But Do You Recall, Why Reindeer Populations Rise and Fall?: A Theoretical Model of Reindeer (Rangifer tarandus) and Warble Flies (Hypoderma tarandi)

Abstract

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 constructed models result in reindeer fluctuations with periods of 12.40-31.99 years. The model constructions are subsequently fit to observed field data of 8 reindeer herds, using both single-input projections and forecasting predictions. The Central Arctic herd of Alaska fits the single-input projection most accurately with an average normalized RMSD (root mean square deviation between observed and expected population sizes) value of 0.21. The Yamal herd of Russia fits the forecasting prediction most accurately with an average normalized RMSD value of 0.28. Changes in latitude, herd status, and herd ecotype show statistically significant correlations with changes in the normalized RMSD values. The present 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. The implications and applicability of these results are discussed, with particular attention to local livelihoods, zoonotic disease, and conservation.