Greater Sage-Grouse Population Dynamics and Probability of Persistence

Micheal A. Schroeder
Garton, E.O.
J.W. Connelly
J. S. Horne
C.A. Hagen
A. Moser
University of California Press
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We conducted a comprehensive analysis of Greater Sage-Grouse (Centrocercus urophasianus) populations throughout the species’ range by accumulating and analyzing counts of males at 9,870 leks identified since 1965. A substantial number of leks are censused each year throughout North America providing a combined total of 75,598 counts through 2007, with many leks having 30 years of information. These data sets represent the only long-term database available for Greater Sage-Grouse. We conducted our analyses for 30 Greater Sage-Grouse populations and for all leks surveyed in seven Sage-Grouse Management Zones (SMZs) identified in the Greater Sage-Grouse Comprehensive Conservation Strategy. This approach allowed grouping of leks into biologically meaningful populations, of which 23 offered sufficient data to model annual rates of population change. The best models for describing changes in growth rates of populations and SMZs, using information-theoretic criteria, were dominated by Gompertz-type models assuming density dependence on log abundance. Thirtyeight percent of the total were best described by a Gompertz model with no time lag, 32% with a one-year time lag, and 12% with a two-year time lag. These three types of Gompertz models best portrayed a total of 82% of the populations and SMZs. A Ricker-type model assuming linear density dependence on abundance in the current year was selected for 9% of the cases (SMZs or populations), while an exponential growth model with no density dependence was the best model for the remaining 9% of the cases. The best model in 44% of the cases included declining carrying capacity through time of --1.8% to -- 11.6% per year and in 18% incorporated lower carrying capacity in the last 20 years (1987–2007) than in the first 20 years (1967–1987). We forecast future population viability across 24 populations, seven SMZs, and the range-wide metapopulation using a hierarchy of best models applied to a starting range-wide minimum of 88,816 male sage-grouse counted on 5,042 leks in 2007 throughout western North America. Model forecasts suggest that at least 13% of the populations but none of the SMZs may decline below effective population sizes of 50 within the next 30 years, while at least 75% of the populations and 29% of the SMZs are likely to decline below effective population sizes of 500 within 100 years if current conditions and trends persist. Preventing high probabilities of extinction in many populations and in some SMZs in the long term will require concerted efforts to decrease continuing loss and degradation of habitat as well as addressing other factors (including West Nile virus) that may negatively affect Greater Sage-Grouse at local scales.
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University of Arizona

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