Of wolves and bears: Seasonal drivers of interference and exploitation competition between apex predators

Competition between apex predators can alter the strength of top-down
forcing, yet we know little about the behavioral mechanisms that drive
competition in multipredator ecosystems. Interactions between predators
can be synergistic (facilitative) or antagonistic (inhibitive), both of
which are widespread in nature, vary in strength between species and
across space and time, and affect predation patterns and predator-prey
dynamics. Recent research suggests gray wolf (Canis lupus) kill rates
decrease where they are sympatric with brown bears (Ursus arctos),
however, the mechanisms behind this pattern remain unknown. We used data
from two long-term research projects in Scandinavia (Europe) and
Yellowstone National Park (North America) to test the role of interference
and exploitation competition from bears on wolf predatory behavior, where
altered wolf handling and search time of prey in the presence of bears are
indicative of interference and exploitation competition, respectively. Our
results suggest the mechanisms driving competition between bears and
wolves were dependent on the season and study system. During spring in
Scandinavia, interference competition was the primary mechanism driving
decreased kill rates for wolves sympatric with bears; handling time
increased, but search time did not. In summer, however, when both bear and
wolf predation focused on neonate moose, the behavioral mechanism switched
to exploitation competition; search time increased, but handling time did
not. Interference competition, however, did affect wolf predation dynamics
in Yellowstone during summer, where wolves prey more evenly on neonate and
adult ungulates. Here, bear presence at a carcass increased the amount of
time wolves spent at carcasses of all sizes and wolf handling time for
small prey, but decreased handling time for the largest prey. Wolves
facilitate scavenging opportunities for bears, however, bears alter wolf
predatory behavior via multiple pathways and are primarily antagonistic to
wolves. Our study helps clarify the behavioral mechanisms driving
competition between apex predators, illustrating how interspecific
interactions can manifest into population-level predation patterns.