No abstract

1989. Demographic responses of arctic hares (Lepus timidus) to experimental reductions of red foxes (Vulpes vulpes) and martens (Martes murtes). Can. J. Zool. 67: 658 -668. This study aimed, through a field experiment, to test predictions from two hypotheses: (i) predation is a factor limiting arctic hare (Lepus timidus) populations, and (ii) predation shifts to arctic hares when voles (Microtus and Clethrionomys) decline to scarcity. Our approach was to compare demographic characteristics of hare populations on two large islands (Rinon, 2350 ha, and Bergon, 1800 ha) in the northern Baltic, where foxes (Vulpes vulpes) and martens (Martes murtes) were alternately present in normal numbers or reduced by snaring and shooting over winter. Hare densities in March, as indexed by transect counts and estimated by Jolly -Seber analyses, averaged two to three times higher on each island during years with reduced numbers of foxes and martens. Rates of population increase on Rinon, and thus population trends, were correlated with annual survival of adults and yearlings, but more strongly with survival of juvenile hares from birth to March (indexed by recruitment). Survival estimates were incomplete for Bergon and could not be similarly analyzed. Hare survival rates were lowest on Rinon during the 3 years when vole populations had declined to cyclic lows, but adult and yearling survival dropped only slightly during the vole low that coincided with reduced fox and marten numbers. We conclude that fluctuations were imposed on ino on's hare population by the vole cycle's effect on predation rates, whereas on both islands major differences in population levels between groups of years were largely due to the number of foxes and martens present. Demographic differences between hare populations of Rinon and Bergon, apparently unrelated to predation, included dispersal rates, mean body weights of females, and mean first-litter conception dates. Densities, survival rates, and natality are compared with selected arctic hare populations elsewhere. MARCSTROM, V., KEITH, L. B., ENGREN, E., et CARY, J. R. 1989. Demographic responses of arctic hares (Lepus timidus) to experimental reductions of red foxes (Vulpes vulpes) and martens (Martes murtes). Can. J. Zool. 67 : 658-668. Une expCrience en nature a permis de mettre B 1'Cpreuve deux hypothkses : (i) la prkdation est un facteur limitant pour les populations de Likvres arctiques (Lepus timidus) et (ii) les likves deviennent l'objet de la prkdation lorsque les populations de campagnols (Microtus et Clethrionomys) sont dCcimCes. Nous avons comparC les caractCristiques dCmographiques des populations de likvres de deux grandes iles (Rinon, 2350 ha, et Bergon, 1800 ha) du nord de la Baltique oh les ranards (Vulpes vulpes) et les martres (Martes murtes) ont Ct C tour B tour prksents en nombres normaux et en nombres rCduits par le pikgeage et la chasse au tir durant l'hiver. La densit6 des likvres en mars, CvaluCe par des dknombrements le long de transects et par des analyses Jolly -Seber, Ctait ...