Snowshoe Hare Population Response to Artificial High Densities

Abstract

Demographic response of snowshoe hare (Lepus americanus) populations to artificial high densities was studied during summer and autumn near Rochester, Alberta. Experimental populations were established by transplanting hares onto islands in lakes in May and June. High-density populations on Dapp Island (15.4 hectares) in 1970 and Birch Island (3.6 hectares) in 1971 were adversely affected by food shortages. Rates of pregnancy and adult survival rates in these high-density populations did not differ from unmanipulated natural populations near Rochester, but juvenile survival was markedly lower. In 1972, a fence was constructed across Birch Island to separate two introduced high-density populations. One was supplementally fed commercial rabbit pellets ad libitum and the other was forced to subsist on an insufficient quantity of native vegetation. Again, survival among adults was comparable in the fed and unfed high-density island populations and unmanipulated populations at lower densities. Pregnancy rates were lower and no young were found in the unfed island population. In comparisons with low-density Rochester populations, no adverse demographic responses such as reduced pregnancy or higher juvenile mortality was recorded in the fed high-density population. The high disappearance rate of juveniles from all high-density populations with limited food appeared to have been directly linked to food scarcity which affected young more severely than adults. We could find no evidence that either parasitic or viral diseases, or social stresses, as indexed by incidence of scarring and adrenal weights, were acting as direct mortality factors in the food-limited high-density experimental populations.