Hungry wolves and dangerous prey: a tale of prey switching

Abstract

For 40 years wolves have subsisted on elk in Riding Mountain National Park – an intensively monitored system. During 24 continuous months of fieldwork, I uncovered a novel observation of prey switching. Wolves switched to a more dangerous alternative, moose, from the historical and vulnerable prey, elk. Generalist predators are ubiquitous but testing mechanisms of population-level prey switching in charismatic megafauna is rare. I empirically test foundational theory with long-term data on predator consumption and prey population abundance to identify processes underlying prey switching. First, the urgency of acquiring energy mediates space-use behaviour of wolves; hunger drove a fine-scale switch in prey tracking. Hunger increased the preference for moose catchability and decreased tracking of elk. Movement patterns of hungry wolves are indicative of area-restricted search behavior to promote encounter rates. Second, I tested the ideal gas law’s ability to describe encounter rates between predators and prey. Wolf movement rate governed the effective speed, and consequently, encounters in the system. Winter kills increase with effective speed and total light for moose, but not for elk. There was a seasonal switch in these prey-specific relationships, where only elk kills increased with effective speed and total light in summer. Third, I express a functional response of a single predator to the abundance of two prey types, dissimilar in energy and time costs to predators when searching, attacking, and consuming prey. This model operationalizes the balance of costs and gains inherent in foragers pursuing their next meal. I simulate how optimal foraging decisions produces prey switching. Fourth, I empirically test this model with data collected on wolf diets and prey abundance in Riding Mountain National Park since 1970. Despite moose being most abundant and consumed, wolves still maintain their preference for elk over moose. The switching observations at both the population and pack level support optimal foraging predictions; wolves prefer elk but their preference for elk declined as the total prey biomass available declined. From search to consumption, my research captures echoes of broad scale population contexts resonating within the fine-scale behaviours of a predator in a multi-prey system.