OOS 20-7
Recruitment and population dynamics in elk following the return of the wolf to a multi-predator multiple-prey system
The reintroduction of wolves (Canis lupus) to Yellowstone provides the unusual opportunity for a quasi-experimental test of the effects of wolf predation on their primary prey (elk – Cervus elaphus) in a multi-predator, multi-prey system where top-down, bottom-up, and abiotic forces on prey population dynamics were closely monitored before and after reintroduction. Here, we examined data from 33 years for 12 elk population segments spread across southwestern Montana and northwestern Wyoming in a large scale before-after-control-impact analysis of the effects of wolves on elk recruitment and population dynamics.
Results/Conclusions
Recruitment, as measured by the midwinter juvenile:female ratio, was a strong determinant of elk dynamics, and declined by 35% in elk herds colonized by wolves as annual population growth shifted from increasing to decreasing. Negative effects of population density and winter severity on recruitment, long recognized as important for elk dynamics, were detected in uncolonized elk herds and in colonized elk herds prior to wolf colonization but not after wolf colonization. Growing season precipitation and harvest had no detectable effect in either wolf treatment or colonization period although harvest ratios of juveniles:females declined by 37% in wolf-colonized herds. Liberal estimates of wolf predation rates on juvenile elk could explain 52% of the total decline in juvenile:female ratios in counts, implying the actual influence of direct wolf predation on population dynamics was considerably weaker due to imperfect elk sightability, reduced female survival, compensatory predation mortality, and increasingly, predation on bison (Bison bison) in some parts of the ecosystem. Increased grizzly bear (Ursus arctos) predation of elk calves might explain the otherwise unexplained declines in juvenile elk, but exponential growth in the bear populaion since wolf reintroduction would be necessary and was not detected. Collectively, these long-term, large-scale patterns align well with prior studies that found relatively weak effects of direct wolf predation on elk survival, and decreased reproduction and recruitment with exposure to predation risk from wolves.