Extreme weather synchronises Arctic populations

An Arctic fox in Svalbard. Fox population decline was a year behind the herbivores, as it first fed on those species' carcasses. Credit: Brage Bremset Hansen

BRISTOL: Extreme weather events can put the birth and mortality rates of four different Arctic species – reindeer, rock ptarmigan, sibling vole and arctic fox – in sync with each other, according to new research.

The synchronising effect of extreme weather has previously been shown in separate populations of the same species. However, the new study, published in Science, is the first compelling evidence that climate can have a synchronising effect across different species within a community.

The Norwegian authors link synchronised population declines in Norway’s Svalbard archipelago in the high Arctic with rain-on-snow events that make winter foraging difficult for herbivores.

With more rain-on-snow events predicted for the region in the future, the authors say their results may be an indicator of how important future climate change could be in shaping these terrestrial Arctic communities.

“There will be local extinctions”

“It’s very difficult to predict anything about future climate change and how it will affect different species,” said Brage Bremset Hansen of the Norwegian University of Science and Technology and lead author on the study.

“If these icing events become more frequent, it may happen that there will be local extinctions. These reindeer are isolated into quite separate populations due to fjords and glaciers, so in areas where icing events are more frequent, these local sub-populations may be more affected.”

Hansen and his colleagues looked at years of population data on the reindeer, ptarmigan (a game bird), vole (a small rodent) and fox populations on the island of Spitsbergen, and correlated these with climate data from a nearby weather station. Due to the relative simplicity of the over-wintering community on this island, the researchers were able to rule out factors such as predation – which can also drive changes in population size among species – and isolate the synchronising effects of extreme weather events.

First evidence across species

The authors found that winters with more frequent rain-on-snow events decreased plant availability for the three herbivore species and drove population sizes down. The increased mortality among herbivores provided abundant carcasses for Arctic foxes to scavenge, so the decline in the fox populations wasn’t seen until the following year.

The synchronising effect of climate across populations of the same species is well recognised among population ecologists and is known as the Moran effect. However, this is the first evidence in support of the Moran effect across four different species.

Eric Post, a biologist at Penn State University in the U.S., also works on the population dynamics of Arctic vertebrates. He was not involved with this study, but recognised the contribution this research makes to his field.

“This is compelling evidence for the Moran effect across an entire community of species,” said Post. “This is important from a climate change perspective, because if you do get this entrainment or synchronisation by climate, what this suggests is that climate can drive multiple populations down simultaneously. This results in the potential for local extinction, or in the case of what’s going on in Svalbard, community-wide population suppression.”

Gradual increase in reindeer population

Svalbard is relatively warm for its latitude, and therefore receives more rain than other areas of the Arctic. If a warming Arctic climate is predicted to bring with it more rain and therefore more extreme ice-on-snow events, are the results from Svalbard an indicator of what’s to come for other Arctic communities?

Nicholas Tyler, from the Centre of Saami Studies at the University of Tromsø in Norway, has looked extensively at unfavourable snow and ice conditions as a factor in declining reindeer and caribou populations and has studied populations in this region. He suggests that there is little evidence to support the view that icing events are a principal cause of major declines in population size of reindeer.

“The reindeer populations in Svalbard are oscillating,” said Tyler, “but, generally there is a gradual population increase, despite more frequent rain on snow events in recent years.”

Tyler also warns not to extrapolate the findings too far:

“The authors of this study have described the synchronising effects of weather on this community beautifully. However, different populations have extremely variable responses to the same climate signals. People tend to make great sweeping statements about the effects of climate change on species, but what they really mean is that they have observed the effect a local change in weather has had on a local population.”