Limiting factors for northern flying squirrels (Glaucomys sabrinus) in the Pacific Northwest: A spatio-temporal analysis

Northern flying squirrels may be important indicators of the long-term health and management of Pacific Northwest forests, including development of successful regional strategies for managing northern spotted owl habitat under the Northwest Forest Plan. I used live-trapping, radio-telemetry, and within-stand measures of squirrel habitat to (1) assess mid-term effects of variable-density thinning on squirrel populations in young, managed forest, (2) quantify multi-dimensional space use by individual squirrels, (3) contrast squirrel populations across 33 natural and managed stands that were representative of wide gradients in within-stand structural complexity found in Pacific Northwest forests, and (4) test hypotheses evaluating the relative importance of four primary limiting factors for flying squirrel populations---food, competition, predation, and dens. Squirrel populations remained low 12 years after stands were treated with variable-density thinning. At the same time, high-density populations were found elsewhere across western Washington. Most space use by individual squirrels occurred above the forest floor and there were marked differences in movement patterns and behaviors between the breeding and non-breeding seasons and between forest supporting low- and high-density squirrel populations. Forests that supported high squirrel abundances generally exhibited high amounts of multi-dimensional structure in the midstory and overstory layers, low to moderate amounts of understory, and few canopy gaps. Three variables, variance in overstory tree d.b.h., area intercept at 10-m above ground, and amount of canopy gaps &ge 100m2 could correctly classify 97% of the stands as supporting either high or low squirrel abundances. The structural complexity of a forest and how individual structural components are apportioned within multi-dimensional space may determine the capacity of a forest to support abundant squirrel populations. A predation-structural complexity hypothesis is consistent with results reported from this and several past studies in the region and suggests that predation may be a primary limiting factor for squirrels, with food, dens, and competition playing hierarchically less important roles in most regional forests. Efforts to restore structural complexity across a dynamic forest landscape may need to consider the spatial and temporal permeability of forests for flying squirrels, especially before long-term ecological benefits of management activities like variable-density thinning are realized.Key Words: northern flying squirrel, northern spotted owl, Strix occidentalis caurina, Pacific Northwest, radio telemetry, forest structure, livetrapping, variable-density thinning, structural complexity, landscape permeability, predator-prey ecology.