Year of Award

2010

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

Mark Hebblewhite

Commitee Members

Kerry Foresman, Paul Krausman, Tom Stephenson

Keywords

forage-predation trade-off, habitat ecology, wildfire, bighorn sheep, Sierra Nevada bighorn sheep

Abstract

We studied changes in vegetation and habitat selection by endangered Sierra Nevada bighorn sheep (Ovis candensis sierrae; hereafter Sierra bighorn) for 2 years following wildfire on winter ranges in eastern California. We hypothesized that wildfire would change both forage availability and predation risk. Green forage biomass on Sierra bighorn winter ranges rebounded quickly from wildfire. Within 2 years green forage biomass was equal in burned and unburned areas, although total forage biomass was greater in unburned areas. Plants in the burn had 3% greater crude protein but equivalent digestibility and phenology. Forage composition in burned areas was forb dominated compared with unburned areas that were shrub dominated. Visibility, a measure of predation risk, was 9% greater in burned areas at a 5 m radii compared with unburned areas. We found no change in fecal nitrogen between Sierra bighorn in burned and unburned areas but there was a shift to higher diet composition of forbs in the burn. We evaluated Sierra bighorn resource selection using seasonal resource selection functions that included spatiotemporal models of forage biomass and spatial models of predation risk by cougars (Puma concolor), the main predator of Sierra bighorn. In the first year post-wildfire, Sierra bighorn increased selection for new growth herbaceous biomass in response to the reduced biomass caused by wildfire. While wildfire initially reduced total forage biomass it also created pockets of the highest new forb biomass in areas of high cougar use. These pockets attracted Sierra bighorn causing an increase in overlap with cougars in winter 2008. Sierra bighorn showed consistent selection to be near escape terrain and remained closer to escape terrain in areas of high cougar use compared to areas with low cougar use. By spring 2008 and winter and spring of 2009 Sierra bighorn strongly selected total forage biomass where cougar use was low and in areas of high cougar use, Sierra bighorn avoided total forage biomass. As a result Sierra bighorn overlap with cougar use was reduced. We advise management to consider the effects of fire on both forage availability and predation when implementing prescribed burns to benefit ungulates.

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© Copyright 2010 Lacey Greene