Publication Date

12-2011

Type of Culminating Activity

Thesis - Boise State University Access Only

Degree Title

Master of Science in Biology

Department

Biology

Major Advisor

Jennifer Forbey, Ph.D.

Abstract

For herbivores, maximizing energy intake is limited by the relatively low nutritional quality of plants and high concentrations of potentially toxic plant secondary metabolites (PSMs). Although several studies have demonstrated that herbivores prefer diets with the highest nutrient and lowest PSM constituents, these studies are primarily confined to laboratory feeding trials due to the difficulty of observing diet selection in free ranging herbivores. Field observations have demonstrated that pygmy rabbits (Brachylagus idahoensis) and greater sage-grouse (Centrocercus urophasianus), both facultative specialists on sagebrush (Artemisia spp.), forage on specific sagebrush plants more than others within a foraging patch. I used these species to test the hypothesis that sagebrush plants heavily browsed (browsed) by pygmy rabbits and sage-grouse are higher in crude protein and lower in PSMs and are preferred to rarely browsed plants (unbrowsed). To test the hypothesis, I analyzed crude protein, digestibility, monoterpene concentration, and total phenolic concentration of leaves from paired browsed and unbrowsed plants. I used model selection to identify which combination of nutritional and chemical variables best describe diet selection by pygmy rabbits and sage-grouse. In general, crude protein content of the plant and several monoterpenes were indentified as variables that explained foraging by pygmy rabbits while only monoterpenes explained selection by sage-grouse. Amounts of total and digestible crude protein were higher and several monoterpenes were lower in browsed sagebrush compared to unbrowsed plants for pygmy rabbits. I also offered captive pygmy rabbits a choice of leaves from browsed and unbrowsed plants. Pygmy rabbits ate 2.5 times more of the leaves from browsed plants than leaves from unbrowsed plants. These results indicate that pygmy rabbits are making foraging decisions based on relative plant quality. Plants browsed and unbrowsed by sage-grouse had similar amounts of crude protein and browsed plants had higher concentrations of several PSMs. The cues pygmy rabbits and sage-grouse use to determine high quality plants may be different between species and are important for understanding how they perceive food quality across a landscape. Defining characteristics that determine higher quality plants among available plants will help guide management and conservation of specialist herbivores such as the pygmy rabbits and sage-grouse.

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