DOI: 10.5061/dryad.djh9w0w94
title: Gyrfalcon prey abundance and their habitat associations in a changing Arctic

description: 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.
description: The study conducted multi-species point count surveys to detect
 bird species and Arctic ground squirrels over three years (2019-2022).
 Surveys began 30 minutes before sunrise and ended when at least ten points
 were surveyed or by noon. Each point was surveyed once. The surveys
 followed a time-removal approach, with specific time intervals acting as
 replicate survey periods. Observations were made for ten minutes, with a
 two-minute quiet period beforehand. The first detection of each individual
 within 400 meters was recorded, excluding subsequent detections of the
 same individual. Data collected included species, time, distance, bearing,
 group size, and sex. Individuals not actively using the area (e.g.,
 flyovers) were excluded. Observers also measured temperature, wind speed,
 and direction at each point. Surveys were not conducted in winds over 24
 km/h or in heavy rain or fog. From 2016 to 2022
 (excluding 2020 due to Covid-19), the study monitored 97 Gyrfalcon
 territories on nesting cliffs, conducting two surveys per territory each
 year between 1 May and 2 July. Surveys were performed using an R44
 helicopter, following established protocols. A territory was considered
 potentially occupied if a Gyrfalcon adult, egg, or nestling was observed
 during the survey. Sixty surveys were not completed due to weather, and
 those were treated as missing data. The study also used prior surveys
 (2011-2015) to confirm whether territories were historically Gyrfalcon
 territories, as some were unoccupied or occupied by other raptors during
 the 2016-2022 period. The study used vegetation data
 from the Arctic Boreal Vulnerability Experiment (ABoVE) to describe land
 cover on the Seward Peninsula. The ABoVE project classified vegetation
 into 15 types based on dominant vegetation derived from satellite imagery
 at a 30-meter grid resolution. The dataset from 2014 was used, and
 relevant habitat types for each species (Willow Ptarmigan, Rock Ptarmigan,
 and Arctic ground squirrels) were selected based on their preferred
 habitats. Vegetation types such as tundra, tussock, low shrub, and tall
 shrub, along with elevation, were chosen for the models. Data for each
 survey point included vegetation cover within a 0.64-km² area and mean
 elevation using a digital elevation model (DEM). Areas above 500 m
 elevation were excluded. To estimate prey density
 inside each Gyrfalcon territory, we excluded cells within each territory
 with vegetation percentage values that fell outside the ranges used in our
 prey abundance models, and with elevations exceeding 500 m. We calculated
 the area size of each territory correcting for these removed cells. We
 summed expected density of each prey from the grid cells inside each
 Gyrfalcon territory and divided by the corrected territory area. For
 Willow Ptarmigan and Rock Ptarmigan, densities were expressed as males per
 square kilometer, whereas for Arctic Ground Squirrels, densities were
 reported as squirrels per square kilometer.
description: # Gyrfalcon prey abundance and their habitat associations in a changing
 Arctic
 [https://doi.org/10.5061/dryad.djh9w0w94](https://doi.org/10.5061/dryad.djh9w0w94) **Author:** Michaela Gustafson **Contact:** [michagustaf@gmail.com](mailto:michagustaf@gmail.com) **Manuscript DOI:** 10.1002/ECE3.70763 **Study area:** Seward Peninsula, Alaska USA **Dates:** 2016, 2019-2022 **Species:** Willow Ptarmigan (*Lagopus lagopus*), Rock Ptarmigan (*Lagopus muta*), Arctic Ground Squirrels (*Urocitellus parryii*), Gyrfalcon (*Falco rusticolus*) **Last updated**: 11 December 2024 ## Description of the data and file structure This study investigates the habitat-abundance relationships of three herbivore species—Willow Ptarmigan, Rock Ptarmigan, and Arctic Ground Squirrels to understand how their distributions may change in response to habitat shifts due to climate change. Data were collected across 983 point count locations during the summers of 2019, 2021, and 2022. These observations were analyzed using N-mixture models to account for imperfect detection, providing more accurate estimates of species abundance in different habitat types. The study also involved mapping the herbivore densities within 97 Gyrfalcon breeding territories using the same point count data. Additionally, a multi-season occupancy model was employed to assess whether Gyrfalcons were more likely to occupy territories with higher prey densities, using occupancy data obtained from helicopter surveys conducted between 2016 and 2022. These combined efforts allowed for the evaluation of how herbivore distributions and abundances relate to Gyrfalcon territory occupancy and the broader trophic web dynamics in the Arctic. ### Files and variables #### File: ropt.csv **Description:** Male Rock Ptarmigan observations and standardized covariates used in analysis. Rock Ptarmigan observations collected May-July 2019, 2021, and 2022. ##### Variables * id: unique site id * 1: time interval (1st 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 2: time interval (2nd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 3: time interval (3rd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 4: time interval (4th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 5: time interval (5th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * observer: unique ID for primary observer of the survey. DS is Observer 1, KW is Observer 2, and MG is Observer 3. * wind: ***standardized*** value of wind(mph) for that site during time of survey. Units: mph * julian: day of year (not true julian date, but the count starting from January 1st each year) for that site during time of survey. ***Values standardized***. Units: day * min_after_sun: ***standardized*** minutes after end of civil twilight or when the sun reaches 0 degrees on the horizon that the survey at that site took place. Negative indicates survey was conducted before 'sunrise'. Units: