2012 Progress report for 1000920 – “Evaluating the influence of climate change and bear predation on Shiras moose declines”
Declines in moose populations across much of the Rocky Mountains has fostered concern over current harvest regimes and heightened efforts to identify contributing factors. Recent declines have been documented in
Wyoming, Utah, and Montana, and are believed to be occurring in Idaho and parts of Colorado. Despite the widespread nature of the declines in Shiras moose abundance and recruitment, few evaluations have been conducted to explain why many populations are declining while some remain stable. Large-scale factors such as climate are likely to play a potential role in population performance, while other factors such as predation or habitat disturbance are likely to influence populations on a local basis. Indeed, the well studied Jackson herd in northwest Wyoming experiences predation by both wolves and grizzly bears, and has exhibited precipitous declines in both calf recruitment and abundance during the past 2 decades. Nonetheless, other populations in
Wyoming also are declining, despite conservative harvest and a near absence of wolves and grizzly bears.
Therefore, it is likely that regional factors such as climate may serve as a widespread limiting factor, thereby synchronizing declines in numerous populations. The regional nature of the declines led the Wyoming Game and
Fish Department, in collaboration with the Wyoming Cooperative Fish and Wildlife Research Unit, to initiate a large-scale analysis to evaluate the potential causes of the declines.
Approach
Since early 1980s, agencies from Wyoming, Utah, and Colorado have routinely monitored herd composition each winter via helicopter surveys for most herd units throughout the state. To increase the geographic scale of our analyses, the range in variation of calf productivity, and improve our ability to detect biologically meaningful patterns, we obtained data on calf-cow ratios for a total of 18 herd units: 9 in Wyoming, 8 in Utah, and 1 in Colorado. Factors related to long-term trends in calf recruitment likely hold insight into identifying the primary factors responsible for population declines, given that inter-annual variability in recruitment of young determines population trajectory. Efforts to date have been to account for variation in calf recruitment that may be underpinned by large-scale factors such as climate and plant phenology. We extracted temperature and precipitation data that was mapped from the parameter-elevation on independent slopes model
(PRISM) for seasonal ranges occupied by moose in the three states. In addition, we extracted weekly data on the normalized difference vegetation index (NDVI) for the same seasonal ranges, which provides an index to greenness of vegetation. We used program TIMESAT to calculate metrics that represented spring phenology from the resulting NDVI curves. We then used linear mixed-effect models and an information theoretic approach to evaluate the potential influence of seasonal climate and plant phenology on calf recruitment. We weighted estimates by the inverse of the variance in calf recruitment, and included herd unit as a random effect.
Effects of climate and plant phenology
Of the 18 herd units, 8 exhibited significant negative trends in calf recruitment during the past 3 decades
(Figure 1). From the 1980s to present, average calf recruitment was 47 calves:100 adult females (range = 15-100).
Warming temperatures and declining precipitation during spring and summer, and more rapid green-up were evident during the past 3 decades throughout moose range in Wyoming, Utah, and Colorado (P < 0.05; Table 1 &
2).
Currently we are working to finalize analyses and compile results in manuscript form. At present, current results are suggestive of negative effects of warming temperatures and declining precipitation during the spring and summer on calf recruitment the following year. That is, climate related effects are occurring from the preceding year that calves were raised. Rate of spring green-up was the only plant phenology metric that was related to calf recruitment. Results indicate that more rapid rates of green-up during the year that a cohort of calves was raised are having a negative effect on calf recruitment. Together, the effects of climate and plant phenology are suggestive of negative effects of changing environmental conditions on nutritional quality of forage and potentially heat stress on the ability of female moose to produce and successfully rear calves.
We currently are finalizing and compiling results to produce the final manuscript and submit for publication in peer-reviewed outlets. We anticipate having documents finalized by the end of the calendar year.
In addition, we recently presented a status update to the Moose Working Group in Lander, Wyoming and at the
American Society of Mammalogists meetings in Reno, Nevada.

Table 1. Parameter estimate, standard error, and test statistics describing temporal trend in seasonal patterns in average daily temperature (°C) and cumulative precipitation (cm) for Shiras moose range in 18 herd units in the
Rocky Mountains, 1980-2009. Temporal trends were modeled as a fixed effect of year with a random intercept for herd unit.
Climate Season
β
SE F -value P -value
Temperature Winter
Spring
0.064
0.051
0.0062
0.0071
103.88
51.62
<0.001
<0.001
Precipitation
Summer
Autumn
Winter
0.061
0.052
-0.048
0.0054
0.0057
0.039
125.20
82.70
1.53
<0.001
<0.001
0.22
Spring
Summer
-0.10
0.0022
-0.064
0.24
22.20
<0.001
7.18
0.0076
Autumn -0.043
0.23
3.59
0.059
Table 2. Parameter estimate, standard error, and test statistics describing temporal trend in spring plant phenology based on the normalized difference vegetation index (NDVI) for Shiras moose range in 18 herd units in the Rocky Mountains, 1989-2009. Temporal trends were modeled as a fixed effect of year with a random intercept for herd unit.
Phenology metric β SE F -value P -value
Start of spring
Length of spring
-0.29
-0.30
0.12
0.12
5.38
6.15
0.021
0.014
Rate of spring green-up 0.0002
0.00004
15.46
<0.001

Figure 1. Temporal trends in calf-cow ratios for 18 herd units of Shiras moose in the Rocky Mountains of Wyoming,
Utah, and Colorado, USA for units with stable and declining trends.