The Heat Is Online

Weather extremes drive synchronized population crashes among animals

Extreme Climate Events Create Synchronized Population Fluctuations, Jan. 18, 2013

Scientists have known that climate change affects the population dynamics of single species, such as reindeer or caribou, but the effects of such climate change at the community level have been much harder to document and quantify.

A new study from the Norwegian University of Science and Technology (NTNU) has found that extreme climate events cause a synchronized population fluctuation among all vertebrate species in a relatively simple high arctic community. The findings, published in a recent issue of Science, may be a bellwether of the radical changes in ecosystem stability that could possibly result from anticipated future increases in extreme events.

The team from the Centre for Conservation Biology at NTNU investigated how climate and weather events influenced an overwintering vertebrate community on the high arctic island of Spitsbergen. The scientists chose this particular ecosystem because it is relatively simple. The ecosystem is comprised of three herbivores in winter – the wild Svalbard reindeer (Rangifer tarandus platyrhynchus), the Svalbard rock ptarmigan (Lagopus muta hyperborea), and the sibling vole (Microtus levis) – and one shared consumer, the arctic fox (Vulpes lagopus).

Rain-on-snow events are the driving factor for the community’s population fluctuations. An extreme climatic occurrence that causes icing on the deep-frozen tundra, rain-on-snow causes ice that keeps reindeer from grazing on their winter pasture and reduces food availability for both the rock ptarmigan and the sibling vole. This causes extensive, simultaneous population crashes in all three species in the winter and spring.

Unlike the herbivores, the arctic fox population, which relies mainly on reindeer carcasses as its terrestrial winter food source, did not see a population size decline until a year after the herbivore die-offs. The synchronized die-offs reduce the number of live prey for the foxes; however, the reindeer carcasses generate an abundance of food the foxes can survive on during icy winters and the following spring and summer. This food abundance leads to high fox reproduction.

In the following winter, however, there will be almost no reindeer carcasses available. The reindeer who survived the previous winter are more robust and have reduced competition for food resources. Simultaneously, none of the other herbivores are able to recover in the summer after the icing, with the net result being low fox reproduction and a strong reduction in the arctic fox population size one year after the herbivore die-offs.

“We have known for a long time that climate can synchronize populations of the same species, but these findings suggest that climate and particularly extreme weather events may also synchronize entire communities of species,” says Brage Bremset Hansen, from NTNU’s Centre for Conservation Biology. “Svalbard’s relatively simple ecosystem, which lacks specialist predators, combined with large weather fluctuations from year to year and strong climate signals in the population dynamics of herbivores, are the likely explanations for how such clear climate effects can be observed at the ecosystem level.”

Hansen said that in other, more complex systems community-level effects of climate can be present. They are more likely masked by other factors that tend to obscure the synchronizing effects of climate, complicating the picture.

In most of the Arctic, compared with Svalbard’s oceanic climate that is mild for the latitude, extreme rain-on-snow events are rare. The frequency of such rain-on-snow events leading to icing is closely linked to a rapidly warming arctic climate, however. This leads the team to warn that changes in winter climate and extreme events may have important implications for ecosystem functioning and stability in the circumpolar Arctic in the future.

“Previous studies have shown that rain-on-snow and icing can also cause vegetation damage and reduce survival of soil microbiota,” says Hansen. “But more importantly, we suspect that the strong effects of icing on the overwintering vertebrate community have the potential to indirectly influence other species and cascade throughout the food web. The die-offs among resident herbivores shape predator abundance, which could in turn affect the migratory prey that reside in the area in the summer, such as sea birds and barnacle geese.”