Ducking the effects of climate change

by Cody Dey

Sea ice is an essential component of Arctic marine environments. It provides transportation routes for people, creates structure on which Arctic plankton can live, and helps regulate the global climate. Unfortunately, sea ice has been one of the first major casualties of global climate change. In the polar regions, sea ice is thinner and less widespread compared to any previous point in recorded history, and melts earlier in the spring.

So what does this mean for Arctic animals whose life cycle is linked to sea ice, such as polar bears and Arctic seabirds? This question forms the basis of my research at the University of Windsor and is important in understanding how Arctic ecosystems will adapt to climate change.

We recently explored how a shorter ice season will impact northern common eiders (Somateria mollissima borealis). This large seaduck is a key part of the Arctic ecological community: adult and juvenile eiders are prey for Arctic predators such as foxes, gulls, and various raptors. Additionally, the collection of eider eggs and down (which female eiders place in their nest for warmth) is a cultural tradition for many Inuit, who eat the eggs and use the down for clothing and blankets. Even eider poop is thought to play an important role in fertilizing Arctic islands: islands with large eider colonies have more plant and insect life than islands without breeding eiders!

Common eiders (David McGeachy, used with permission).

In Hudson Strait — the Arctic waterbody between northern Quebec and Baffin Island — sea ice loss is having three major impacts on eiders.

First, earlier ice melt in the spring gives female eiders longer to forage on mollusks, which live on the sea bed and would otherwise be blocked by ice. As a result, hens are able to lay more eggs when it comes time to breed.

Additionally, more feeding opportunities mean that more eider hens can reproduce each year. Laying eggs and caring for young is hard in the harsh Arctic environment, so most eider hens only reproduce every 2 to 3 years. However, increased access to food during the pre-breeding period has allowed a larger number of females to reproduce.

Common eider hen and chicks (Sjoerd Dujins, used with permission).

Unfortunately, not all of the climate change effects on eiders are positive. Earlier sea ice melt has also led to an increase in polar bear predation of eider nests. In some areas, polar bear predation is now a major issue for eiders, with over 90% of nests being predated each year in some colonies in Nunavut, Greenland, and Svalbard.

Polar bear foraging (Evan Richardson, used with permission).

Using a combination of field studies, analysis of long term datasets, and computer models, we generated forecasts of how these three factors would affect eider populations. To do so, we used an approach called “agent-based modelling,” which involves creating digital polar bears and eider hens that are programmed based on our knowledge of their behaviour. These agents were then let loose in a digital landscape that represented Hudson Strait, and allowed to interact over a 50-year period.

Somewhat unexpectedly, our models predicted a stable eider population size. The positive effects of climate change (laying more eggs and an increase breeding individuals) were almost completely cancelled out by an increase in nest predation by polar bears!

Predictions of changes in common eider population size. While increases in clutch size and breeding incidence lead to increases in the number of eiders, more polar bear predation lead to decreases in the number of eiders. When these effects are considered together, they almost completely cancel out. Data from: Dey et al. 2018. Biol Cons 220:94–103.

However, our models did predict other changes in eider populations. The largest eider colonies were visited by polar bears more often than the small colonies, because bears were able to eat a bigger meal and thus remembered the location of these large colonies in subsequent years. Over time, eider hens stopped nesting in these large colonies to avoid having their eggs eaten by bears. Instead, they nested in smaller groups that were spread out across the landscape.

One of the predicted effects of polar bear predation on common eiders is that they will stop nesting in large colonies. Instead, eiders will spread out in the landscape to avoid polar bears. Data from: Dey et al. 2018. Biol Cons 220:94–103.

These results are consistent with what other researchers have observed in the field. Polar bears preferentially visit certain eider colonies that are easy to access and have many nests. In response, eiders slowly change where they nest. Unfortunately, this makes it harder for northern people to collect eggs and down, both because polar bears might get there first (leading to the loss of many eggs and the down being dispersed by the hungry bears), and because there will be fewer large eider colonies from which Inuit have been harvesting for generations.

My research, along with other recent studies, demonstrates that climate change can have complex effects on Arctic animals. While we are definitely losing certain types of habitat such as sea ice, climate change is also leading to new species interactions, such as the increasing interaction between polar bears and common eiders. In some cases, these effects may cancel out and species populations may be stable– but in other cases they may not. Ecologists will need to use all their tools to understand new species interactions in the Arctic if we want to help protect northern ecosystems and understand the global impacts of climate change.

Cody Dey is a Liber Ero Fellow at the University of Windsor.