Response of an ecological indicator to landscape composition and structure: Implications for functional units of temperate rainforest ecosystems

We evaluated the northern flying squirrel (Glaucomys sabrinus griseifrons), a known associate of key habitat features and processes of old-growth forest, for its capacity as a broad-scale ecological indicator of temperate rainforest ecosystem condition in southeastern Alaska, USA. We utilized a spatially explicit, resource-selection function to evaluate its distribution relative to landscape composition and structure at local (within home-range) and broad (home-range selection) spatial scales, followed by a moving-window analysis to model patch occupancy across this landscape. We found strong support for the influence of type, size, and compositional elements: large, old-growth patches were selected at both spatial scales, and regenerating forest patches ≤40 yrs old were selected against at the broader scale. More importantly, we found that occupancy was related to critical thresholds in composition: patches required ≥73% old-growth forest cover or a minimum total area of 73 ha of old-growth forest to be occupied by flying squirrels. A non-uniform pattern of selection for patches with higher structural connectivity at the local scale, but not at the broader scale, was likely a result of the “empty forest” phenomenon, in which remnant patches were inaccessible in this fragmented landscape. These results are consistent with recent studies of this and related species and suggest that occurrence of northern flying squirrels in southeastern Alaska is influenced by a number of landscape structure and compositional variables that relate critically to late-seral forest conditions. These findings shed new light on the utility of this species as an ecological indicator and its functional relevance to the resilience of fragmented forest ecosystems.