Implementation Plan for Augmentation of the Western Gray Squirrel Population, Fort Lewis, Washington

Category: Wildlife Research

Published: October 2007

Pages: 36

Author(s): W. Matthew Vander Haegen, Sara C. Gregory, and Mary J. Linders

Executive Summary

The western gray squirrel is listed as threatened in Washington State and currently exists in 3 disjunct populations. Of these, the Puget Trough population faces the greatest extinction risk as a result of declining numbers and altered habitat. The majority of the Puget Trough population occurs on Fort Lewis, a 35,000 ha military reservation that contains some of the last patches of oak-pine forest in western Washington. The recovery plan for western gray squirrels in Washington lists augmentation of the population in the Puget Trough as a high priority. Augmentation is necessary both to increase the genetic diversity of the Puget Trough population and to expand the area of occupied habitat to provide a buffer against catastrophic loss due to disease, wildfire, or other causes.

An extensive review of tree squirrel reintroduction efforts suggests that translocations are a viable tool for recovery of tree squirrel populations. Based largely on established reintroduction protocols, augmentation of the western gray squirrel population on Fort Lewis will follow a detailed outline of preparation, implementation, and monitoring. There will be four general phases to the augmentation:

Phase 1. Identify areas appropriate for release of new animals. Suitability of areas for release will be determined based on historic records for the species, locations of extant western gray squirrels, and current habitat conditions.

Phase 2. Select source populations, numbers of animals to be translocated, and a timeline for translocation.

Phase 3. Capture and relocate animals from the source population to the release location.

Phase 4. Monitor translocated animals over a suitable period of time to assess the effectiveness of the augmentation. Concurrent with monitoring will be active research to assess demographic parameters, movement, and habitat use by western gray squirrels on the study area along with focused research on their spatial and behavioral interactions with non-native eastern gray squirrels.

Four potential release areas have been identified on Fort Lewis. Three of these currently have western gray squirrels or are adjacent to occupied habitat; the fourth is disjunct from occupied habitat but within the known dispersal distance for the species. Active habitat management on Fort Lewis has improved habitat conditions and continued improvements are planned.

Based on a range-wide assessment of western gray squirrel genetics and the size of the extant populations and their habitat, the preferred option for augmentation would be to obtain animals from both the Klickitat and North Cascades populations in Washington and from populations in northern Oregon. Obtaining animals from multiple source populations will maximize the potential to introduce animals with sufficient behavioral plasticity and genetic diversity to prosper in south Puget Trough, while minimizing the demographic impact on source populations.

Numbers of animals translocated to Fort Lewis will vary by release unit. The initial release in the Squirrel Triangle Unit will consist of 12 animals. This unit has an extant population of western gray squirrels and the primary goal of the release is to augment the number of breeding females and add to the genetic diversity of the population. This first release is planned for fall of 2007 and will include animals from both the Klickitat and North Cascades populations. Subsequent releases (2008-2010) will occur in units with suitable habitat but no known extant populations; the number of squirrels released will be based on the area of suitable habitat.

Initial monitoring of translocated squirrels will be integrated with ongoing research of the extant western gray squirrel population. All translocated squirrels will be marked with ear-tags and fitted with radio-collars prior to release. Movements of translocated squirrels, their survival, causes of mortality, and reproductive effort will be documented. This intensive monitoring planned for the first 5 years of the project will allow rapid assessment of the fate of translocated squirrels, providing the opportunity for mid-course corrections of the augmentation strategy.

Suggested citation

Vander Haegen, W. M., S. C. Gregory, and M. J. Linders. 2007. Implementation Plan for Augmentation of the Western Gray Squirrel Population, Fort Lewis, Washington. Washington Department of Fish and Wildlife, Olympia. 34pp.