Fates of disappearing individuals in fluctuating populations of Microtus townsendii

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Abstract
Fates of disappearing individuals were determined for two natural populations of Microtus townsendii by livetrapping of evacuated areas and marginal habitat, and by marking individuals with radioactive tags to locate carcasses. Two hundred and nineteen out of the 454 individuals that disappeared were radioactively tagged during this 3-year study conducted in the University of British Columbia Research Forest near Haney, B.C. Only 30 radioactive tags were recovered on the live-trapping areas. Most disappearances in this vole population must have been caused by dispersal or predation.Most disappearance from increase and peak populations was due to dispersal. Two summer periods of declining numbers were observed; during one such period many individuals' remains were found on the study areas, but during the other period of declining numbers few remains were found. There was no increase in dispersal rate as measured by the above techniques during either decline period. It is concluded that during the first brief period of declining numbers individuals were dying in their nests from physical factors associated with a severe summer drought and during the second period of declining numbers individuals were either moving long distances without going into traps or being taken by predators.