Many studies show that predators generally prey more on certain demographic classes, (e.g. juveniles and older individuals). However, such preferences (e.g. the demographic classes predators kill vs. what is available to them) are likely to vary if predators switch to preying more upon other demographic classes when preferred prey become scarce, or if severe weather conditions make some demographic classes more accessible to predators. Despite this, little is known about the extent to which such preferences vary over time. Improving our understanding of preference dynamics is not only valuable from a behavioural perspective, but also because changes in the demographic composition of individuals killed by predators can have an important mediating influence on the impact of predation on prey dynamics. Furthermore, changes in preference can also affect kill rates when demographic classes have markedly different body sizes. Consequently, preference dynamics may influence predator-prey and community dynamics. Here we used elk carcass recovery data to assess wolf preference dynamics for different demographic classes of elk in Yellowstone National Park, over a 12-year period. We assessed the extent to which preferences (calculated as Manly’s alpha) varied among years and in relation to the relatively availability of that demographic class in the population.
Results/Conclusions
Wolves showed the strongest preference for calves, followed by senescent females, then senescent males. They avoided prime-aged males and females. Preference for calves scarcely varied among years (coefficient of variation (CV) of 0.05) and the strength of this preference did not fluctuate with the relative availability of calves in the population. Conversely, preferences for senescent females and males were more variable (CV of 0.24 and 0.30 respectively) and the strength of these preferences were negatively related to the availability of these demographic classes in the population. While a strong preference for juveniles and senescent individuals is unsurprising, our work suggests that rather than switching to more abundant demographic classes, wolves continue to selectively kill senescent individuals, even when they are relatively rare in the population. This presumably reflects that prime-aged elk are largely inaccessible to wolves, such that wolves are forced to trade-off a potentially higher intake rate against the increased risk of injury/difficulty associated with killing prime-aged individuals. Thus changes in prey population structure likely alter the amount of prey accessible to wolves, independently of prey density. Consequently, prey population structure may be an important predictor of kill rates and should be taken into consideration in future predator-prey studies.