Western Arctic caribou herd winter lichen, snow depth, and mortality
Data files
Dec 06, 2025 version files 55.33 MB
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README.md
5.16 KB
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WAH_lichen_analysis.R
6.07 KB
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wah_lichen_snow_mort.csv
55.32 MB
Abstract
This repository contains data and code to generate results and figures from 'Winter survival shaped by forage abundance and snow depth for a long-distance migratory ungulate.' The dataset includes winter lichen cover, snow depth, and mortality data for adult female caribou from the Western Arctic Herd in northwestern Alaska. Each row represents unique locations from 372 caribou collected between 2009 and 2024, including information on snow depth, lichen cover, and mortality. Environmental data were derived from GPS collar locations; however, the locations themselves have been excluded due to restrictions on sharing data from harvested wildlife in Alaska.
https://doi.org/10.5061/dryad.qbzkh18tz
Description of the data and file structure
The dataset is a single comma-separated table, where each row corresponds to a unique caribou ID and time. Movement data from adult female caribou (>2 years old) were collected using GPS telemetry collars (Telonics TGW-4680), following State of Alaska IACUC-approved protocols. Collars recorded locations every 2, 4, or 8 hours. Mortality events were identified when collars stopped moving but continued transmitting, creating clusters of positions at the same location. Mortality sites were visited when possible to recover collars and locate remains.
The dataset was analyzed to assess overwinter survival, including mortality during winter, maximum number of winter days, average snow depth, and mean lichen cover.
Files and variables
File: wah_lichen_snow_mort.csv
| Field name | Format | Description |
|---|---|---|
| ID | Character | Individual identifier |
| snow_depth_NARR | Numerical | Snow depth (centimeters) |
| lichen | Numerical | Percent lichen cover |
| DateTime | yyyy-MM-dd HH:mm:s s | Location, date, and time |
| Year | yyyy | Calendar year |
| yday | Numerical | Day of year |
| Date | yyyy-MM-dd | Date |
| biological_year | yyyy | Encompasses the full winter period (e.g., for winter dates between 01 Dec 2009 - 31 Mar 2010, biological_year would be 2009) |
| Fate | Categorical | Animal’s fate at the end of the study period: "DEAD" (animal died), "DROPOFF" (collar dropped off), "CollarFail" (collar failure), "LIVE ANIMAL" (animal still alive) |
| startDate | yyyy-MM-dd | Date animal was collared |
| endDate | yyyy-MM-dd | Last date of data collection for a particular animal |
| deathDate | Logical (TRUE/FALSE) | Indicates whether the date is the day an animal died. TRUE if Date equals endDate and Fate is “DEAD”. |
| died | Logical (TRUE/FALSE) | Indicates whether the animal died in the winter during the study period. TRUE if any deathDate is TRUE. |
| deathLoc | Logical (TRUE/FALSE) | Indicates whether the location is the final recorded location on the day an animal died. TRUE if Fate is “DEAD”, Date equals endDate, and it is the animal’s last location |
| winter_days | Numeric | Number of days since start of winter (01 Dec, biological_year) |
Code/Software
WAH_lichen_analysis.R: This R script loads caribou overwinter survival data, summarizes snow and lichen conditions by individual winters, performs statistical tests, fits multiple Cox proportional hazards models, compares them with AIC, and generates hazard ratios and survival predictions across snow depth and lichen levels. It also visualizes predicted hazard ratios with confidence intervals for different lichen cover values.