Gyrfalcon prey abundance and their habitat associations in a changing Arctic

Arctic habitats are changing rapidly and altering trophic webs and
ecosystem functioning. Understanding how species’ abundances and
distributions differ among Arctic habitats is important in predicting
future species shifts and trophic-web consequences. We aimed to determine
the habitat-abundance relationships for three small herbivores on the
Seward Peninsula of Alaska, USA by fitting data from 983 point counts
(collected during 2019, 2021, 2022) with N-mixture models that account for
imperfect detection. These herbivore species, Willow Ptarmigan [Lagopus
lagopus], Rock Ptarmigan [L. muta], and Arctic ground squirrels
[Urocitellus parryii]), are fundamental to tundra food webs, and primary
prey for Arctic raptors including Gyrfalcons (Falco rusticolus). Second,
we aimed to map herbivore densities within Gyrfalcon breeding territories.
Third, we aimed to evaluate whether Gyrfalcons were more likely to occupy
territories with higher prey densities using a multi-season occupancy
model coupled with occupancy observations from helicopter surveys
conducted during 2016-2022 at 97 Gyrfalcon territories. We found that male
Willow Ptarmigan were more abundant in areas with greater cover of tundra,
tall shrubs, and tussock tundra. Conversely, male Rock Ptarmigan were more
abundant in areas with greater cover of sparse vegetation and tundra.
Arctic ground squirrels were more abundant at higher elevations with
greater cover of sparse vegetation and low shrubs. Willow Ptarmigan were
widespread within Gyrfalcon breeding territories, whereas Rock Ptarmigan
and Arctic ground squirrels had patchier distributions with few areas of
high abundance. Lastly, Gyrfalcons were more likely to occupy territories
with higher densities of Willow Ptarmigan and Arctic ground squirrels. As
the Artic continues to warm, Rock Ptarmigan and Arctic ground squirrels
may be vulnerable to ongoing shrub encroachment, whereas Willow Ptarmigan
may benefit. By tying abundances of three prey to Gyrfalcon occupancy, our
results contribute to understanding potential impacts on higher levels of
this Arctic trophic web.