Sharks: The Ultimate Keystone Species?

By Lambert Strether of Corrente.

I thought I would look at another keystone species, the shark, but I won’t go look systematically at these fascinating creatures, because I have a couple of items floating in the zeitgeist I want to get to, on keystone species and “trust the science” generally. (Here is a good site from the How Stuff Works podcast). Unsystematically, then, sharks are, obviously, streamlined:

But they not only look like streamlined aircraft, they swim like aircraft, too:

A shark is more like an airplane. It doesn’t have a swim bladder, so it uses its forward movement to control vertical position. The tail is like the shark’s propeller — the shark swings it back and forth to move forward. In an airplane, this forward movement pushes air around the wings. In a shark, this forward movement pushes water around the fins. In both cases, this movement of matter creates lift — the fluid is different, but the principle is exactly the same.

Sharks have two sets of paired fins on the sides of their body, in the same general position as the main wings and horizontal tail wings of a plane. The shark can position these fins at different angles, changing the path of the water moving around them. When the shark tilts a fin up, the water flows so there is greater pressure below the fin than above it. This creates upward lift. When the shark tilts the fin down, there is greater pressure above the fin than below it. This pushes the shark downward.

The shark also has one or two vertical dorsal fins on its back and sometimes a vertical anal fin on its underside. These fins work like the vertical stabilizer wing on an airplane. They help the shark keep its balance as it moves through the water and they can be moved from side to side to turn the shark left and right.

This fin arrangement gives sharks amazing maneuverability. They can cruise at high speeds, stop suddenly and make sharp turns in every direction. This is one of the reasons they are such effective hunters. They move more quickly and with greater control than any of their prey — most of the time, a shark’s prey doesn’t even know what hit it.

That was the best True Fact I found, but others include that yes, sharks have negative buoyancy and so need to keep moving to survive, they have personalities, they have the ability to learn, they use magnetic fields for long distance navigation, and as a species they are 450 million years old, despite several extinction events. Until now, of course.

Nobody would call sharks cuddly. (Blake, “from downtown,” ‘here from Mitch and Murray”, is described as shark-like.) But their danger to human beings (64 shark attacks world-wide[1]) has been blown utterly out of proportion by our wonderful media. From the Wall Street Journal:

The American relationship with sharks was changed irrevocably during the summer of 1916. The East Coast was gripped by both a heat wave and a polio epidemic, leaving the beach as one of the few safe places for Americans to relax. On July 1, a man was killed by a shark on Long Beach Island off the New Jersey coast. Over the next 10 days, sharks in the area killed three more people and left one severely injured. In the ensuing national uproar, President Woodrow Wilson offered federal funds to help get rid of the sharks, an understandable but impossible wish.

The Jersey Shore attacks served as an inspiration for Peter Benchley’s bestselling 1974 novel “Jaws,” which was turned into a blockbuster film the next year by Steven Spielberg. Since then the shark population in U.S. waters has dropped by 60%, in part due to an increase in shark-fishing inspired by the movie. Appalled by what he had unleashed, Benchley spent the last decades of his life campaigning for shark conservation.

(I think “in part” is doing a lot of work, there.) On the bright side, Discovery Channel has very popular franchise called “Shark Week,” described here by Forbes:

This year, the viewer will tag along with A-List stars including Mike Tyson, Will Smith and Shaquille O’Neil. And, fans will get to see how the current pandemic is offering researchers a once in a lifetime opportunity to study how the global lockdown and reduced amount of human interaction and activity in our oceans has impacted the hunting patterns of sharks.

Arguably, the popularity of “Shark Week” increased because of the Covid pandemic:

Due to the large amounts of time spent at home, people were hungry for entertainment. Lectures and webinars on shark species, science, conservation and research became much more popular and younger generations were able to interact with scientists and conservationists all around the world.

Still, despite Forbes’ best efforts (“I just watch it for the science”), “Shark Week” really looks like it’s all about the jaws. Here is the program line-up, which I have helpfully highlighted:

Whether the shark’s status as a dreaded poster animal will arrest its decline is an open question; Peter Benchley was right to worry. From Nature:

Overfishing is the primary cause of marine defaunation, yet declines in and increasing extinction risks of individual species are difficult to measure, particularly for the largest predators found in the high seas… We find that, since 1970, the global abundance of oceanic sharks and rays has declined by 71% owing to an 18-fold increase in relative fishing pressure. This depletion has increased the global extinction risk to the point at which three-quarters of the species comprising this functionally important assemblage are threatened with extinction.

That’s not good news. Overfishing is attributed to three practices:

1) Adjuvants for vaccines. From Discovery:

Sharks produce a natural oil in their livers called squalene, which is an ingredient currently used in flu vaccines. [Shark Allies, a] California-based conservation group, warned that if the world’s population received one dose of a COVID-19 vaccine containing squalene, around 250,000 sharks would need to be killed, depending on the quantities used. But if two doses are needed, this would likely increase to half a million sharks killed and harvested for their liver oil. since five COVID-19 vaccine candidates include adjuvants with squalene harvested from sharks.

2) Shark-fin soup. From Smithsonian:

Shark fins are tempting targets for fishermen because they have high monetary and cultural value. They are used in a popular dish called shark fin soup, which is a symbol of status in Chinese culture…. Many fishermen prefer to practice shark finning instead of bringing whole sharks to the market because the fins are far more valuable than the rest of the body, sometimes selling for as much as $500 a pound ($1,100 a kilogram)… Another major factor is that shark fisheries—and finning in particular—are having catastrophic effects on shark populations around the world. Approximately 100 million sharks are killed globally each year, and one of the major incentives for this is the shark fin trade. Between 1.3 and 2.7 million [hammerhead sharks] are killed every year in the shark fin trade… Today, some shark populations have decreased by 60-70% due to human shark fisheries.

