Biogeosciences News

Reindeer Have to Eat Up to Survive the Winter

Arctic biologists use 25 years of data to find that warmer autumns might be enough to increase the odds of reindeer’s winter survival on Svalbard.

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The Arctic archipelago of Svalbard is an extreme environment whose short summers and harsh winters challenge the survival of everything that lives there. Right now, as ice blankets the dark landscape, the Norwegian islands’ endemic subspecies of reindeer are fighting for their lives. Arctic biologists want to know what helps them survive.

Svalbard reindeer—distinguished from their continental relatives by shorter legs and more rounded heads—are a hardy bunch, snacking on low vegetation during the archipelago’s brief growing season to pack on pounds necessary to survive the winter. When ice coats the landscape from January through March, they have only their fat reserves to rely on.

Now biologist Leif Loe at the Norwegian University of Life Sciences in Oslo; Steven Albon at the James Hutton Institute in Aberdeen, Scotland; and a team of climate modelers are asking the question, Is Svalbard’s warmer autumnal growing season enough to tip the scales for reindeer survival?

On Spitsbergen, the largest island in Svalbard, scientists have been tracking the same herd of about 850 reindeer for more than 2 decades. By observing climatic and environmental factors and how they affect population dynamics, scientists laid the groundwork for understanding what controls the success or failure of a herd. The new research from the team led by Loe, published in Global Change Biology in November, shares how individual animals’ survival is affected by one important factor: access to food in the fall.

Reindeer population research on Svalbard has long focused on the effect of ice storms, called rain on snow (ROS) events, which coat the landscape in ice and prevent deer from accessing ground-level vegetation. Without access to food, reindeer can die of starvation, sometimes en masse.

The Autumn Effect

Every April for 25 years, biologists painstakingly tagged individual reindeer and measured their body mass. In 2009, they added GPS tracking collars to 65 reindeer. This long-term data set gives scientists unprecedented insight into how Arctic warming affects individual behaviors as well as population changes, providing nuanced information that helped Loe’s team tease out the causes of population growth or loss.

The team found that although ROS events certainly play a role, they’re not the only control on a Svalbard reindeer’s survival. Instead, Loe turned his focus to the region’s longer, warmer autumns and their effect on food supply. “These [ROS] icing events on vegetation tend to get a lot of attention, including from some of our previous work,” he said. “It wasn’t until we started to look at the pattern of snow in autumn that we realized how strong and important the autumn effect was.”

The “autumn effect” describes the positive relationship between warmer autumns and reindeer’s access to vegetation. With delayed snowfall, the reindeer can eat exposed plants later in the year. In addition, with higher summer temperatures, scientists have seen up to a “doubling of plant biomass,” Loe explained, “so warmer temperatures have a double-positive effect. Reindeer are able to feed up and store more fat prior to the autumn, and if the snow comes late, they can start to use their fat reserves later.”

By pairing field observations with detailed climate models, Loe and his colleagues found that reindeer with higher body masses had better odds of surviving winter and that body masses generally increased with warmer autumns because snow coated the vegetation later in the season. Critically, this relationship between food access and winter survival was just strong enough to balance the risk of ice storms.

“Warmer autumns more than offset the negative effects of icy winters,” Loe said, with the delay in autumn snow leading to about 20% population growth. In fact, warmer autumns are “enough to counteract all but the most extreme icing events,” the paper states.

Survival Is Still Uncertain

That the autumn effect is correlated with better survival rates is not the whole story, researchers pointed out. “In some cases, the climate change card is pulled too easily. There are lots of important underlying factors,” Loe said. The herd’s age structure and population density, as well as delayed effects from the previous year, can affect population changes. And, of course, warmer temperatures don’t always suit reindeer. “On very warm summer days, they are less active,” he explained. “They seek out cooler ground and snow patches to rest on. There are both positive and negative effects of continued warming.”

Arctic biologist Mathilde Le Moullec of the Norwegian University of Science and Technology, who was not involved in the study, was also careful to point out that because climate change affects so many environmental factors, each season sees a different combination of effects.

“If you really want to grasp what’s happening in the system, you need these annual measurements,” Le Moullec said. “We’re really starting to get an understanding of what’s happening with climate change, because…as climate warms, it’s not just that the mean [temperature] increases, but also [the climate’s] variability increases. So when you have a bad combination year,” like early autumn snow and multiple ice storms in winter, “the population can still crash.”

—Becca Dzombak (@bdzombak), Science Writer

Citation: Dzombak, B. (2021), Reindeer have to eat up to survive the winter, Eos, 102, https://doi.org/10.1029/2021EO153981. Published on 29 January 2021.
Text © 2021. The authors. CC BY-NC-ND 3.0
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