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Ecology and Conservation of Rattlesnakes in Sagebrush Steppe Ecosystems: Landscape Disturbance, Small Mammal Communities, and Great Basin Rattlesnake Reproduction

Investigators and Affiliations

Christopher L. Jenkins, Graduate Student, 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

U.S. Department of Energy Idaho Operations Office
INL Education Outreach Program (Bechtel)
Idaho State University (CERE Lab and Graduate Research Committee)
Bureau of Land Management
Idaho Department of Fish and Game

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 DOE 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 recommendations 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 included the following:

  • Quantifying spatial variation in rattlesnake life histories.
  • Determine if spatial variation in rattlesnake life histories correlate with coarse scale patterns in habitat and small mammal biomass.
  • Determining if rattlesnakes are selecting habitats with greater small-mammal biomass.
  • Determining if disturbance to sagebrush steppe systems affects small-mammal biomass.
  • Determine if changes in small mammal communities influence body condition of female rattlesnakes.

Accomplishments

This research was conducted as part of a doctoral program and has been completed.

Dissertation Abstract

Widespread disturbance in sagebrush steppe ecosystems is threatening Great Basin rattlesnake populations. The sagebrush steppe ecosystem is experiencing a variety of disturbances including mining, human development, livestock grazing, invasive plants, and changing fire regimes. Great Basin rattlesnakes (Crotalus oreganus lutosus) are capital breeding snakes that acquire energy over multiple years for reproduction. Disturbances in sagebrush steppe may be influencing rattlesnake reproductive output by limiting the amount of energy (i.e., food) they can acquire during the active season. The goal of this dissertation was to determine to what extent and how disturbance influences populations of Great Basin rattlesnakes.

The following were sampled substrate, vegetation, small mammals, and operative temperatures and conducted mark-recapture, radio telemetry, and a common garden experiment on rattlesnakes. These studies occurred at three large overwintering complexes (Crater Butte, Cinder Butte, and Rattlesnake Cave) on the INL. The INL is a DOE nuclear research facility. Portions of the INL are grazed by livestock and some fires have occurred in the area.

Results suggest that broad patterns in landscape disturbance are indirectly influencing rattlesnake reproduction by altering prey availability. First, a significant microgeographic variation in reproduction was found. Specifically, the Crater Butte population had lower reproductive output due to lower body condition, slower growth, later ages to maturity, longer intervals between pregnancies and lower fecundity. Second, an approach was developed to determine the factors influencing reproduction that links broad scale landscape disturbance such as grazing and fire to rattlesnake ecology through a series of trophic interactions. Finally, using this approach, it was determined that prey availability was higher in a landscape with less disturbance and greater precipitation. Snakes using areas with higher prey availability meandered more during movements and gained less weight. When comparing two of the sites, Crater Butte had more landscape disturbance, lower prey availability, snakes moved more linearly, gained less weight, and had lower reproductive output relative to Rattlesnake Cave. In addition, Rattlesnake Cave received approximately 4 centimeters more precipitation from May to September than the rest of the sites. There was no difference in estimated available foraging times between study sites or disturbance categories and no evidence for local adaptation of growth rates although due to low sample sizes there was relatively low power (0.30) for detecting a difference.

Results from these studies suggest that natural and human caused patterns on the landscape influence prey availability and subsequently that rattlesnake ecology is influenced by prey availability. Specifically, relatively high precipitation likely provides high prey availability at Rattlesnake Cave relative to Crater Butte. Disturbance lowers prey availability levels at both sites. Likely in response to low prey availability, snakes are making more linear movements as they search for prey and are gaining less weight. Less weight gain is likely resulting in lower body condition and growth. Snakes in areas where they gain less weight also have lower reproductive output. These findings have applied implications for the conservation of sagebrush steppe, predators, and rattlesnakes. For example, wildlife management programs interested in maintaining rattlesnake populations need to consider broad patterns of landscape disturbance and their resulting impacts on prey availability.
 

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