Weekend Reads | New DNA Evidence Tells Us How to Save the Orcas

by Kevin Schofield

This weekend's read is a pair of reports about our friendly neighborhood resident orcas, often referred to as "killer whales." Orcas are known to swim the waters of all the oceans of the world, but there are at least nine different subgroups (called "ecotypes" by scientists) that frequent specific geographic areas and have specific differences in body shape, behavior, and diet. More recently, scientists have also documented differences in DNA across the ecotypes, which has led to questions about whether any of the orca ecotypes are actually their own subspecies — or a separate species altogether. The first of this weekend's two papers, by researchers at the National Marine Fisheries Service, the University of British Columbia, and Oregon State University, looks at whether the two ecotypes that frequent Puget Sound should be classified as separate subspecies or species.

Three orca ecotypes live in the north Pacific Ocean: "resident" orcas that spend the summers swimming around Vancouver Island and the San Juan Islands; "transient" orcas (also known as "Bigg's orcas") that come and go year-round, and "offshore" orcas that rarely make appearances in our local waters. It's well-documented that the residents feed almost entirely on chinook salmon, while the transients hunt other marine mammals. Both residents and transients live in familial "pods," but resident and transient pods tend to avoid each other, and there are no documented cases of interbreeding between the two ecotypes. So despite the fact that all orcas worldwide are currently lumped into the same species designation, Orcinus orca, we know there are subpopulations that hang out in different places, eat different food, and are genetically isolated. However, that doesn't mean their DNA is frozen in time; DNA mutations happen all the time across all species, and some of them get passed down to descendants. And over time, the accumulated mutations in a genetically isolated population can "drift" far enough from the rest of the species that it becomes its own thing — either a subspecies of the original, or a new species.

However, the rules as to whether a new subspecies or species has been created are a bit fuzzy. The broadly accepted definition among scientists for a species is "a separately evolving lineage composed of a population or collection of populations." The definition of a subspecies is similar, but instead emphasizes that a population "appears to be a separately evolving lineage" because of changes based upon geography, ecology, or other natural forces that have restricted gene flow with the rest of the species. In short, a species is definitely evolving separately, and a subspecies appears to be evolving separately (even if there isn't overwhelming evidence that it actually is, yet).

Scientists measure whether there is independent evolution by looking at four characteristics:

• Distinctness: Do the ecotypes have distinct genetics from each other?
• Differentiation: Is there gene flow across ecotypes, or is an ecotype genetically isolated?
• Diagnosability: By observing genetic and physical characteristics, can an individual's ecotype be predicted accurately?
• Divergence: Is the amount of difference between ecotypes sufficiently large to indicate there are separately evolving lineages?

The researchers lay out the evidence around these four questions, which at this point they say makes a strong case that both transient and resident orcas are each their own species. The physical differences are already well-known, after about 50 years of research on orcas: body shape and color, their whistles and clicks, pigmentation on their eye and saddle patches, their dorsal fin, and skull and jaw shape. Behaviorally, there are also many differences, including hunting and foraging practices and primary diet. But the area where the evidence is developing rapidly is DNA analysis, which for most mammals was nearly nonexistent 30 years ago and today is both prolific and relatively inexpensive. Researchers have now documented enough unique mutations that just by looking at an individual orca's DNA sample they can accurately predict whether it is resident or transient. But they can go much further, because they can assemble the "family tree" of orcas from around the world such that they can pinpoint how long ago one group split off from another and became genetically distinct. It turns out that residents and transients are in branches that separated about 350,000 years ago; transients had their last major branching about 125,000 years ago, while residents branched off into their own grouping about 100,000 years ago.

Further, researchers have found that both transients and residents have continued to evolve, with specific genetic mutations unique to their lineage since it branched off. And some of those mutations are meaningful and significant, such as those related to regulating chemical processes for digesting and metabolizing mammal prey (for transients) and salmon (for residents). The differences in their diets isn't just a matter of preference; their bodies are now hard-wired to eat specific foods.

And the number and types of mutations for each lineage tell even more about their history, including the size of the population. The researchers believe the transient orca population was as large as 5,500 or 6,000 until it started to decline about 10,000 years ago; while the resident population declined from about 6,000 to around 600 somewhere between 40,000 and 10,000 years ago. But there is also evidence that within the past 750 years, both populations "crashed" at some point to under 100 individuals.

Based upon all of this evidence, the researchers conclude that transient and resident orcas are each their own genetically isolated and independently evolving species. Why do we care? Because not only are species protected under federal laws, such as the Endangered Species Act, but their status as separate species also informs our understanding of what we need to do to preserve them. For instance, if the chinook salmon run out, resident orcas won't switch to hunting mammals, because their digestive system won't support it anymore; the fate of the residents is completely tied to the restoration of chinook salmon runs in the Pacific Northwest.

And that brings us to the second paper: a fresh look at the long-term survival prospects for the Southern Resident orcas, those that tend to hang out around the southern end of Vancouver Island and the San Juan Islands (in contrast to the Northern Residents that frequent the northern end of Vancouver Island and inland waters farther up the Canadian coast). The researchers argue that whereas most species extinction events are "dark," in that they happen with no one paying close attention, the southern resident orcas are experiencing a "bright extinction" that is noticeable and is being meticulously measured and documented.

The southern residents are particularly prone to bright extinction, researchers suggest, because it is a small population (around 75 individuals at the moment) with long life spans and low birth rates. While on one hand, the population numbers don't change wildly from one year to the next, it's easy to miss the large impact of those small changes: A single birth or death in a population of 75 means an increase or decrease of 1.4% in the total population. But they also note that the small size and genetic isolation of the Southern Resident orca population reduces its genetic diversity, and thereby its biological resilience to disease, food scarcity, and other threats.

In addition, the Southern Residents have recently been the victim of "stochastic effects," a fancy way of saying "bad luck." For example, recently, the Southern Residents have seen more male births than female ones, even though the odds of the gender of any individual birth is still 50-50. But having fewer female orcas in the population makes it more difficult to sustain an overall birth rate that would allow the population to recover. The Southern Residents have also seen some individual orcas struck by vessels, and they have suffered from human-caused environmental degradation (including PCB pollution and increased vessel noise).

The researchers' conclusion: the Southern Resident orca population is on the verge of collapse. They predict it will see a gradual decline for two more generations — about 40 years — and then it will experience an accelerating decline until the population disappears altogether.

The researchers argue that saving the Southern Residents will require more drastic measures, including, most importantly, fortifying the plan to allow chinook salmon runs to recover so there is a food supply for the orcas. They also suggest that we might need a more radical approach to medical interventions to protect the health of the orcas, including veterinary treatments for the sick or injured and a proactive vaccination program to protect them from common diseases. They predict that over one-quarter of the naturally occurring mortality of the orcas could be deferred with the right kinds of medical interventions.

On the bright side, the researchers point out that in recent years there have been several successful cases of preventing "bright extinctions." Those include the California condor, whooping cranes, mountain gorillas, and several species of birds and butterflies. Other efforts, such as the race to save the northern white rhino, are still ongoing.

Nevertheless, as the researchers point out, "the capacity to determine what we can do often outstrips our ability to decide what we will do; a dilemma that leads to delays in threat reduction measures and perpetuates extinction debt, especially in long-lived species," such as the orcas.

Revised taxonomy of eastern North Pacific killer whales (Orcinus orca): Bigg's and resident ecotypes deserve species status

Warning sign of an accelerating decline in critically endangered killer whales (Orcinus orca)

Featured image via Daniella Beccaria/Shutterstock.com.