Western gray squirrel ecology and augmentation of the population in the South Puget Trough: Progress Report


Published: January 2009

Pages: 16

Author(s): W. Matthew Vander Haegen and Gene R. Orth


Western gray squirrel (Sciurus griseus) populations in Washington have declined over the last century and their range has diminished. Of the three disjunct populations of western gray squirrel remaining in Washington, the Puget Trough population faces the greatest risk of extinction. The majority of the Puget Trough population occurs on Fort Lewis and the adjoining McChord Air Force Base; combined, these military reservations include over 35,000 ha and contain some of the last patches of oak-pine forest in western Washington. The Recovery Plan for this State-threatened species lists augmentation of the population in the Puget Trough as a high priority. In 2007, Washington Department of Fish and Wildlife and the Department of Defenseâ€"Fort Lewis, engaged in a cooperative study of the ecology of western gray squirrels on the Fort and implemented a plan to augment this endangered population. Research on the resident population has focused on quantifying population parameters including survival, causes of mortality, productivity, habitat use, and movements. This information will be critical for assessing why the Puget Trough population has contracted over the last few decades and for focusing management efforts to allow the population to recover. Augmentation has focused on bringing animals from other populations in Washington in order to increase the genetic diversity of the resident population and expand the occupied area. From January through December 2008 we captured a total of 29 new resident western gray squirrels during routine and grid trapping. This brings to 59 the total number of resident western gray squirrels captured on the study area since October 2006. Of the squirrels captured in 2008, 15 were fitted with radio-collars and 14 were ear-tagged only and released. We collected 2624 telemetry locations of 36 radio-collared western gray squirrels in 2008 and we documented 114 nests. Eight western gray squirrels died during 2008; 5 were depredated and 3 animals died from tularemia. Twelve of 13 females tracked through the breeding period attempted to rear young; litter size varied from 2 to 4. Nineteen animals were translocated from the Methow valley and from Klickitat County to Fort Lewis in September 2008. As of 31 December, 13 translocated animals were known to be alive, 3 confirmed mortalities, and 3 animals with unknown fate due to failure of their radio-collars. One of the 9 animals translocated in 2007 was known to be alive, 4 were confirmed mortalities, and 4 animals had unknown fate due to failure of the radio-collars. Plans for 2009 include continued monitoring of radio-collared animals, vegetation sampling within core-use areas, intensified monitoring of reproductive success, and further documenting the extent of tularemia in the both the eastern and western gray squirrel populations. Additional translocations will take place in fall 2009 as part of the species recovery effort.

Suggested citation

Vander Haegen, W. M. and G. R. Orth. 2009. Western gray squirrel ecology and augmentation of the population in the South Puget Trough. Progress report. Washington Department of Fish and Wildlife, Olympia. 14pp.