Expanding Killer Whale Distribution in the Arctic with Melting Ice

          As the world’s oceans warm from global climate change, marine organisms have to adjust to altered habitat conditions and subsequent changes in population dynamics. As temperate waters warm, certain species are able to move further towards the poles where suitable conditions now exist. With the invasion of species into new waters, previously established marine populations are in threat of being outcompeted for resources or becoming prey to unfamiliar predators. These consequences of climate change are apparent in Artic regions, where rising temperatures are decreasing ice coverage, allowing normally ice-avoiding species to disperse further North than in previous decades. As high latitudes are expected to show the most pronounced effects of climate change, scientists are beginning to study how organisms and ecosystems are changing in these increasingly ice-free areas.

One species benefiting from decreased ice coverage in Artic waters is Orcinus orca, or the killer whale. Though found almost globally, killer whale populations differ from one another with their specific environmental adaptations, dietary habits, and geographic range, resulting in what are called different ‘ecotypes.’ Residential ecotypes exclusively prey on fish, while the transient ecotypes, or ‘visiting’ populations, will only prey on marine mammals such as seal, sea birds, and other whales. Though these transient ecotypes hunt along ice edges, sometimes even flipping small ice floats to overturn resting seal, they tend to avoid the larger, heavier ice surfaces.

Two killer whales about to flip over an ice float a seal is resting on. (http://e360.yale.edu/images/features/antarctica_killer_whales_seal_e360.jpg)

Because of this ‘ice-avoidance’ behavior, killer whale distribution is usually restricted geographically and seasonally, dependent upon the surface ice cycle; though sea ice is prevalent year-round in some areas, surface ice generally breaks apart or melts in August, and reforms by November. However, with increasing temperatures, this melt-reform cycle no longer occurs in some areas, decreasing habitat availability to ice-adapted marine mammals and potentially increasing accessibility to species such as the killer whale. As killer whales are an apex predator, or a predator at the very top of the food chain, their expanded distribution in the Arctic may have a significant impact on pre-existing population dynamics. It is therefore important to see whether sea ice reduction really is redistributing the transient ecotypes, a question Jeff Higdon and Steven Ferguson addressed in their paper ‘Loss of Arctic sea ice causing punctuated change in sightings of killer whales (Orcinus orca) over the past century’ (2009.)

In their study, the authors compiled historic data on July sea ice concentration in the Canadian Arctic from 1900 to 2006 and reported killer whale sightings in the Hudson Bay region for the same time period. They found that, since 1900, the number of killer whale sightings per decade increased exponentially while median ice concentration over time showed a significantly declining trend. In sum, they found that ice concentration in the Hudson Strait was negatively correlated with the number of killer whale sightings reported per decade, as well as with the range of sightings. This means that with receding ice coverage in the Hudson Strait, killer whales are no longer spatially restricted, seen more frequently in areas further west than ever before.

Median ice concentration from 1902 to 2004 taken from three areas of the Hudson Strait (Western, Central, and Eastern.) All three areas show a significant decline in ice concentration.

Due to its apex status, the killer whale’s increased range poses as a top-down ecosystem force. In other words, its recent expansion in places like the Hudson Strait will result in a significant increase of predation on whales and seals, which will decrease their predation on other species. Since sea ice is predicted to decrease even more within the next few decades, the killer whale’s distribution will most likely continue to expand, impeding upon the habitats of other apex predators such as the polar bear, who rely on high ice concentrations. Higdon and Ferguson conclude by predicting that with this expanded distribution, killer whales will most likely replace polar bears as the dominant predator of the Arctic, changing the marine mammal community in this environment.

With global climate change, the distribution and abundance of organisms will change drastically. This study is just one example of how a predator’s range expansion has the potential to drastically change an entire ecosystem’s community dynamics, and similar shifts should be expected in the future. It has been projected that within the next 50 years, surface ice coverage in the Arctic may eventually be non-existent, making it even more pertinent to study the potential effects this will have upon species’ presence and abundance. Though it now seems inevitable that species such as the killer whale will continue moving North, learning of the consequences now may better prepare us for future management of fisheries that will be impacted as well as form potential conservation plans for species in these rapidly changing environments.

Reference:

Higdon, J.W, Ferguson, S.H. Loss of Arctic sea ice causing punctuated change in sightings of killer whales (Orcinus orca) over the past century. 2009. Ecological Applications

19(5): 1365-1375