A young orca from the southern population chasing its dinner. (Image Credit: JC Winkler, CC BY 2.0, Image Cropped)

Long-distance migration of prey synchronizes demographic rates of top predators across broad spatial scales (2016) Ward et al, Ecosphere, DOI: https://doi.org/10.1002/ecs2.1276

The Crux

Populations that experience some kind of connection are classified as “meta-populations”, as they are all interconnected in some way and can influence one another. Although these populations may be geographically and reproductively isolated, meaning that they are in different places and the organisms from the different populations don’t breed with one another, certain environmental factors may cause these populations to grow or shrink in similar ways.

The key to understanding how this synchrony between the varying populations happens is understanding what connects them. Killer whale (orca) populations in the northeast Pacific Ocean inhabit three distinct areas, with orcas from the northern and southern populations never coming into contact with one another. They do, however, feed on the same salmon populations that migrate from where the southern population lives to the where the northern lives. The authors wanted to find out if this connection via a food source could result in the demographic rates of these distant populations syncing up.

Did You Know: Demographic Rates

In ecology, a fundamental part of understanding how food webs work is understanding how a population grows or decreases in size. In order to do that we use demographic rates, with a demographic rate being the amount that a population changes over time. The easiest way to think of this is with birth and death events, like this study considers. If a population has more births than deaths, it will grow over time and have a positive demographic rate, while a population with more deaths than births has a negative demographic rate.

What They Did

The authors used mark-recapture data collected from 346 whales over a 30+ year span to estimate temporal synchrony of fecundity and survival. Basically, did the orcas from the two populations give birth and/or die at the same time. They used detection models from photo records of the whales taken near Washington state and southeast Alaska, and implemented a variety of statistical methods to determine the probabilities of birth and death for the the orcas in the two populations.

Because birth rates are age-dependent (number of years), and survival/death rates are life-stage dependent (young or old), the authors also broke the populations up by both age in years and life-stage.

What Did They Find Out?

Both survival and fecundity varied quite a bit within populations through time, meaning that the demographic rate of a population wasn’t always the same year-to-year. That being said, both the survival and fecundity of the southern population was correlated with that of the northern population, meaning that as demographic rates changed in one population they changed in the same way (and about the same amount) in the other population.

This is striking, because these two populations never come into contact with one another and live in different environments. In addition, the northern population tends to do better than the southern, in that overall survival for the different ages and sexes was always higher in the northern populations (the exception being the juveniles, where survival was 50% for both populations). The predicted fecundity of the northern population was also higher in the northern population.

So What?

This paper offered compelling evidence for population connectivity across enormous spatial scales. The primary food source for these orcas, Chinook salmon, migrate from their spawning grounds in the rivers of the northern United States all the way up the Canadian coast to the southern coast of Alaska. The fact that these demographic rates sync up so much is evidence that these orcas are not only consuming the same kind of food, but likely the exact same prey populations.

When a prey species like the Chinook salmon is affected by some kind of environmental disturbance it has consequences for the rest of the environment. Although we may only consider grizzly bears catching salmon on a waterfall when we think of these migrating fish, this paper has shown that not only will the predators at the source be affected by changes in the salmon numbers, but organisms thousands of kilometers away will also be affected by that change.