Population Trends of Bighorn Sheep and Mountain Goats in the Greater Yellowstone Area

Scientific Disciplines
Biological Sciences - Terrestrial
Wildlife conservation
Montana State University
Yellowstone national park
greater yellowstone area
bighorn sheep
mountain goat
herd growth rates
Ovis canadensis
Oreamnos americanus
Volume 18, No. 1-4

Population trends of bighorn sheep and mountain 
Goats in the greater yellowstone area
Elizabeth P. Flesch*, Ecology Department, Montana State University, Bozeman, Montana 59717
Robert A. Garrott, Ecology Department, Montana State University, Bozeman, Montana 59717
Bighorn sheep (Ovis canadensis) and mountain goats (Oreamnos americanus) are 
important components of the large mammal community in the Greater Yellowstone Area 
(GYA) and are of considerable public interest.  However, foundational ecological research 
concerning these species is limited.  We analyzed historic bighorn sheep and mountain goat 
population counts collected by management biologists using ln-linear regression to estimate 
herd growth rates (λ).  The analyzed dataset consisted of 538 bighorn sheep counts since 

1971 and 120 mountain goat counts since 1966.  Most mountain goat count units experienced 
a positive growth rate and increased their distributions over recent decades.  Bighorn sheep 
growth rates were more variable among the 26 recognized herd units in the GYA.  We used 
the historic count data to evaluate the hypothesis that sympatry of non-native mountain goats 
with bighorn sheep adversely affected bighorn sheep populations.  This was accomplished by 
comparing the growth rates of sympatric herds with that of allopatric herds.  There was no 
evidence that sympatric herd growth rates were significantly lower than allopatric herd growth 
rates.  We caution, however, that many counts in consecutive years suggested larger changes 
in abundance than what would be reasonable to expect from biological processes.  We suspect 
that variability in counts likely reflects varying detection probability and the overall difficulty 
of counting mountain ungulates.  Therefore, conclusions derived from these data should be 
further evaluated with more detailed demographic studies in the future.