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Climatic drivers of Dall sheep survival rates
Madelon
van de Kerk, University of Washington, madelon@uw.edu
(Presenter)
Laura
Prugh, University Of Washington, lprugh@uw.edu
Dall sheep population declines have been attributed primarily to harsh spring weather and snow conditions. However, the mechanisms behind the declines are not known, and no quantitative analyses have examined why some populations appear to be resilient while others appear to be vulnerable. We investigated Dall sheep survival rates among several populations and how they were affected by a variety of environmental factors.
We used GPS and VHF telemetry data from Dall sheep collected from 9 different sites for a total of 229,815 sheep days. We estimated annual survival from May to May for sheep of different age classes (lamb: 0-1 yrs, adult: 1-12 yrs, old: >12 yrs) using Cox proportional hazard regression. We then used a model selection approach to examine how survival was affected by mean temperature, mean precipitation, mean and maximum NDVI, and total number of freeze/thaw events in summer and winter. We combined covariates that were uncorrelated into additive and interactive models and ranked the models based on the AIC. We compared the model fit of the base model with and without a random effect of site to see if there were any significant differences in survival rates among sites. We then compared the fit of the top model with and without a random effect of site to investigate if the included covariates adequately accounted for these differences.
We estimated the overall survival rates by age class to be 29% (SE: 14) for lambs, 80% (SE: 3) for adults, and 50% (SE: 19) for old sheep. Including a random effect of site in the base model (with no environmental covariates) improved the fit, indicating substantial differences in survival rates among sites. Based on a total model set of 453 models, the top model included an interactive effect of the mean precipitation in summer and the total number of winter thaw events. Including a random effect of site in the top model did not improve the fit, indicating that the variation among sites was adequately captured by the included environmental factors.
Adult survival was generally high and stable, except in years with dry summers and multiple thaws in winter. Lambs survival, and to a lesser extend the survival of old individuals, was good in years with wet summers and multiple thaws in winter as well as years with dry summers and few thaws in winter. Their survival was relatively poor under the opposite circumstances; years with wet summers and few thaws in winter as well as years with dry summers and multiple thaws in winter.
Associated Project(s):
Poster Location ID: 26
Session Assigned: Wildlife and Ecosystem Services
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