Extreme warming events may sound like bad news to reindeer, but they could help increase population stability (Image Credit: Christopher Michel, CC BY 2.0, Image Cropped)

More frequent extreme climate events stabilize reindeer population dynamics (2019) Hansen et al., Nature Communications, https://doi.org/10.1038/s41467-019-09332-5

The Crux

Whilst climate change has been causing (and will cause) a myriad of environmental problems, it’s important to remember that not all species will be negatively affected by more extreme weather events. One example is reindeer on the Arctic island of Svalbard, according to this week’s paper.

Taken at face value, an increased frequency of extreme warming events may not sound like a good idea for a cold-adapted species. But despite the fact that it can lead to rain falling and freezing over snow, rendering massive patches of food inaccessible, the authors show that this can actually lead to increased population stability.

What They Did

The population of reindeer on Svalbard is very well monitored, so the researchers were able to compare the effect of increased rain on snow (ROS) events on the population’s annual growth rate. They also calculated the effect ROS events had on different age groups, and on the effect it had when the population was at different densities.

They used this data to simulate the effect of increased ROS events on the population, comparing this to current trends. They were then able to compare the possibility of extinction over the two scenarios.

Did You Know: The Social Impact of Reindeer The Nordic countries have traditionally practiced open herding of reindeer, whereby herds are owned, but not fenced in, and as such are more vulnerable to predators. This has led to a rocky relationship with predators in these areas, and as a result species like wolves, bears, and wolverines were driven out of the areas. They have started to filter back in over the last 30-40 years, but this rewilding provokes constant controversy.

What They Found

As expected, years with high-frequency ROS events led to a drop in reindeer population growth rates. However this drop was markedly less in years with an already low population (more food to go around) and affected very young and very old (more sensitive to low resources) age groups.

Yet this led to increased stability within the population. The loss of very young and very old individuals led to a population which had more individuals in their prime, which increased population recovery rates after crashes, as did the lowered density of the population. So whilst the population was lower with increased extreme warming events, its probability of going quasi-extinct (defined here as the population dropping to less than 100) was ten times lower.

Problems

Whilst the message of this paper is an important one, it’s important to note that Svalbard is a somewhat unique ecosystem. There are no wolves on the island, and the reindeer’s only concern outside of resources is man. I would love to see results of a similar study on mainland Europe.

So What?

Reindeer hunting on Svalbard is carefully monitored. Knowing that populations could become more stable over the coming decades could significantly impact hunting quotas. It could also have profound impacts on summer vegetation on the island as well.

But on a wider scale, this represents an important message – that the long term effects of climate change need to be studied with an open mind. Particular attention needs to be paid to the interaction of density dependence and extreme events.