Connecting migration to demography through seasonal energy available to reindeer populations

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Rebecca Viejou and John M. Fryxell, Department of Integrative Biology, University of Guelph, Guelph, ON, Canada, Audun Stien, Department of Arctic and Marine Biology, University of Tromsø, Norway, Torkild Tveraa, Manuela Panzacchi, Bram Van Moorter and Olav Strand, Norwegian Institute for Nature Research, Norway

Background/Question/Methods
Given that migratory behavior is expected to result in a fitness payoff to individuals, migrations should result in higher birth rates and/or densities at the population level. For ungulates, a likely mechanism for the fitness payoffs of migration is increased energy access. The overall research goal of this study is to determine how migration behavior and demography are connected via seasonal energy access. We developed spatial models of the digestible energy available to reindeer across Norway during the summer and winter and propose a path model that links migration distance and seasonal energy access to intrinsic birth rate and equilibrium density for 42 semi-domestic reindeer populations with wide variation in human-driven migration.
Results/Conclusions
We found that the equilibrium density of reindeer increased by 0.23 ± 0.04 reindeer/km2 for every 10 km migrated. The relationship between migration and equilibrium density was mediated by access to energy on the winter range, with winter energy access increasing by 0.054 ± 0.006 MJ/m2 for every 10 km migrated and equilibrium density increasing by 4.3 ± 0.6 reindeer/km2 for every additional MJ/m2 accessed on the winter range. Equilibrium density was not related to energy on the summer range. Conversely, the intrinsic birth rate of reindeer populations increased by 0.43 ± 0.10 for every additional MJ/m2 available on the summer range, but summer energy access was related to spring onset and the distance from the ocean rather than migration distance and intrinsic birth rate was not related to energy available on the winter range. These results provide support for the idea that in a temperate environment summer resources drive reproduction and winter resources constrain survival. While alternative explanations for variation in demographic parameters across semi-domestic reindeer populations have been proposed, this study found support for the simple explanation that herders are managing populations in proportion to the resources available on their ranges and suggests that herding districts with enough space for migrations to suitable winter ranges do experience a demographic payoff. This fundamental pattern should be accounted for before populations can be compared regarding controversial issues such as harvesting practices and overgrazing.