But Do You Recall, Why Reindeer Populations Rise and Fall?: A Theoretical Model of Reindeer and Warble Flies, Ingrid Koester, UG '21 (2313694)

Reindeer (Rangifer tarandus) are arctic large-bodied herbivores that have long-term population fluctuations. Although multiple underlying mechanisms might contribute to these fluctuations, the parasitic warble fly (Hypoderma tarandi) may be an overlooked contributor. The present research investigates such possibilities by first constructing a basic theoretical model of this host/parasite system using the work of Crofton and May as a foundation, and then introducing additional nuance through host age-structure, parasite predators, and climatic data. The model constructions are subsequently fit to observed field data of 8 reindeer herds, using both single-input projections (i.e. fitting the model to the entire time-series) and forecasting predictions (i.e. fitting the model to half of the time-series). The average normalized RMSD (root mean square deviation between observed and expected population sizes) value across all herds is 0.3797 for the single-input projections and is 0.7036. for the forecasting predictions. Changes in latitude, herd status, and herd ecotype show statistically significant correlations with changes in the normalized RMSD values. This study demonstrates that warble flies may be critical regulators of long-term reindeer population cycles, as the models produce stable oscillations and have predictive ability for some reindeer herds, particularly for herds with migratory tundra as its dominant ecotype. Future research should confirm these results through empirical parasite removal experiments.