Ecology and conservation of rattlesnakes in sagebrush steppe ecosystems: landscape disturbance, small mammal communities, and Great Basin rattlesnake reproduction

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1. Widespread disturbance in sagebrush steppe is threatening many species including reptiles. Great Basin rattlesnakes (Crotalus oreganus lutosus) are capital breeding snakes that acquire energy over multiple years for reproduction. Disturbances may be influencing rattlesnake reproductive output by limiting the amount of energy they can acquire during the active season. The goal of this dissertation was to determine how disturbance influences reproduction in Great Basin rattlesnakes. Specific objectives were to 1) document microgeographic variation in rattlesnake reproductive characteristics, 2) determine if disturbance is related to reproduction indirectly through prey availability and/or the amount of potential foraging time, and 3) determine whether local adaptation in growth has occurred in rattlesnake populations. The author 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 Idaho National Laboratory (INL). Results suggest that patterns in landscape disturbance are indirectly influencing rattlesnake reproduction by altering prey availability. First, the Crater Butte population had lower reproductive output due to lower body condition, slower growth, and lower fecundity than the other two populations. Second, prey availability was lower at Crater Butte and in disturbed areas. Snakes using areas with lower prey availability made more linear movements and gained less weight. Weight gain in individually tracked snakes was correlated to prey availability. When making an overall comparison of the two sites, the author found that Crater Butte had more landscape disturbance and lower prey availability and that snakes moved more linearly, gained less weight, and had lower reproductive output relative to Rattlesnake Cave. There was no evidence that potential foraging times or local adaptation were influencing the observed variation in reproductive output between Crater Butte and Rattlesnake Cave. Results from these studies suggest that disturbance is indirectly influencing predator populations by altering prey availability. This result is consistent with studies of snakes in other ecosystems and studies of other small mammal predators in the sagebrush steppe. 2. 2. Disturbances are natural occurrences in all ecosystems and often help maintain ecosystem function. Disturbances can have indirect effects on species by influencing species at other trophic levels. The author develops a conceptual framework that outlines multiple ways by which disturbance could influence rattlesnake reproduction. A study was conducted to determine whether disturbance influences prey availability and/or potential time available for foraging for Great Basin rattlesnakes (Crotalus oreganus lutosus). In addition, a common garden experiment was conducted to determine whether snake growth has become locally adapted to the environment. The author found that more disturbed areas were characterized by high bare soil and grass cover and by low biological crust and shrub cover, shorter shrubs, and lower small mammal species richness, abundance, and biomass. Rattlesnakes made relatively linear movements in areas with low prey availability and gained significantly less weight. No consistent evidence for snake preference of areas with high prey availability was found, but snakes used approximately the same core activity areas between years. There was no evidence for disturbance having an influence on the potential time available for snake foraging and no evidence for local adaptation of growth rates of snake populations. However, the least disturbed study site with high prey availability also experienced higher summer precipitation. The results from this study suggest that human-caused disturbance can have indirect effects on predator populations by altering prey availability. These results have important applied implications for the conservation of predators, rattlesnakes, and sagebrush steppe ecosystems. Specifically, mitigating for these indirect ecological interactions by protecting current habitat and restoring degraded habitat is important for conserving snakes, other predators, and sagebrush steppe communities.

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ELECTRONIC FILE - Zoology