The roles of climate and alternative prey in explaining 142 years of declining willow ptarmigan hunting yield

Long time series are important because they extend back to an era when animal populations were less influenced by habitat loss and climate change. Annual fluctuations in harvest yields are good proxies for large changes in population size and may reveal underlying ecological processes. From a variety of sources, we built a 142-year long time series representing the mean daily catch (CPUE) of willow ptarmigan Lagopus lagopus in southeastern Norway. CPUE decreased over the 142 years, from approximately 35 birds shot per day in early years to around two in the last years. There were three periods in the time series: a first period with 3-5 year cycles of high peaks and low troughs (1872-1900), a short second period with similarly high peaks, variable depth of troughs and variable cyclicity (1901-1916), and a third long period with much lower peaks and faded cycles (1917-2013). Yearly variation in CPUE was best explained by an interacting effect of small rodent peak years and period, with a reduced positive effect of rodents in the last period, and a positive effect of the North Atlantic Oscillation index in spring and early summer. None of the weather variables with significant time trends explained any variation in CPUE and we could therefore not attribute the decline in CPUE directly to climate change. We postulate that a long-term dampening of the amplitude in small rodent cycles combined with an increase in red fox numbers, have increased predation on alternative prey like ptarmigan, and prevented the populations from reaching their earlier peaks. Even though the present population of willow ptarmigan is only a fraction of what it was in former days, we recommend light hunting to motivate for monitoring and to keep public attention on the bird.

Automated summary

Long time series data of willow ptarmigan catch in southeastern Norway reveals a decline in population size over the past 142 years. The yearly variation in catch is primarily influenced by small rodent peak years and the North Atlantic Oscillation index. Weather variables with significant time trends do not directly explain the decline, suggesting other factors such as predation and changes in prey availability may be contributing.