Disentangling the effects of climate, density dependence, and harvest on an iconic large herbivore's population dynamics

Abstract

Understanding the relative effects of climate, harvest, and density dependence on population dynamics is critical for guiding sound population management, especially for ungulates in arid and semiarid environments experiencing climate change. To address these issues for bison in southern Utah, USA, we applied a Bayesian state-space model to a 72-yr time series of abundance counts. While accounting for known harvest (as well as live removal) from the population, we found that the bison population in southern Utah exhibited a strong potential to grow from low density (β0 = 0.26; Bayesian credible interval based on 95% of the highest posterior density [BCI] = 0.19-0.33), and weak but statistically significant density dependence (β1 = -0.02, BCI = -0.04 to -0.004). Early spring temperatures also had strong positive effects on population growth (Pfat1 = 0.09, BCI = 0.04-0.14), much more so than precipitation and other temperature-related variables (model weight > three times more than that for other climate variables). Although we hypothesized that harvest is the primary driving force of bison population dynamics in southern Utah, our elasticity analysis indicated that changes in early spring temperature could have a greater relative effect on equilibrium abundance than either harvest or. the strength of density dependence. Our findings highlight the utility of incorporating elasticity analyses into state-space population models, and the need to include climatic processes in wildlife management policies and planning.

Estimating ungulate abundance while accounting for multiple sources of observation error

“…With increased accuracy in abundance estimates, managers are better equipped to assess the status of a population relative to objectives; thereby ensuring knowledgeable decisions are made regarding management actions. In addition, improved estimates of abundance over time and their associated measures of uncertainty (e.g., CIs) can be used in state‐space models of population dynamics to separate observation error from the underlying ecological process (e.g., Rotella et al ) that affect population dynamics the most (Koons et al ). Finally, such data can be combined with information on reproductive success and survival to develop integrated population models and provide robust guidance for management planning (e.g., Johnson et al , Péron et al ).…”

The effect of climate on population growth in a cold‐adapted ungulate at its equatorial range limit

“…We followed the approach of Koons et al. () in defining a prior distribution for β N moose to exclude values that exceed the mathematical limitations of the effect of population density on growth rate in the Gompertz model (Dennis et al. ).…”

“…Also following Koons et al. (), we estimated an informative prior for β rmax (, standard deviation [SD] = 0.08) based on the mean of five previous studies which provided estimates of r max in moose (Bergerud , Keith , Van Ballenberghe , Cederlund and Sand , Sinclair ). Informing the parameters with reliable prior information aids identifiability of other model parameters including process and observation errors (Lebreton and Gimenez , Koons et al.…”

“…Informing the parameters with reliable prior information aids identifiability of other model parameters including process and observation errors (Lebreton and Gimenez , Koons et al. ).…”

Horn size and nutrition in mountain sheep: Can ewe handle the truth?

“…Female harvest as a management strategy remains unacceptable to many individuals, despite populations continuing to thrive and remain robust with some substantive female harvests in, for example, bison ( Bison bison ; Koons et al ), North American elk (Walker , Proffitt et al ), white‐tailed deer (Miller and Marchinton , Walker , Harper et al ), pronghorn (O'Gara and Morrison ), and moose ( Alces alces ; Boertje et al ) in North America, or red deer and moose in Europe (Solberg et al , Clutton‐Brock et al , Milner et al , Rivrud et al ). In contrast, harvest of female mountain sheep is rarely employed as a management tool (Wild Sheep Foundation Professional Biologist Meeting Attendees , Monteith et al ), perhaps because of a reluctance of management agencies to face social animosity or unacceptance of such management by the public.…”