Paper: Ferguson et al. (2017), Demographic, ecological, and physiological responses of ringed seals to an abrupt decline in sea ice availability. PeerJ 5:e2957; DOI 10.7717/peerj.2957
Climate change is happening at a global scale as the planet warms. One of the main story lines of climate change is the loss of sea ice in polar regions, particularly as it relates to habitat for mammals. Questions remain, however, as to what that means for a given species, habitat, or ecosystem. Many researchers endeavor to predict those outcomes, but this isn’t an easy task. One group of scientists, Ferguson et al., are attempting to determine what happens to ringed seal (Pusa hispida) (Fig. 1) populations in the Arctic. To do so, they need to know about the demographic changes they go through in response to environmental stressors, such as disruption to reproduction, low pup survival, high mortality—anything that affects their population structure.
Seasonality of Arctic Sea ice is critical for ringed seals. They require sea ice in the Spring when they molt—an extended time period during which they shed their fur and skin and grow a new coat—and reproduce and the ice-free season of the summer to forage in open water and build up their fat stores. During winter, they give birth and nurse but are restricted to smaller ranges with less available food. While they are well-adapted to the seasonality of sea ice, sea ice is declining in response to warming. Ferguson et al. decided to examine ringed seals in Hudson Bay, one of the southernmost reaches of their distribution as well as the extent of sea ice. The area goes through a complete cycle of ice loss and re-formation.
Ferguson et al. gathered a variety of existing datasets and set about looking for patterns. There were three datasets:
- Sea Ice Data: Trends in sea ice breakup and formation from the Canadian Ice Service 1979-2014.
- Climate Data: Major climate indices—the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and El Nino-Southern Oscillation (ENSO)—from the Climate Prediction Center through NOAA.
- Biological data: Data including body condition, reproductive condition, pup recruitment, and stress level (amount of cortisol) from 1,425 seals harvested as part of Inuit subsistence hunting in Hudson Bay between 2003 and 2013.
They wrote general linear models between environmental and biological variables to establish relationships. Basically, they wrote a bunch of equations relating the datasets to each other to determine the relationships between sea ice, climate patterns, and seal population health.
By analyzing the data sets, researchers found that sea ice is gradually breaking up earlier and freezing up later, with greater variability in ice break-up than freeze-up. This means that the ice season is getting shorter! Between 1979 and 2014, there was no relationship between any of the climate indices and the dates of break-up or freeze-up. What this means is that the shortening of the sea ice season isn’t related to natural climate patterns, but instead likely a direct result of human induced climate change. The longest ice-free season occurred in 2010 with ice break-up arriving in May and not freezing again until January of 2011 (Fig. 2).
In addition, researchers found that ringed seal body condition declined from 55.4% blubber mass in 2004 to only 40.3% in 2012, with the caveat that it did increase again to 48.1% in 2013. The decline in body condition was related to the increased period of open water (shorter sea ice season). Cortisol concentration (a measure of stress) increased over time in ringed seals, and decreased with the NAO index. In 2010, cortisol levels were high and showed high variability. Ovulation rates were low the following year (2011), likely attributable to the high stress ringed seals experienced in 2010 (Fig. 3).
What that might mean and the Bigger Picture
This study showed declining ringed seal body condition is concurrent with sea ice decline, one of the many consequences of climate change. In addition, the study documents a relationship between the 2010 climatic event and ringed seal demographic changes, as body conditions were reduced, seals were stressed, and ovulation in seals decreased, leading to fewer pups in the following years (Oh no! They’re so cute!!! See Fig. 4). This climatic event is linked to large-scale climatic patterns and cycles of good and bad times for seals are therefore expected to continue. On the positive side, they also showed some recovery in the years following 2010. That cohort will be smaller and the population will thereby be a bit diminished, but there is still some hope to be found.
The authors went on to discuss the mechanism by which ringed seal body condition was reduced by and stress increased by sea ice loss (or other factors). The longer open water period of 2010 may have affected the ringed seals’ access to prey items as well as the abundance and distribution of the prey. It may also have disrupted their molting with such an early break-up in ice, predisposing them to disease—which they did exhibit symptoms of (based on anecdotal evidence of anomalous lethargic behavior). Illness may in turn increase their risk of predation by polar bears. All told, the mechanism behind their decline in body condition with a greater open water period is not well understood yet.
According to atmospheric patterns, episodic events such as the one in 2010 are expected to occur every 10-15 years but be unpredictable. It is the combination of climate-change-driven gradual sea ice loss and these unpredictable episodic events that is most likely to have major implications for ringed seal body condition and in the long-term their abundance and distribution.
This research provides more information on what may be expected in the future, but prediction of the future is still elusive. Further research will be required to determine the mechanism behind body condition declines with sea ice loss, how seal populations will adapt to climate change, and how the larger ecosystems will then be affected.
Engage: What do you think? Are ringed seals set to be the new face of species impacted by climate change? Sound off in the comments!