Non-native grazers affect physiological and demographic responses of
Greater Sage-grouse
Abstract
1. Non-native ungulate grazing has negatively impacted native species
across the globe, leading to massive loss of biodiversity and ecosystem
services. Despite their pervasiveness, interactions between grazers and
native species are not fully understood. We often observe declines in
demography or survival of these native species, but lack understanding
about the mechanisms underlying these declines. Physiological stress
represents one mechanism of (mal)adaptation but data are sparse. 2. We
investigated glucocorticoid levels in a native avian herbivore exposed
to different intensities of non-native grazing in the cold desert Great
Basin ecosystem, USA. We measured corticosterone, a glucocorticoid in
birds, in feathers for a large sample (n = 280) of female Greater
Sage-grouse (Centrocercus urophasianus) from three study areas in
Northern Nevada and Southern Oregon with different grazing regimes of
livestock and feral horses. 3. We found greater feral horse density was
associated with higher corticosterone levels, and this effect was
exacerbated by drought conditions. Livestock grazing produced similar
results; however there was more model uncertainty about the livestock
effect. Subsequent nesting success was lower with increased feather
corticosterone, but corticosterone levels were not predictive of other
vital rates. 4. Our results indicate a physiological response by
sage-grouse to grazing pressure from non-native grazers. We found
substantial among-individual variation in the strength of the response.
These adverse effects were intensified during unfavorable weather
events, highlighting the need to reevaluate management strategies in the
face of climate change.