protein nutrition of snowshoe hares (Lepus americanus) in the Kluane Lake region, Yukon from 1977 to 1986. Plant secondary compounds in winter food plants were monitored from ether (resin) and methanol (phenolic) extracts. (2) Extracts of phenols and resins in concentrations normally found in natural food decreased protein digestibility of processed rabbit food by 40-80% in laboratory experiments. Extracts at half normal concentration did not affect digestibility of protein compared to controls. (3) Digestibility of dry matter of winter food plants was 40% in the more preferred species and 20% in less preferred species. (4) Faecal protein from wild hares was used as an index of mean food quality consumed by hares. Faecal protein was lower in winter than summer. In the winters of peak numbers (1978-82), some animals obtained food of a quality lower than necessary to maintain body weight, indicating food limitation. (5) On the food-supplemented experimental areas, protein indicated there was adequate food in all years except 1980. Since the hare population declined in these areas in 1982 in the presence of adequate food, we conclude that food limitation was not a necessary cause of the decline on the scale of these experiments. (6) Plant secondary compounds declined through the increase, peak and decline phases of the hare cycle. Phenols increased 2 years after hare numbers dropped due to new growth of juvenile shoots. These results are contrary to the hypothesis that fluctuations in secondary compounds cause the hare cycle. (7) The proportion of juvenile twigs in the available food, containing high amounts of secondary compounds, remained low during the decline and first year of low hare density. This is contrary to the plant structure hypothesis which predicts that juvenile twigs should be predominant in the decline and first 2-3 years of the low phase. (8) We suggest that predation is a necessary cause of the hare cycle and in some circumstances, it may be the only cause. The hare cycle is constant in period but variable in amplitude. Food limitation may occur as a consequence of high amplitude rather than a cause of it. (9) It remains to be tested whether predation can act alone or synergistically with food limitation on a larger scale.

Population Biology of Snowshoe Hares. III. Nutrition, Plant Secondary Compounds and Food Limitation

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