The neglected season: Warmer autumns counteract harsher winters and promote population growth in Arctic reindeer.

Leif Egil Loe,Audun Stien,Gabriel Pigeon,Adele K Reinking,Nir Horvitz,Erik Ropstad,Liv Monica Trondrud,Atle Mysterud,Vebjørn Veiberg,Glen E Liston,Torkild Tveraa,Lars Kvam Movik ,R Justin Irvine ,Brage Bremset Hansen ,Åshild Ønvik Pedersen ,Aline Magdalena Lee ,Wayne M Getz ,Mads C Forchhammer ,Kristin J Barker ,Steve Albon

doi:10.1111/gcb.15458

Abstract

Arctic ungulates are experiencing the most rapid climate warming on Earth. While concerns have been raised that more frequent icing events may cause die-offs, and earlier springs may generate a trophic mismatch in phenology, the effects of warming autumns have been largely neglected. We used 25years of individual-based data from a growing population of wild Svalbard reindeer, to test how warmer autumns enhance population growth. Delayed plant senescence had no effect, but a six-week delay in snow-onset (the observed data range) was estimated to increase late winter body mass by 10%. Because average late winter body mass explains 90% of the variation in population growth rates, such a delay in winter-onset would enable a population growth of r=0.20, sufficient to counteract all but the most extreme icing events. This study provides novel mechanistic insights into the consequences of climate change for Arctic herbivores, highlighting the positive impact of warming autumns on population viability, offsetting the impacts of harsher winters. Thus, the future for Arctic herbivores facing climate change may be brighter than the prevailing view.

Highlights

Climate change affects ecosystems and animal populations worldwide via altered temperature and precipitation regimes (Scheffers et al, 2016), causing changes in the timing, duration and characteristics of seasons
Combining rare long-term life-history data of an Arctic herbivore with a spatially explicit snow modelling approach, we demonstrated that the positive effect of delayed snow on late winter body mass, to a large extent counteracts the negative effects of harsher, icy winters and higher population densities
The most likely explanation for the population growth is that a delay in snow-onset provides a mechanism by which a warmer climate could enhance the plane of nutrition

Summary

| INTRODUCTION

Climate change affects ecosystems and animal populations worldwide via altered temperature and precipitation regimes (Scheffers et al, 2016), causing changes in the timing, duration and characteristics of seasons. Delayed autumns are likely to enhance the nutritional landscape of herbivores (Hurley et al, 2014; Parker et al, 2009), which may explain why warmer autumns are associated with higher body mass of Svalbard reindeer Rangifer tarandus platyrhynchus (Albon et al, 2017) The focus, both in the scientific literature and in media, has been on the negative effects of winter warming in the Arctic, because conditions are often harsher for resident herbivores due to more extreme ‘rain-on-snow’ (ROS) events which may encase forage in ice (Putkonen & Roe, 2003; Rennert et al, 2009). By evaluating the effects of changes in autumn snow and forage conditions on Arctic ungulates, our study contributes to our understanding of the impact of continued climate warming on these keystone species

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION