Why do migrants move downhill? The effects of increasing predation and density on red deer altitudinal migration in temperate Carpathian forests
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Resource selection by ungulates is driven by trade-offs between foraging and predation avoidance or by intraspecific competition. Ungulates use migratory flexibility to optimize access to spatially and temporally variable resources across seasons, sometimes even adaptively switching between migrant and resident strategy as conditions change. After an 80% increase in red deer (Cervus elaphus) population and simultaneous recovery of wolves (Canis lupus) in the Kremnica Mountains, Slovakia, a significant portion of the deer population started to migrate downhill (< 700 m) to marginal habitats during winter. Building on available spatial data on forage availability, predation risk, and deer abundance, we tested for differences in habitat selection of migrant and resident male red deer to assess possible reasons for this change. On high-altitude (700–1100 m) summer ranges, deer were not forced to trade-off forage to avoid predation within their home ranges. However, during winter, residents remaining on high-altitude ranges selected for areas with highly abundant forage only under low predation risk or at high deer abundance. Downhill migration exposed migrants to 15% lower forage availability but simultaneously reduced wolf predation risk by 39% relative to residents. Consequently, the limited access to forage resources at low-altitude ranges have reduced antler growth, especially in young males. Our study represents one of few that address the role of predation risk in driving seasonal migrations in temperate systems where snow is not likely to be the major driver of migration to low-altitude winter ranges.
KeywordsPredator-prey interaction Ungulates Seasonal migration Wolf predation Slovakia
We thank Dr. J. Ciberej, Dr. M. Jurčík, and Dr. D. Rajský for supervising the immobilisations. We also thank the University Forest Enterprise and the Urban Forests of Kremnica for the field assistance and National Forestry Centre for providing GIS forestry data. Special thanks to Dr. J. Sládek, Dr. M. Svitok, Dr. P. Garaj Sr., Dr. K. Střelcová, and Dr. J. Luptáková for their advices and to field assistants Ľ. Ferlica, Dr. M. Ostrihoň, and Dr. P. Garaj Jr. All applicable national and international guidelines for the care and use of animals were followed.
This paper was produced within the project APVV-14-0637 supported by the Ministry of Education, Science, Research and Sport of the Slovak Republic and projects ITMS 26220120006 and ITMS 26220120049 supported by the Operational Programme Research and Development within the European Regional Development Fund.
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