Habitat Suitability and Conservation of the Giant Gartersnake (Thamnophis gigas) in the Sacramento Valley of California

Resource managers often have little information regarding the habitat requirements and distribution of rare species. Factor analysis-based habitat suitability models describe the ecological niche of a species and identify locations where these conditions occur on the landscape using existing occurrence data. We used factor analyses to assess the suitability of habitats for Thamnophis gigas (Giant Gartersnake), a rare, threatened species endemic to the Central Valley of California, USA, and to map the locations of habitat suitable for T. gigas in the Sacramento Valley. Factor analyses indicated that the niche of T. gigas is composed of sites near rice agriculture with low stream densities. Sites with high canal densities and near wetlands also appeared suitable, but results for these variables were sensitive to potential sampling bias. In the Sacramento Valley, suitable habitats occur primarily in the central portion of the valley floor. Based upon the results of the factor analyses, recovery planning for T. gigas will require an on-the-ground assessment of the current distribution and abundance of T. gigas, maintaining the few remaining natural wetlands and the practice of rice agriculture in the Sacramento Valley, and studying the effects of agricultural practices and land use changes on populations of T. gigas.

Fig. 1

The approximate historic range (re-drawn from Rossman et al., 1996) of Thamnophis gigas (dark gray), the location of the Sacramento Valley (dotted line), the locations of sites sampled from 1996 through 2005 (white polygons), and the locations of individual T. gigas (black dots).

Fig. 2

Input maps used to examine the niche of Thamnophis gigas in the Sacramento Valley of California.

Fig. 3

Results of the Ecological-Niche Factor Analysis (ENFA) for Thamnophis gigas in the Sacramento Valley of California. (A) Graph of the correlation of the environmental variables with the marginality (x-axis) and first specialization (y-axis) axes from the ENFA; (B) factorial map of the niche of T. gigas on the marginality and first specialization axes of the ENFA. The light gray points in (B) indicate the available environment in the Sacramento Valley, and the black points indicate locations at which T. gigas occurred.

Fig. 4

Habitat suitability map for Thamnophis gigas in the Sacramento Valley based upon squared Mahalanobis distances. White dots indicate locations of T. gigas captures used for validation of the Mahalanobis distances model. The dashed line indicates the boundary of the Sacramento Valley. Numbers in parenthesis indicate the range of in each mapped category.

Contributor Notes

¹ U.S. Geological Survey, Western Ecological Research Center, Dixon Field Station, 6924 Tremont Road, Dixon, California 95620; E-mail: (BJH) bhalstead@usgs.gov. Send reprint requests to BJH.

Associate Editor: G. Haenel.

Received: 28 Oct 2009

Accepted: 09 Aug 2010