Long-term Studies of Willow Ptarmigan and Gyrfalcon in the Yukon Territory: A Collapsing 10-Year Cycle and Its Apparent Effect on the Top Predator.

Abstract

—From the late 1950s to the present, several study plots across the Yukon have been variously surveyed annually for territorial Willow Ptarmigan (Lagopus lagopus). Beginning in the mid-1970s, Gyrfalcon (Falco rusticolus) breeding numbers in the same tundra systems have been monitored annually. These data are held in a long-term data base. Monitoring has supported studies of winter survival strategies, tests of population change theory, and reproductive strategy. Willow Ptarmigan are seen as a ‘keystone’ in the tundra community. Understanding and tracking very basic trophic interrelationships with the Gyrfalcon, the top predator, has been a major effort at community study. Stable, regular, synchronous, 10-year cycles have been demonstrated in both species. However, beginning in 2000, monitoring surveys have been suggesting the regular cycling of ptarmigan abundance may be faltering—the population peaks seem to be disappearing, although there is no evidence of imminent local extinctions. The potential consequence to the tundra ecosystem is suggested in disruption at the top of the food chain—Gyrfalcons are breeding much later, may be producing fewer young, and seem to be declining in abundance. Simple modeling suggests Gyrfalcon productivity through the few years of peak ptarmigan productivity may be critical. It will be important to maintain longer monitoring to demonstrate conclusively this change as well as causes. Received 31 December 2010, accepted 20 June 2011. MOSSOP, D. H. 2011. Long-term studies of Willow Ptarmigan and Gyrfalcon in the Yukon Territory: A collapsing 10-year cycle and its apparent effect on the top predator. In R. T. Watson, T. J. Cade, M. Fuller, G. Hunt, and E. Potapov (Eds.). Gyrfalcons and Ptarmigan in a Changing World. The Peregrine Fund, Boise, Idaho, USA. http://dx.doi.org/10.4080/gpcw.2011.0206