Northern Flying Squirrel (Glaucomys sabrinus)
The primary threats to this species are incompatible forest management practices, large high-severity wildfires, and climate change.
Forest management practices and large high-severity wildfires reduce canopy closure and large conifer and hardwood tree densities, reducing habitat quality and quantity. It has been shown that northern flying squirrel abundance is reduced by forestry practices that influence the structure or age of residual stands, both in the short and the longer term, and abundance has also been shown to be reduced in partially harvested areas compared to uncut stands. Prescribed fire may have short-term (less than 8 years) negative effects on abundance by reducing the frequency and biomass of their primary food source, truffles. Climate-driven changes in fire regimes are projected to include increases in fire frequency, area, and intensity which are expected to result in loss of late-seral habitat. Because northern flying squirrels are thought to be resource and dispersal limited these threats make small and isolated populations more vulnerable to local extinction.
The northern flying squirrel is a nocturnal arboreal squirrel occurring throughout the northern latitudes of the United States and Canada. There is considerable evidence that flying squirrels are more abundant in forests with an abundance of large live trees, large dead trees (snags), well-developed understories, and many large logs on the ground. In the Sierra Nevada, where conditions are relatively xeric (dry) compared to elsewhere within the species’ range, higher densities of flying squirrels are often found in relatively close proximity (less than 150 meters) to perennial streams. It is believed that the correlation between this species and perennial streams in the Sierra Nevada is due to higher soil moisture and other microclimate conditions in riparian areas that increases the abundance of the squirrels primary food source, truffles (fruiting body of subterranean fungi).
Mean home range sizes of approximately 9 to 29 hectares (22 to 72 acres) have been recorded in the Sierra Nevada, with individual home ranges overlapping both within and between sexes, suggesting individuals share foraging areas. On the Sierra National Forest in the central Sierra Nevada, these squirrels were found selecting taller and larger than expected diameter conifers (red firs in particular) and snags for nesting. On the Plumas National Forest in the northern Sierra, one study found that they selected hardwoods (black oaks) more often than expected compared to all other available trees and snags. Larger trees, snags, and hardwoods are more likely to have woodpecker or fungal cavities, or other decay features that allow for internal nesting, and thereby offer protection from predation and environmental extremes. They do not create their own cavities, relying on naturally created cavities or cavities created and abandoned by woodpeckers. Northern flying squirrels play several important roles in forest ecosystems. In addition to serving as a primary prey for several old forest dependent species, including California spotted owls, Pacific fishers, and American martens, flying squirrels eat, store, and distribute truffle spores throughout the forests they inhabit. Truffles are critical to forest health because these fungi form mycorrhizal relationships with trees, a symbiotic (mutually beneficial) relationship that provides the fungi with carbohydrates and the tree with otherwise unavailable water and nutrients from the soil. Not only do trees with mycorrhizal fungi grow larger and faster, they are less susceptible to drought and disease.
The northern flying squirrel is a primary prey species for several rare old forest species and they are a primary vector for distributing the spores of mycorrhizal fungi, which increase tree nutrient and water uptake and forest resilience to climate change; therefore, maintaining, expanding, and increasing habitat quality, quantity, and connectivity for the northern flying squirrel is essential to maintaining diverse, healthy, and resilient forests in the Sierra Nevada.
- Survey for flying squirrels within 150 meters of perennial streams and on mesic sites prior to implementing activities that may affect the species to determine if species specific measures should be implemented.
- Avoid group-selection harvesting within 200 meters of perennial streams and within 50 meters of mesic sites occupied by flying squirrels to minimize potential edge effects.
- Avoid conducting management activities in occupied habitat when females are lactating (May 1 to August 1).
- Maintain closed-canopies in occupied habitat.
- Maintain large trees with rot or cavities.
- Minimize the loss of large snags from prescribed fire on occupied sites by raking and removing adjacent fuels.
- Protect and restore large red fir forest component on appropriate sites.
- Protect and restore large black oak forest component on appropriate sites.
- Increase canopy cover and the number of large trees and snags on unoccupied sites within 150 meters of perennial streams, on mesic sites, and adjacent to occupied sites.
The Northern flying squirrel is considered a management indicator species for all ten National Forests in the Sierra Nevada. The species is currently being monitored on the Plumas and Lassen National Forests though a multi-year monitoring effort. It is not considered a game species and it is illegal to kill or capture a Northern flying squirrel.
Meyer, M.D., M.P. North, and S.L Roberts. 2008. Truffle abundance in recently prescribed burned and unburned forests in Yosemite National Park: implications for mycophagous mammals. Fire Ecology Special Issue 4:24-33. (2.4 MB PDF)
North, M. 2002. Seasonality and abundance of truffles from oak woodlands to red fir forests. p 91-98 in Jared Verner (tech. editor). Proceedings of a Symposium on the Kings River Sustainable Forest Ecosystems Project: Progress and Current Status. USDA Forest Service, Pacific Southwest Research Station General Technical Report, PSW-GTR-183. (164 KB PDF)
Pyare, S., and W.S. Longland. 2002. Interrelationships Among Northern Flying Squirrels, Truffles, and Microhabitat Structure in Sierra Nevada Old-growth Habitat. Canadian Journal of Forest Restoration 32, 1016-1024. (78KB PDF)
Rosenberg, D.K., R.G. Anthony, 1992. Characteristics of Northern Flying Squirrel Populations in Young Second and Old-growth Forests in Western Oregon. Canadian Journal of Zoology 70,161-166. (486KB PDF)
Smith, J.R. 2009. Spatial organization, habitat preference, and management of northern flying squirrels, Glaucomys sabrinus, in the northern Sierra Nevada. Thesis, University of California, Davis. 45 Pages (372 KBPDF)
Waters, J.R., et.al., 2000. Northern Flying Squirrel Mycophagy and Truffle Production in Fir Forests in Northeastern California. Pacific Southwest Research Station, U.S. Forest Service, GTR-178. (498KB PDF)
Wilson, J.A., D.A. Kelt, and D.H. Van Vuren. 2008. Home range and activity of northern flying squirrels (Glaucomys sabrinus) in the Sierra Nevada. Southwestern Naturalist 53:21-28.
California Department of Fish and Game Natural History Information --This California state website contains rather limited and old information but is a good basic background composite for the species. Choose from a drop-down list to select the animal you are interested in.