Moose Habitat Selection and Movement Dynamics in a Changing Thermal Landscape under Climate Change: Accounting for Circadian Rhythmicity using Harmonic Terms

By Niels van der Vegt

Abstract
This thesis investigates how Swedish moose (Alces alces) respond behaviorally to extreme heat events, with a focus on movement dynamics and habitat selection during heatwaves across a latitudinal and ecological gradient. Using GPS collar data from 300 female moose collected at 3-hour intervals across four distinct ecological strata in Sweden, the study applies time-varying Step Selection Functions (SSFs) augmented with harmonic terms to model diel behavior, and uses space-time regression kriging to estimate fine-scale temperature at animal locations.

The results show that moose consistently reduce daytime movement during heatwaves, indicating clear heat-avoidance behavior. Selection for habitats, particularly dense forests and wetlands, increased during hot periods in several strata, especially around midday. Contrary to expectations, moose did not consistently seek proximity to water or higher elevations, and in some alpine regions, actively avoided wetlands during heat. This suggests that shade and vegetative cover are more critical thermal refuges than water access or elevation. Circadian rhythmicity in movement was preserved across all regions, although slightly modulated under heat stress. Harmonic modeling revealed that diel cycles persist even under continuous daylight, indicating temperature-driven behavioral entrainment in the absence of photoperiod cues. However, the evidence for compensatory foraging strategies or consistent water-seeking behavior during heatwaves was limited.

Methodologically, this study highlights the utility of harmonic terms in SSFs for capturing diel behavioral variation and underscores the importance of high-resolution spatial and temporal data in ecological modeling. Limitations include the coarse GPS fix rate, limited habitat resolution, and computational challenges with harmonic modeling.

This research contributes to a growing understanding of how large northern herbivores adapt their behavior under climate change, offering insight into wildlife management strategies amid rising global temperatures.