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 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.