Spatiotemporal variation in abundance of key tundra species: from local heterogeneity to large-scale synchrony

Abstract

Summary of thesis:

From local heterogeneity to large-scale synchrony of wildlife abundance in the high Arctic.

The Arctic is a ‘hot-spot’ for climate change, and understanding its impacts on tundra ecosystems will help predict consequences on global biodiversity. In my PhD, I studied how climate determine variation in tundra wildlife abundance through time and at different spatial scales. Climate can synchronize the growth from distant populations, which can lead to large-scale extinctions under extreme climate events. My work focused on two key species in Svalbard: the wild Svalbard reindeer and the polar willow. I first tested different monitoring tools locally, for then to apply these methodologies across large-scales. During three summers, I collected polar willows and monitored reindeer with ‘distance sampling’ and total counts by sailing and hiking all around Svalbard. These reindeer monitoring methodologies were proven highly accurate. I also found that the annual ‘tree-ring’ growth of the tiny polar willow reliably traced vegetation production back in time. Reindeer were distributed according to vegetation productivity, and the total population size appeared twice as large as previously assumed, even though recovery from over-hunting a century ago is still ongoing. Across Svalbard, I found contrasting effects of summer versus winter climate warming. Warm summers increased plant growth in all sites, causing spatial synchrony in growth. Reindeer benefit from such increased plant production but suffer from warm winters, when rain-on-snow events encapsulate plants in thick ice-layers. Rain-on-snow thus partly explained spatial synchrony in reindeer abundance fluctuations, but varying strength of summer versus winter warming has resulted in locally contrasting reindeer population trends. Such spatial heterogeneity may counteract synchronizing effects of climate on abundance fluctuations across large areas, thereby avoiding mass extinctions. The polar willow was only influenced by rain-on-snow in coastal sites exposed to icing-events. However, since we are heading toward a rain-dominated Arctic, rain-on-snow may become a synchronizing agent of wildlife fluctuations across the food chain.