Red Squirrel Population Dynamics I. The Effect of Supplemental Food on Demography

(1) We examined the effect of food availability on the population dynamics of red squirrels. A temporary, ad libitum food addition was conducted during two summers in Douglas fir (low population density) and white spruce (high population density) forest habitats from 1985 to 1988 in south-central British Columbia, Canada. (2) The addition of supplemental food resulted in a strong increase in recruitment, primarily by juvenile immigrants in autumn. Increased recruitment caused a fourfold increase in population density in fir habitat and a twofold increase in spruce habitat. Both recruitment to the food-supplemented grids and the decline in density following the removal of food were density-dependent. (3) Supplemental food had no clear effect on juvenile or adult weights, juvenile growth rates, juvenile or adult survival, reproductive parameters or sex ratios. (4) The difference in red squirrel population density in the two habitats was due to higher average food availability in white spruce habitat. The availability of excess food during the food addition period indicated that factors other than food availability limited the population at high density.