Trophic facilitation by introduced top predators: grey wolf subsidies to scavengers in Yellowstone National Park

1. The reintroduction of grey wolves Canis lupus (L.) to Yellowstone National Park provides a natural experiment in which to study the effects of a keystone predator on ecosystem function. 2. Grey wolves often provision scavengers with carrion by partially consuming their prey. 3. In order to examine how grey wolf foraging behaviour influences the availability of carrion to scavengers, we observed consumption of 57 wolf-killed elk Cervus elaphus (L.) and determined the percentage of edible biomass eaten by wolves from each carcass. 4. We found that the percentage of a carcass consumed by wolves increases as snow depth decreases and the ratio of wolf pack size to prey size and distance to the road increases. In addition, wolf packs of intermediate size provide the most carrion to scavengers. 5. Applying linear regression models to the years prior to reintroduction, we calculate carrion biomass availability had wolves been present, and contrast this to a previously published index of carrion availability. Our results demonstrate that wolves increase the time period over which carrion is available, and change the variability in scavenge from a late winter pulse dependent primarily on abiotic environmental conditions to one that is relatively constant across the winter and primarily dependent on wolf demographics. Wolves also decrease the year-to-year and month-to-month variation in carrion availability. 6. By transferring the availability of carrion from the highly productive late winter, to the less productive early winter and from highly productive years to less productive ones, wolves provide a temporal subsidy to scavengers.