The Paper: Atwood TC, Peacock E, McKinney MA, Lillie K, Wilson R, Douglas DC, et al. (2016) Rapid Environmental Change Drives Increased Land Use by an Arctic Marine Predator. PLoS ONE 11(6): e0155932. doi:10.1371/journal.pone.0155932.
The Times They Are A’Changin’
When the times change, it’s time for you to change. Or is it?
Everyone knows that polar bears have become the poster children for species threatened by climate change. And it’s for good reason that they are. Polar bears rely on sea ice for access to prey, finding mates, and creating dens. The persistence of the species depends on the state of sea-ice and more generally a healthy marine ecosystem in the Arctic. Unfortunately, the volume and extent of sea ice have been decreasing by 28% and 14% per decade. Is there a way for polar bears to adapt to the changing sea ice coverage in this sensitive habitat?
To respond to changes to their environment due to climate change, animals may need to modify their behavior (i.e. change timing of certain activities or shift distribution). Polar bears are already at the extreme and cannot change their distribution by moving further North (they already live at the pole!). Instead, polar bears may need to change certain behaviors to adjust to the changing environment or exhibit what scientists refer to as behavioral plasticity. Many behaviors are highly evolved and if too deeply established, they could become maladaptive and harm the population, contributing to its decline. Many populations of polar bears have spent the entire year on sea ice. As sea ice retreats to a certain point where bears can no longer access enough prey, they can no longer thrive. Some bears from some populations have been observed leaving the sea ice for shore during the summer and fall. Leaving the sea ice may not be a good strategy though, as it may confine bears to areas with new risk factors or few food sources. To contribute to species persistence, any behavior change will require an increased chance of survival through access to resources or minimized risk. Knowing how polar bears respond to sea-ice displacement may help predict how they’ll survive in their changing world. Fortunately, some researchers are studying this!
Who? The team of brave scientists (Atwood et al. 2016) and a population of polar bears that have historically spent the entire year on the sea ice.
Where? The southern Beaufort Sea, where sea ice extent has declined rapidly over the last 15 years and the melt season lengthened. (Fig. 2) This area is largely industrial, with oil and gas exploration and extraction. In addition, three communities in this area harvest bowhead whales in the fall (Fig. 3). The remains from these harvests are consistently aggregated at Cross Island and Barter Island and sporadically at Point Barrow.
Why? To evaluate 1) has the use of terrestrial habitat increased over time? 2) what characteristics of sea ice affect the timing of movement to land and back to ice? 3) where the polar bears go while on shore?
How? Adult female polar bears were fitted with satellite radio collars and their position was tracked to determine if polar bears were residing on terrestrial habitat in the summer. The goal was find out specifically when bears came ashore, how long they stayed, and when they returned to the sea ice. From 2010-2013, aerial surveys were conducted to characterize their distribution along shore (counts were conducted from the air). Sea ice characteristics were obtained from the National Snow and Ice Data Center (NSIDC) (yes, it exists!) and compared with the timing of polar bear use of terrestrial habitat. They compared data between two time periods (1986-1999 and 2000-2014).
Are Polar Bears Behaviorally Plastic?
The scientists did find that more bears have been leaving the ice for the terrestrial environment over the years (Fig. 4). Between 1986 and 1999, only 5.8% of bears came on shore (and stayed for at least 21 days), but between 2000-2014, that percentage increased to 20%, with a maximum of 37% in 2013. The majority of bears are staying on the sea ice during summer, though.
They’re also staying on shore longer. The average length of stay in 1986-99 was 20 days but from 2000-2014 was 56 days. Polar bears are arriving on shore (by about 5 days earlier per decade) staying longer (by about 7 days per decade), and departing later (by about 7 days per decade). Bears that come ashore are timing their arrival and departure around when the sea ice falls below or rises above 15% concentration as well as what proportion of the ice is at that 15% concentration and how far beyond the shelf sea ice at 15% concentration extends. The bears appear to wait until ice drops below that concentration, making it unstable to occupy at which time they transition to shore. They may come ashore before the widespread seasonal reduction in ice to avoid swimming long distances.
While on shore, they are not randomly distributed, but rather concentrated in close proximity to villages where bowhead whale remains are actively stocked. These villages participate in subsistence harvesting of bowhead whales (Fig. 3). Any portions of the whale not used (the remains) are added to a pile near the village (stocking). Following this stocking, the number of bears located nearby increases (Fig. 2). Before stocking, 64% of bears were observed within 16 km of the stocking sites, but this jumped to 78% afterward. The majority of bears were found near Barter Island (40%) and Cross Island (33%) where stocking is most consistent, while fewer bears were near Point Barrow (2%) where stocking is sporadic.
All told, it appears it may be adaptive for polar bears to come ashore, at least in the short-term. Summer sea ice that is retreating over the polar basin is not providing as many opportunities to encounter seals, leading to declines in polar bear health for those that stay on the sea ice. Those that stay on the ice may have the behavior too deeply rooted or be finding enough to survive and therefore staying where they know rather than exposing themselves to unknown risks. Currently, those that come ashore increase energy intake and minimize energy use by feeding on readily available whale remains.
Implications and Future Questions
As environmental conditions in the Arctic continue to change rapidly, behavioral plasticity is expected to be the initial response, likely to be followed by the transmission of these new behaviors. This may ultimately lead to evolution of the behavior of polar bears over time. That’s good news, right? We can stop worrying about the polar bears because they’ll adjust? There is reason for some optimism yes, but they are still at risk from these climate-driven environmental changes. If the environment changes faster than the transmission of an adaptive behavior occurs, the population of bears will not be able to keep up and will decline. Plus there may be more risks they face as they come ashore. They evolved to live on the ice. In an ideal world, they’d get to continue living there.
Something else that comes out of this study is an application for stakeholders. Managers, conservationists and industry professionals may be able to monitor the timing and rate of sea ice loss to better predict and prepare for polar bear arrival on shore. As more bears come ashore and stay longer, there is increased possibility of wildlife-human conflict that may affect the wildlife populations, cause economic declines, and endanger the safety of the public.
As always, every answer spawns many more questions. For example:
Are there greater or new risks involved when accessing terrestrial food resources, such as competition with grizzly bears?
How will exposure to humans impact the ability of bears to survive?
Will concentrations of bears near whale stocking areas increase the transmission of disease?
Can the terrestrial landscape support all the polar bears if they are forced to leave the sea ice?
Tell Me What You Think: Is this change in polar bear behavior overall good or bad? Or is it too early to tell? Let me know in the comments!
You can learn more about these issues and this research in the PBS series The Great Polar Bear Feast!