minutes * lowshrub: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * tundra: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: %  * sparseveg: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * elev: ***standardized*** elevation for each point. unstandardized elevation value is the mean elevation for land within 800m2 of survey point. Units: meters #### File: widedf.csv **Description:** Gyrfalcon occupancy data ##### Variables * terrXyear: Unique territory and year of survey ID * territory: Territory name * year: year survey occurred * pres_1: Gyrfalcon occupancy status during the 'early' survey that occurred in May * pres_2: Gyrfalcon occupancy status during the 'late' survey that occurred in June * jday_1: day of year (from January 1st) the early survey took place. Units: day * jday_2: day of year (from January 1st) the late survey took place. Units: day *Cells that contain NA means a survey was not conducted for that site during that year and during that survey round (either early or late). #### File: squir.csv **Description:** All Arctic ground squirrel observations and standardized covariates used in analysis. Arctic ground squirrel observations collected May-July 2019, 2021, and 2022. ##### Variables * id: unique site id * 1: time interval (1st 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 2: time interval (2nd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 3: time interval (3rd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 4: time interval (4th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 5: time interval (5th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * wind: **standardized** value of wind (mph) for that site during time of survey. Units: mph * julian: day of year (not true julian date, but the count starting from January 1st) for that site during time of survey. ***Values standardized***. Units: day * min_after_sun: ***standardized*** minutes after end of civil twilight or when the sun reaches 0 degrees on the horizon that the survey at that site took place. Negative indicates survey was conducted before 'sunrise'. Units: minute * observer: unique ID for primary observer of the survey. DS is Observer 1, KW is Observer 2, and MG is Observer 3. * lowshrub: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * tundra: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * sparseveg: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * elev: ***standardized*** elevation for each point. unstandardized elevation value is the mean elevation for land within 800m2 of survey point. Units: meter #### File: wipt.csv **Description:** Male Willow Ptarmigan observations and standardized covariates used in analysis. Willow Ptarmigan observations collected May-July 2019, 2021, and 2022. ##### Variables * id: unique site id * 1: time interval (1st 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 2: time interval (2nd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 3: time interval (3rd 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 4: time interval (4th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * 5: time interval (5th 2-minute period). The number in that row x column is the number of Rock Ptarmigan observed at that site during that specific time interval *** Data was filtered to only include male Rock Ptarmigan that were observed at or less than 400 m from the observer. Distances were measured using a rangefinder. * observer: unique ID for primary observer of the survey. DS is Observer 1, KW is Observer 2, and MG is Observer 3. * wind: ***standardized*** value of wind(mph) for that site during time of survey. Units: mph * julian: day of year (not true julian date, but the count starting from January 1st) for that site during time of survey. ***Values standardized***. Units: day * min_after_sun: ***standardized*** minutes after end of civil twilight or when the sun reaches 0 degrees on the horizon that the survey at that site took place. Negative indicates survey was conducted before 'sunrise'. Units: minute * tallshrub: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * tundra: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * tussock: ***standardized*** value of the percent of that land cover type within an 800m2 area of the survey point. Units: % * elev: ***standardized*** elevation for each point. unstandardized elevation value is the mean elevation for land within 800 m x 800 m of survey point. Units: meter ## Code/software Software necessary: Program R, RStudio, and RTools [https://www.r-project.org/](https://www.r-project.org/) [https://posit.co/download/rstudio-desktop/](https://posit.co/download/rstudio-desktop/) [https://cran.r-project.org/bin/windows/Rtools/](https://cran.r-project.org/bin/windows/Rtools/) Packages necessary are listed in the code scripts.  Program and package versions used are listed in the manuscript. **File List:** **06_WillowTimeModel.qmd**: code containing model analysis and model fit for Willow Ptarmigan observations **07_RockTimeModel.qmd**: code containing model analysis and model fit for Rock Ptarmigan observations **08_SquirrelTimeModel**: code containing model analysis and model fit for Arctic ground squirrel observations **12_GyrOccupancyAnalysis.qmd**: code containing occupancy model analysis for Gyrfalcon territories **13_GyrOccEval.qmd**: code containing model fit and evaluation for the Gyrfalcon occupancy model ## Access information Other publicly accessible locations of the data: * [https://github.com/michaelagustafson/abundance_distribution_manuscript](https://github.com/michaelagustafson/abundance_distribution_manuscript) Data was derived from the following sources: * Land cover data derived from: [https://daac.ornl.gov/ABOVE/guides/Annual_Landcover_ABoVE.html](https://daac.ornl.gov/ABOVE/guides/Annual_Landcover_ABoVE.html) Wang, J.A., D. Sulla-Menashe, C.E. Woodcock, O. Sonnentag, R.F. Keeling, and M.A. Friedl. 2019. ABoVE: Landsat-derived Annual Dominant Land Cover Across ABoVE Core Domain, 1984-2014. ORNL DAAC, Oak Ridge, Tennessee, USA. [https://doi.org/10.3334/ORNLDAAC/1691](https://doi.org/10.3334/ORNLDAAC/1691)