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Yellowstone's Destabilized EcosystemElk Effects, Science, and Policy Conflict$
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Print publication date: 2006

Print ISBN-13: 9780195148213

Published to Oxford Scholarship Online: September 2007

DOI: 10.1093/acprof:oso/9780195148213.001.0001

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The Census Period: 1923–2003

The Census Period: 1923–2003

(p.15) 2 The Census Period: 1923–2003
Yellowstone's Destabilized Ecosystem

Frederic H. Wagner

Oxford University Press

Winter elk numbers on the northern range have been counted through most of its history, with ground counts up to 1956 (except 1935) and aerial counts up to the present. Several sightability bias tests have indicated that the censuses count approximately 70-75% of the herd. Regression of annual, instantaneous rates of change (r) between censuses of years t and t+1 on censuses of years t produces an inverse correlation with r reaching zero (equilibrium) at 16,800 (22,000-25,000 corrected for the sightability bias), and indicating density dependence. Below 16,800, the herd increased or held its numbers when hunting kills outside the park and park removals fell below 15-20%, but declined when removals exceeded this magnitude. While evidence of equilibration supports one tenet of the population aspect of the natural-regulation hypothesis, the 6x increase from a census of 3,172 in 1968 to 18,913 in 1988 (‘eruption’) falsifies two of the population tenets of the hypothesis.

Keywords:   ground counts, aerial censuses, sightability bias, density dependence, equilibrium, annual instantaneous rate of change, natural regulation, eruption

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