Contrasting effects of summer and winter warming on body mass explain population dynamics in a food-limited Arctic herbivore.

Steve D Albon,R Justin Irvine,Odd Halvorsen,Rolf Langvatn,Torkild Tveraa,Audun Stien,Brage B Hansen,Aline M Lee,Vebjørn Veiberg,Leif E Loe,Elizabeth I Duff,René Van Der Wal,Eirin M Bjørkvoll,Erik Ropstad

doi:10.1111/gcb.13435

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.

Highlights

IntroductionThe resulting increase in available forage for vertebrate herbivores (Hill & Henry, 2011; Van der Wal & Stien, 2014) should enhance body growth, fecundity and survival (Parker et al, 2009), and lead to increases in population size locally (geese: Morrissette et al, 2010; Jensen et al, 2014; musk oxen: Forchhammer et al, 2008; reindeer: Tveraa et al, 2013)
There was too little overlap between the October and February body mass time-series to test for a correlation, but the above result suggests that between-year variation in overwinter mass loss was already apparent in February
The incidence of ‘rain-on-snow’ in warmer winters which leads to starvation due to ice-locked pastures (Hansen et al, 2010) depresses late winter body mass

Summary

Introduction

The resulting increase in available forage for vertebrate herbivores (Hill & Henry, 2011; Van der Wal & Stien, 2014) should enhance body growth, fecundity and survival (Parker et al, 2009), and lead to increases in population size locally (geese: Morrissette et al, 2010; Jensen et al, 2014; musk oxen: Forchhammer et al, 2008; reindeer: Tveraa et al, 2013) As some of these herbivores occur at high densities, changes in their numbers may have profound consequences for tundra plant communities (Van der Wal, 2006), predators (Gilg et al, 2009) and ecosystem processes (e.g. CO2 flux: Sjo€gersten et al, 2008) across the Arctic. While ‘bottom-up’ processes may be dominant on Svalbard, elsewhere in the Arctic top-down processes, including predation, may be changing, especially for small vertebrate herbivores (Legagneux et al, 2012, 2014)

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Conclusion