Climate and non-native herbivores influence reproductive investment by Greater Sage-grouse

Highly seasonal environments can increase competition among herbivores for
nutrients, leading to consequences affecting rates of reproduction and
survival. There is concern about the impacts of non-native ungulates on
Greater Sage-grouse in the Great Basin of North America. We estimated
nesting propensity, the annual proportion of females attempting a nest,
for Greater Sage-grouse in relation to the abundance of ungulates using 7
years of data from the northwestern Great Basin, USA. We focused on
nesting because it is the necessary first major investment required for
the production of new recruits. We used a Bayesian multi-stratum model to
investigate the effects of weather and sympatric non-native herbivores,
free-roaming horses and domestic cattle, on reproductive rates and female
survival of adult and yearling sage-grouse. Adults nested at a higher rate
(0.931, 95% CI, 0.904 – 0.953) than yearlings (0.867, 95% CI, 0.802 –
0.922) under average conditions of all other covariates. If the first nest
failed, renesting rates were similar between adults (0.349, 95% CI, 0.292
– 0.410) and yearlings (0.353, 95% CI, 0.217 – 0.507). Females in better
body condition at the start of the season nested at higher rates, and
moderately snowy winters led to the highest nesting propensity during the
following spring. Drier conditions led to low rates of nesting,
particularly in areas with dense cattle grazing. Female survival was lower
for nesting females, indicating a survival cost of reproduction. Areas
with abundant free-roaming horses had slightly higher nesting propensity,
though other research suggests negative impacts later in the breeding
cycle. Sage-grouse face life-history trade-offs that may be shifting due
to changing climatic conditions. Our work suggests that the effects of
competition with non-native ungulates on sage-grouse life-histories may be
exacerbated by adverse weather.