Tracking snowmelt to jump the green wave: Phenological drivers of migration in a northern ungulate

• Authors:
  • Michel P. Laforge
  • Maegwin Bonar
  • Eric Vander Wal

This repository contains code accompanying the paper “Tracking snowmelt to jump the green wave: Phenological drivers of migration in a northern ungulate” in Ecology. Attributes of used and available locations to make the models is saved under “Data/Locations/Locs.RDS”. The actual coordinates of the locations have been removed for data sharing reasons. All code is found in “R code”, which includes the main scripts used for the paper, along with extra code for making supplementary figures and tables, and the function used for k-fold cross-validation. The paper supplements are included as well.

Abstract:

In northern climates, spring is a time of rapid environmental change: for migrating terrestrial animals, melting snow facilitates foraging and travel, and newly-emergent vegetation provides a valuable nutritional resource. These changes result in selection on the timing of important life-history events such as migration and parturition occurring when high-quality resources are most abundant. We examined the timing of female caribou (Rangifer tarandus, n = 94) migration and parturition in five herds across seven years in Newfoundland, Canada as a function of two measures of environmental change—snowmelt and vegetation green-up. We generated resource selection functions to test whether caribou selected for areas associated with snowmelt and green-up during migration and following calving. We found that caribou migrated approximately one week prior to snowmelt, with the flush of emergent vegetation occurring during the weeks following parturition. The results indicate that caribou “jump” the green wave of emergent forage and do so by tracking the receding edge of melting snow, likely reducing movement and foraging costs related to snow cover. Our research further broadens the ecological scope of resource tracking in animals. We demonstrate that resource tracking extends beyond resources directly related to foraging to those related to movement. We also show that snowmelt provides an environmental cue that may provide a buffer against changing environmental conditions.

Key words: Calving, caribou, energetic demands, migration, movement, Newfoundland, normalised difference vegetation index, phenology, resource tracking, snowmelt