Data from: A specialist predator in a food web with cyclic alternative prey: The gyrfalcon-ptarmigan case revisited

Population dynamics of specialist predators are tightly linked to their
main prey and can in simple food webs, lead to complex predator-prey
interactions (e.g., predator-prey cycles). However, the strength of these
interactions may be affected by the availability of alternative prey if it
appears in high numbers and the predator's diet is sufficiently
flexible. A prominent example of a specialist predator-prey interaction is
the interaction between gyrfalcon and ptarmigan known from simple food
webs with few alternative prey. Here we use 23-year long time series from
a more complex food web in northern Fennoscandia to investigate the
relative roles of main (ptarmigan) and cyclically fluctuating alternative
prey (lemmings) in driving gyrfalcon nesting territory occupancy and
production of young. We find that nesting territory occupancy dynamics are
dependent on both main and alternative prey, while fledgling production
within occupied territories is independent of prey abundance dynamics.
Population density of willow ptarmigan had the expected direct and delayed
effect on gyrfalcon territory occupancy dynamics, while onehigh-amplitude
lemming population peak year in the time series boosted colonization of
nesting territories. This study demonstrates that the population dynamics
of alternative prey can play an important role also for specialist
predators. While it might be uplifting that the gyrfalcons being a
regionally red listed species seem to be able to consume alternative prey,
both ptarmigan and lemmings are negatively impacted by ongoing climate
change. Hence, the population status of gyrfalcon should still be a
conservation concern.