Good Moose Weather?

Young_moose_grazing_in_a_field(1)Safari Club International (SCI) Foundation has multiple ongoing projects studying moose populations. Wildlife managers can rely on SCI Foundation-funded work to contribute to the successful management of these large mammals. Research Teams are investigating the effects of predation on calves, the influence of annual rainfall on much-needed warm season forage, wintering habitat conditions and parasite/disease susceptibility to understand why moose numbers are declining in some areas while increasing to the point of saturation in others.

Climate plays a fundamental role in determining range limits and population dynamics of large herbivores. Through much of western Wyoming and northern Utah, Shiras moose populations have been declining in recent decades.  Although predation pressure by large carnivores, mainly wolves and grizzly bears, likely plays a major role in regulating moose populations at low density, the absence of these important carnivores across much of the range occupied by Shiras moose in the Rockies negates predation as the primary cause of observed declines. Because of the potential for broad landscape-scale effects, climatic factors are thought to be involved in the regional declines of Shiras moose, yet the factors that allow one moose herd to do well while another declines are unknown.

SCI Foundation partnered with the Wyoming Cooperative Fish & Wildlife Research Unit and the Wyoming Game & Fish Department to examine effects of climate and plant growth on calf recruitment rates of Shiras moose collected over 3 decades across the Rocky Mountains. Of the 18 herd units with available data, calf-cow ratios declined in 8 of them during 1980-2009, while the remaining herds displayed no trends. Over the 3 decade record, rates of calf recruitment were affected negatively by warm temperatures and dry springs/summers that occurred a year before a cohort of calves being raised, and were lower during years with a rapid rate of spring green-up. These patterns indicate both direct and delayed effects of weather, potential insect parasites and plant productivity on moose calf recruitment, which likely was controlled through the nutritional condition of mothers.


Nutritional stress could have been induced by (1) heat stress and (2) warmer, drier springs that shortened the duration when high-quality forage was available. Both pathways would lower summer fat gain leading to reduced pregnancy or the ability to nurse young calves. Interestingly, annual variation in temperature did not appear to influence populations with stable calf-cow ratios, many of which were recently introduced or expanding populations that may have been buffered against drought because they occupied habitats that still contain abundant forage. This study suggests that maintaining high quality habitat may allow managers to buffer the impact of drought on Shiras moose populations.

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