What Can We Learn From Calf/Cow Ratios?

Scientific Disciplines
Biological Sciences - Terrestrial
Keywords
University of Montana
population growth
alberta
ratios
recruitment
female survival
age ratios
recruitment rates
vital rates
Authors
Years
Volumes
Volume 17, No. 1-4

What can we learn from calf/cow ratios?
Nicholas J. DeCesare,* Wildlife Biology Program, Department of Ecosystem and Conservation 
Sciences, University of Montana, Missoula, Montana 59812, nick.decesare@umontana.edu
Mark Hebblewhite, Wildlife Biology Program, Department of Ecosystem and Conservation 
Sciences, University of Montana, Missoula, Montana 59812 
Mark Bradley, Parks Canada, Jasper National Park, Box 10, Jasper, Alberta, T0E 1E0, Canada
Kirby G. Smith, Alberta Sustainable Resource Development, Edson, Alberta, T7E 1T2, Canada
David Hervieux, Alberta Sustainable Resource Development, Grande Prairie, Alberta, T8V 6J4, 
Canada
Lalenia Neufeld, Parks Canada, Jasper National Park, Box 10, Jasper, Alberta, T0E 1E0, Canada
Trends in population growth can be monitored with data for key vital rates without 
requiring knowledge of abundance. Adult female survival has the highest elasticity for 
ungulate population dynamics, but the more variable recruitment rates can be better 
predictors of local variation in growth rates. Recruitment is often monitored using young 
adult age ratios, which are difficult to reliably interpret given the contribution of multiple 
vital rates to annual ratios. We show how concurrent monitoring of adult female survival 
and age ratios allows both retrospective estimation of empirical population growth rates 
and the decomposition of recruitment-specific vital rates. We demonstrate the estimation of 
recruitment and population growth rates for one woodland caribou population using these 
methods, including elasticity and life-stage simulation analysis of the relative contribution 
of adult female survival and recruitment rates to variation in population growth. We show, 
for this woodland caribou population, that adult survival and recruitment rates are nearly 
equivalent drivers of population growth rates. We recommend the concurrent monitoring 
of adult female survival to reliably interpret age ratios when managing caribou and other 
ungulates.