Science —

Killer whales stress narwhals out just by being in the area

As warmer Arctics let killer whales roam farther, the effects could be dramatic.

Pod of narwhals, northern Canada, August 2005. Image courtesy of Kristin Laidre.
Enlarge / Pod of narwhals, northern Canada, August 2005. Image courtesy of Kristin Laidre.
Dr. Kristin Laidre, Polar Science Center, UW NOAA/OAR/OER - NOAA Photolib Library

The warming of the Arctic is bad news for so many species, but for killer whales, it’s offering up a buffet. While the ice had previously prevented access to large areas—like Hudson Bay—the whales can now find their way into those waters during the summer. In areas that they could already access, they can now arrive earlier and stay later.

Obviously, any ecosystem that suddenly has new predators hanging around is going to be profoundly affected, but it’s possible that we’ve underestimated just how profoundly. A paper in this week’s PNAS finds that narwhals, one of many marine mammal species preyed on by killer whales, change their behavior substantially when they’re so much as sharing a fjord with killer whales. This kind of large, long-lasting behavioral change suggests that the mere presence of new predators changes ecosystems at a level previously unsuspected.

Predators can affect an ecosystem in the obvious way—by eating other animals—but also in less obvious ways, which are known as “nonconsumptive effects.” When prey species know, or think, that they’re at risk of being eaten, they change their behavior. They might avoid certain areas, which limits their access to food. They might be forced to make regular escapes or be permanently on the watch, which means a higher calorie burn. All of these mean fewer reproductive opportunities and more stress.

While the obvious problem of being eaten usually only affects a few individuals, nonconsumptive effects can have an impact on an entire population. In some cases, they can leave a deeper mark on the population than losing a few individuals does.

Unfortunately, studying nonconsumptive effects is really tricky. How do you tell which behaviors are responses to predators? In the ocean, animals cover huge distances, and it’s difficult to tell how many predators might be around in a particular region. So far, there’s data on how certain marine animals respond to a nightmarish event: where a killer whale sticks its nose up next to your floe and snatches your friend. But this isn't the kind of data you need to understand typical nonconsumptive effects.

Greg Breed of the University of Alaska led a team of researchers who studied these effects in narwhals. They used satellite tracking tags to track seven of the estimated 35,000 narwhals living in Admiralty Inlet, a 300 km-long, 50 km-wide fjord in the Canadian Arctic. They also tagged a killer whale, one of a group of around 16 individuals who moved and acted together.

For around 10 days in the late summer of 2009, the group of killer whales roamed in the inlet, allowing the researchers to compare the narwhals’ behavior during and after their presence. Importantly, the narwhals didn’t move as a unit, which means that similarities in their behavior weren’t caused by group behavior.

They found two very distinct patterns of movement in the narwhals. While the killer whales were around, the narwhals hugged the shore, mostly within 500m of the coastline. This seemed to reflect a preference for shallow water less than 100 m deep. Once the killer whales had left, the narwhals left the shoreline, opting for the deeper waters between four and 10km from the coast. The analysis showed that it didn’t seem to matter how close the narwhals actually were to the whales; just the presence of this predator in the inlet was enough to drive the behavior.

It’s not clear what this actually means for the narwhals' health, and researchers don’t speculate on that question. It’s possible that they prefer the deeper waters because of food opportunities, so hugging the shoreline might be bad for them. It's potentially a question for future research to look into.

That's one of the main reasons this research is important: it should help to steer future investigations. It’s important to try to anticipate the effect of killer whales’ greater geographical access on other marine species. Studies so far have tended to focus on straightforward predation effects (how many animals will be eaten?), but this research suggests that we need to understand the nonconsumptive effects better, too.

There’s also a risk of misinterpreting other kinds of data if nonconsumptive effects aren’t taken into account. If tracking data shows that animals tend to hang out in certain areas, it’s easy to assume that the animals prefer those areas because there’s more or better food, when in fact the animals might be there because it’s safer. The impacts of misunderstandings like these could be huge—for instance, we could end up protecting a less-than-ideal habitat because we thought a species liked it more. Gathering data on nonconsumptive effects might be tricky, but it should lead to better science.

PNAS, 2016. DOI: 10.1073/pnas.1611707114  (About DOIs).

Channel Ars Technica