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Idaho NERP Research: The Effect of Landscape Change on the Life History of Western Rattlesnakes (Crotalus Oreganus)

Investigators and Affiliations

Christopher L. Jenkins , Graduate Student, Herpetology Laboratory, Department of Biological Sciences, Idaho State University, Pocatello, ID
Charles R. Peterson, Professor, Herpetology Laboratory, Department of Biological Sciences, Idaho State University, Pocatello, ID

Funding Sources

Idaho Department of Fish and Game
Bureau of Land Management
ISU Department of Biological Sciences
ISU Graduate Student Research Committee
INL – ISU Education Outreach Program

Background

This project was designed to assess the impact of landscape disturbance on western rattlesnakes by examining trophic interactions among habitat, small mammals, and snakes. The synergistic effect of livestock grazing, invasive plants and fire is changing sagebrush steppe ecosystems in the Upper Snake River Plain. It is hypothesized that this phenomenon is affecting the prey base of top-level predators in the system. The main research goal is to determine if changes in habitat are altering prey availability and subsequently life history characteristics of western rattlesnakes.

Information from this project is important to the Department of Energy for several reasons: (1) as an indicator of how habitat change is influencing small mammal biomass; (2) as an indicator of how trophic interactions affect western rattlesnakes; (3) providing implications for the management and conservation of predators on the INL; (4) for utilizing a long term mark recapture data set gathered by the Idaho State University Herpetology Laboratory to further an understanding of community ecology on the INL; (5) assisting in the training of graduate and undergraduate students in environmental research.

Objectives

The overall goal of this project is to determine if current landscape patterns in habitat and prey on the INL are influencing rattlesnake life histories. Specific objectives for 2003 included the following:

Quantifying spatial variation in rattlesnake life histories.
Determining if rattlesnakes are selecting habitats with greater small mammal biomass.
Determining if disturbance to sagebrush steppe systems affects small mammal biomass.

Accomplishments Through 2003

Specific accomplishments for 2003 include the following:

Found significant variation in life history characteristics among three den complexes on the INL (Table 1). More specifically, it was found that snakes at one den complex had life history characteristics that would indicate lower fitness.
Found that small mammal biomass was greater in snake core activity areas than in either migration corridors or random locations (Figure 1).
Found that small mammal biomass was highest in habitats characterized by relatively tall shrub cover, low grass cover, and high biological crust cover (Table 2).

Figure 1. Average small mammal biomass found in random areas, core areas of snake activity, and migration corridors used by snakes during summer 2003. Error bars represent one standard error.

Table 1. Life history characteristics calculated from western rattlesnakes captured between 1994-2002 at three den locations in southeastern Idaho.

	Den Location
Life History Characteristics	Cinder Butte	Crater Butte	Rattlesnake Cave
Age at Maturity	4	6	3
Proportion of Females Pregnant	0.25	0.20	0.24
Number of Young	5.20 + 0.32	4.40 + 0.30	5.92 + 0.28
Condition of Young^a	-0.83 + 0.14	-0.54 + 0.15	1.08 + 0.22
Female Body Condition	-6.08 + 4.80	-6.16 + 2.54	11.88 + 4.76
Ecdysis (sheds/year)	1.66 + 0.15	1.68 + 0.18	2.41 + 0.23
Growth (cm/year)	5.50 + 0.96	3.60 + 0.69	5.95 + 1.14
a. Condition of young and adult females is calculated as the residual mass (i.e., residual values of the regression of mass to length). b. Values in bold type represent the values presumed to be the most advantageous relative to the other den locations.

Table 2. The best model for predicting small mammal biomass in the study area, during the summer active period of snakes (May through September) 2003. The overall R² for the model was 0.26.

Chris Jenkins, ISU PhD student, received the Ted Trueblood Communications Award for best student presentation for his lecture entitled, Life History Variation Among Western Rattlesnake Populations on the INL, presented at the Idaho Chapter of the Wildlife Society in Boise on March 7, 2003.