Do irrigation and predator control reduce the productivity of migratory ungulate herds?

Wilmers, Christopher; Levi, Taal

Migration in ungulates is thought to have evolved as a strategy to increase seasonal access to high-quality forage and to decrease predation risk. Where migratory and nonmigratory ungulates coexist, migrants can outnumber nonmigrants by an order of magnitude. However, migratory ungulates are now in decline worldwide due to anthropogenic factors. Middleton et al. (2013; hereafter referred to Middleton et al.) suggest that the productivity of migratory elk (Cervus canadensis) in the Greater Yellowstone Ecosystem, USA, is declining because of increased predation pressure and a contraction of the growing season in their high-altitude summer range. As an alternative hypothesis, we propose that irrigation and predator control at low elevations have increased the benefits of a nonmigratory strategy. We show that while migratory elk calf : cow ratios decline in drought years, resident elk calf : cow ratios do not respond to drought. This is likely because resident elk increase forage utilization on irrigated landscapes during drought years. Low-altitude nonmigratory elk now outnumber the migratory segment of the population. We hypothesize that the resulting increase in competition for winter range is responsible for the decline in the productivity of the migratory elk herd. Our hypothesis is equally plausible to Middleton et al.’s and is supported by data. We suggest that this debate is best resolved using experimental or meta-analytical approaches where multiple hypotheses are tested against one another. Wildlife managers should consider the potential impacts on migratory elk of predator control and elk irrigation programs.