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. 2020 Jun;89(6):1511-1519.
doi: 10.1111/1365-2656.13200. Epub 2020 Mar 27.

How climate impacts the composition of wolf-killed elk in northern Yellowstone National Park

Christopher C Wilmers  1 , Matthew C Metz  2   3 , Daniel R Stahler  2 , Michel T Kohl  4 , Chris Geremia  2 , Douglas W Smith  2
Affiliations

How climate impacts the composition of wolf-killed elk in northern Yellowstone National Park

Christopher C Wilmers et al. J Anim Ecol. 2020 Jun.

Abstract

While the functional response of predators is commonly measured, recent work has revealed that the age and sex composition of prey killed is often a better predictor of prey population dynamics because the reproductive value of adult females is usually higher than that of males or juveniles. Climate is often an important mediating factor in determining the composition of predator kills, but we currently lack a mechanistic understanding of how the multiple facets of climate interact with prey abundance and demography to influence the composition of predator kills. Over 20 winters, we monitored 17 wolf packs in Yellowstone National Park and recorded the sex, age and nutritional condition of kills of their dominant prey-elk-in both early and late winter periods when elk are in relatively good and relatively poor condition, respectively. Nutritional condition (as indicated by per cent marrow fat) of wolf-killed elk varied markedly with summer plant productivity, snow water equivalent (SWE) and winter period. Moreover, marrow was poorer for wolf-killed bulls and especially for calves than it was for cows. Wolf prey composition was influenced by a complex set of climatic and endogenous variables. In early winter, poor plant growth in either year t or t - 1, or relatively low elk abundance, increased the odds of wolves killing bulls relative to cows. Calves were most likely to get killed when elk abundance was high and when the forage productivity they experienced in utero was poor. In late winter, low SWE and a relatively large elk population increased the odds of wolves killing calves relative to cows, whereas low SWE and poor vegetation productivity 1 year prior together increased the likelihood of wolves killing a bull instead of a cow. Since climate has a strong influence on whether wolves prey on cows (who, depending on their age, are the key reproductive components of the population) or lower reproductive value of calves and bulls, our results suggest that climate can drive wolf predation to be more or less additive from year to year.

Keywords: Canis lupus; age structure; climate change; predator-prey dynamics; prey selection.

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Figures

FIGURE 1
FIGURE 1
Box whisker plots of per cent fat in the femur marrow of individual wolf‐killed elk during early and late winter. Year on the x‐axis refers to the year on 31 December (e.g. 1995 represents the winter of 1995–1996). Data for 30 wolf‐killed elk from 2013–2014 and 2014–2015 are displayed here, although these data were excluded from the analysis (Figure 2)
FIGURE 2
FIGURE 2
Determinants of elk nutritional condition. Model‐averaged standardized coefficient values ± 95% CIs from a logistic regression fitting elk demographic, wolf pack size and climatic variables to data on marrow fat (1 = healthy marrow and 0 = depleted marrow). Results for bulls and calves are relative to cows which served as the base case
FIGURE 3
FIGURE 3
Wolf prey composition. Model‐averaged standardized coefficient values ± 95% CIs from a multinomial regression fitting elk population and climatic variables to data on elk demographic class (bull, cow and calf), with cow serving as the base case in (a) early winter and (b) late winter
FIGURE 4
FIGURE 4
Plots of interaction effects from the multinomial regression fitting elk population and productivity in year t to data on elk demographic class, holding all other covariates to their mean values. We show predictions for the influence of productivity on the log odds of a kill at the mean (8,226 elk) and ±1 SD (±4,449 elk) of the elk population (mean and SD represent the mean and SD for our wolf‐killed elk data) for (a) bulls and (b) calves in early winter, and for (c) bulls and (d) calves in late winter. Shaded areas represent standard error intervals
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References

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