FUNCTIONAL-RESPONSES OF COYOTES AND LYNX TO THE SNOWSHOE HARE CYCLE

Coyotes and lynx are the two most important mammalian predators of sno wshoe hares throughout much of the boreal forest. Populations of hares cycle in abundance, with peaks in density occurring every 8-11 yr, an d experimental results suggest that predation is a necessary factor ca using these cycles. We measured the functional responses of coyotes an d lynx during a cyclic fluctuation of hare populations in the southwes t Yukon, to determine their effect on the cyclic dynamics. We used sno w-tracking and radio telemetry to examine changes in the foraging beha vior of the predators. Coyotes and lynx both fed mostly on hares durin g all winters except during cyclic lows, when the main alternative pre y of coyotes was voles, and lynx switched to hunting red squirrels. Bo th predators showed clear functional responses to changes in the densi ties of hares. Kill rates of hares by coyotes varied from 0.3 to 2.3 h ares/d, with the most hares killed one year before the cyclic peak, wh ile those of lynx varied from 0.3 to 1.2 hares/d, with the highest one year after the peak. Maximum kill rates by both predators were greate r than their energetic needs. The functional response of coyotes was e qually well described by linear and type-2 curves, and that of lynx wa s well described by a type-2 curve. Kill rates by coyotes were higher during the increase in density of hares than during the cyclic decline , while the reverse was true for lynx. Coyotes killed more hares early in the winter, and cached many of these for later retrieval. Lower de nsities of hares were associated with longer reactive distances of bot h predators to hares, but with little apparent change in time spent se arching or handling prey. In summary, our data show that the two simil arly sized predators differed in their foraging behavior and relative abilities at capturing alternative prey, leading to different patterns in their functional responses to fluctuations in the density of their preferred prey.