DOES DENSITY REFLECT HABITAT QUALITY FOR NORTH AMERICAN RED SQUIRRELS DURING A SPRUCE-CONE FAILURE?

Abstract

Predicting animal populations over time often is done using models of density–habitat relationships that assume animal density is a reflection of habitat quality. We explored whether this assumption was true for the North American red squirrel (Tamiasciurus hudsonicus), a species present at different densities in 3 conifer habitats: white spruce (Picea glauca; high resource quality, unstable availability), mixed conifer, and lodgepole pine (Pinus contorta; lower resource quality, stable availability). We documented density, body condition, survival, reproduction, and turnover in these 3 habitats during a failure of spruce-cone crop, when the potential for the relative importance of habitats to change was greatest. During this time squirrel densities in white spruce decreased by 66% to match with those found in pine and mixed-conifer forests. Red squirrels in spruce forests experienced lower survival and fewer females successfully weaned young, and juvenile production was lower. Adult and juvenile immigration was more important than local juvenile production in replacing squirrel mortality in spruce and mixed-conifer forests. Our results indicate that density does not always reflect habitat quality for red squirrels, and we question the historical high-quality rating of white spruce habitat for this species. Further, our findings suggest that movement of ostensibly highly territorial adults in late winter and early spring is an important mechanism in determining annual squirrel densities regardless of habitat type.