title: Contrasting effects of summer and winter warming on body mass explain population dynamics in a food-limited Arctic herbivore.
abstract: The cumulative effects of climate warming on herbivore vital rates and population dynamics are hard to predict, given that the expected effects differ between seasons. In the Arctic, warmer summers enhance plant growth which should lead to heavier and more fertile individuals in the autumn. Conversely, warm spells in winter with rainfall (rain-on-snow) can cause 'icing', restricting access to forage, resulting in starvation, lower survival and fecundity. As body condition is a 'barometer' of energy demands relative to energy intake, we explored the causes and consequences of variation in body mass of wild female Svalbard reindeer (Rangifer tarandus platyrhynchus) from 1994 to 2015, a period of marked climate warming. Late winter (April) body mass explained 88% of the between-year variation in population growth rate, because it strongly influenced reproductive loss, and hence subsequent fecundity (92%), as well as survival (94%) and recruitment (93%). Autumn (October) body mass affected ovulation rates but did not affect fecundity. April body mass showed no long-term trend (coefficient of variation, CV = 8.8%) and was higher following warm autumn (October) weather, reflecting delays in winter onset, but most strongly, and negatively, related to 'rain-on-snow' events. October body mass (CV = 2.5%) increased over the study due to higher plant productivity in the increasingly warm summers. Density-dependent mass change suggested competition for resources in both winter and summer but was less pronounced in recent years, despite an increasing population size. While continued climate warming is expected to increase the carrying capacity of the high Arctic tundra, it is also likely to cause more frequent icing events. Our analyses suggest that these contrasting effects may cause larger seasonal fluctuations in body mass and vital rates. Overall our findings provide an important 'missing' mechanistic link in the current understanding of the population biology of a keystone species in a rapidly warming Arctic.
authors.affiliation: The James Hutton Institute, Aberdeen, AB15 8QH, UK.
authors.forename: Steve D
authors.initials: SD
authors.lastname: Albon
authors.affiliation: The James Hutton Institute, Aberdeen, AB15 8QH, UK.
authors.forename: R Justin
authors.initials: RJ
authors.lastname: Irvine
authors.affiliation: Natural History Museum, University of Oslo, Box 1172 Blindern, NO-0318, Oslo, Norway.
authors.forename: Odd
authors.initials: O
authors.lastname: Halvorsen
authors.affiliation: University Courses in Svalbard (UNIS), P.O. Box 156, NO-9171, Longyearbyen, Norway.|Norwegian Institute for Nature Research (NINA), P.O. Box 5685 Sluppen, NO-7485, Trondheim, Norway.
authors.forename: Rolf
authors.initials: R
authors.lastname: Langvatn
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432, Aas, Norway.
authors.forename: Leif E
authors.initials: LE
authors.lastname: Loe
authors.affiliation: Norwegian University of Life Sciences, P.O. Box 8146, NO-0033, Oslo, Norway.
authors.forename: Erik
authors.initials: E
authors.lastname: Ropstad
authors.affiliation: Norwegian Institute for Nature Research (NINA), P.O. Box 5685 Sluppen, NO-7485, Trondheim, Norway.
authors.forename: Vebjørn
authors.initials: V
authors.lastname: Veiberg
authors.affiliation: Aberdeen Centre for Environmental Sustainability (ACES), School of Biological Sciences, University of Aberdeen, Aberdeen, AB24 3UU, UK.
authors.forename: René
authors.initials: R
authors.lastname: van der Wal
authors.affiliation: Centre for Biodiversity Dynamics (CBD), Department of Biology, Norwegian University of Science & Technology (NTNU), N-7491, Trondheim, Norway.
authors.forename: Eirin M
authors.initials: EM
authors.lastname: Bjørkvoll
authors.affiliation: Biomathematics & Statistics Scotland (BioSS), Aberdeen, AB15 8QH, UK.
authors.forename: Elizabeth I
authors.initials: EI
authors.lastname: Duff
authors.affiliation: Centre for Biodiversity Dynamics (CBD), Department of Biology, Norwegian University of Science & Technology (NTNU), N-7491, Trondheim, Norway.
authors.forename: Brage B
authors.initials: BB
authors.lastname: Hansen
authors.affiliation: Centre for Biodiversity Dynamics (CBD), Department of Biology, Norwegian University of Science & Technology (NTNU), N-7491, Trondheim, Norway.
authors.forename: Aline M
authors.initials: AM
authors.lastname: Lee
authors.affiliation: Norwegian Institute for Nature Research (NINA), Fram Centre, NO-9296, Tromsø, Norway.
authors.forename: Torkild
authors.initials: T
authors.lastname: Tveraa
authors.affiliation: Norwegian Institute for Nature Research (NINA), Fram Centre, NO-9296, Tromsø, Norway.
authors.forename: Audun
authors.initials: A
authors.lastname: Stien
mesh_terms: D000818:Animals; D001110:Arctic Regions; D015992:Body Mass Index; D005260:Female; D060434:Herbivory; D011157:Population Dynamics; D012053:Reindeer; D012621:Seasons; D013538:Svalbard
keywords: 
  Rangifer
  ; Svalbard; climate change; density dependence; extreme events; icing; nutrition; primary production; reindeer; weather