(“One of the major incentives” and “some” are doing a lot of work, there.)

3) Bycatch. Bycatch is fish caught unintentionally by commercial fisheries seeking other species. (Dolphins caught while fishing for tuna are bycatch.) Neither of the above two practices seems to have the numbers needed to bring extinction near. Bycatch may. From the Pew Environment Group Ocean Science Series:

A study commissioned by FAO estimated that in the late 1980s and early 1990s, nearly a third of all reported shark catches were landings from bycatch fisheries (Bonfil 1994). More recently, Stevens et al. (2000) suggested that sharks taken as bycatch could account for as much as 50 percent of all shark landings…. In some regions of the world, even though sharks are not the primary target of fisheries, they make up a majority of the total catch (i.e., all fish caught and discarded). For example, in the Atlantic Ocean (north and south), large pelagic sharks amount to roughly 70.3 percent of the total landings in weight in the Spanish surface longline fleet targeting swordfish (Xiphias gladius) (Mejuto et al. 2006). In the U.S. Atlantic, sharks made up 25 percent of the total catch of the pelagic longline fishery between 1992 and 2003 (Abercrombie et al. 2005). The sharks caught in these fisheries are often unmanaged, because regulations typically focus on the target species (e.g., tunas and swordfish) (Stevens et al. 2000). Bycatch of sharks results in a substantial number of sharks being discarded dead or dying every year. However, quantifying total shark mortality from bycatch is challenging because comprehensive data on these discards are unavailable. Most monitoring focuses primarily on fishing effort and landings of target species, and few fisheries have onboard observer programs (FAO 2009).

So the numbers don’t quite add. What is clear is that sharks, as a species, are in bad shape. From AP:

24 of the 31 species of sharks and rays are threatened with extinction, while three species — oceanic whitetip sharks, scalloped hammerhead sharks and great hammerhead sharks — are considered critically endangered.

“The last 50 years have been pretty devastating for global shark populations,” said Nathan Pacoureau, a biologist at Simon Fraser University in Canada and a co-author of the study.

The effect of removing a keystone species like sharks from marine food webs is not known but likely to be significant. Back to Nature:

The ecosystem consequences of the declines in oceanic shark populations are uncertain because of the complexity and scale of the marine food webs. Nevertheless, the profound effects of depleting predatory species are becoming apparent. For example, the decline in predatory sharks and tunas is associated with increases in mesopredators, including teleosts and smaller-bodied shark species, indicating that fundamental functional changes to these marine food webs are occurring. Of further concern is the associated threat to food security and income in many low-income and developing nations, many of which have fished sharks for generations. Alternative livelihood and income options are needed to ease transitions to sustainability.

I’m going to skip over policy proposals for avoiding or mitigating shark extinction, and go straight to zeitgeist (which would inform policy discussion anyhow).

First corrective to the zeitgeist: “Apex predator” and “keystone species” should not be equated. For example, from the Shark Conservation Fund:

Sharks and rays are keystone species that play an essential role in our oceans and coastal communities. As apex predators, sharks and rays stabilize food webs and act as a barometer for ocean health.

From White Shark Projects:

Sharks are what scientists call a ‘keystone species’. This basically means that they are responsible for keeping the intricate ocean ecosystem in balance. As apex predators sharks keep everything below them in harmony. They do this by keeping the predatory species that they prey on at a healthy but balanced number.

From Exploring Nature:

Sharks play an important role in ocean ecosystems. They are apex predators. This means that they are at the top of their food web and are rarely prey themselves – expect to humans. As predators, they play a role in keeping their prey populations healthy by capturing the slower, weaker fish, turtles, manatee, dolphins, etc. Many are also considered keystone species because of their importance in maintaining a balance in their food webs.

From Study.com:

Ecosystems sometimes rely on one or a few species to hold the rest of the ecosystem together. These species are called keystone species and they perform unique roles that support the entire ecosystem.Tiger sharks are a keystone species because they control the populations of primary consumers. Tiger sharks are top predators in the ocean.

From Conserve Energy Futures (“Top Eco-Conscious blogger“):

[The Shark] is an avid predator that feeds on fishes of all kinds making it the keystone species in deep waters. Sharks have been the regulators of life in the deep water ecosystem given that they feed on living fishes in water, the sick and the weak thus not only keeping the count of fishes manageable but also reducing the chances of diseases from the sick and dead fishes respectively.

From Padi Aware Foundation:

Sharks are considered ‘keystone species’, which means that as top predators, they are extremely important in maintaining the balance in marine ecosystems. Removing too many sharks from an eco-system can lead to a monumental shift in the equilibrium between predators and prey all the way through the food chain.

I know all these sources are pretty mediocre, but that’s the point; the pervasiveness of the equation (and hopefully your kid isn’t using study.com). I do know that the sources are writing about sharks, who are both “keystone species” and “top predators,” so mentioning both phrases is necessary. However, “an avid predator that feeds on fishes of all kinds making it the keystone species” is simply not correct, and to me, all these sources breathe this attitude. Beavers, as we have seen, are a keystone species because they are builders, not predators. What concerns me is how easy it is to transfer this “top predator as keystone species” line of thinking to political economy (Silicon Valley speaks regularly of ecosystems), making Jeff Bezos and Bill Gates “top predators” (as indeed they are), and hence essential to our “ecosystem,” which would collapse without them. (Disposing of “the sick and weak” isn’t very pleasant to read about after Covid, either.)

Second corrective to the zeitgeist: Science advocates should not be triumphalist but humble. There’s been some interesting work done recently on past shark extinctions. Recently, scientists Leah Rubin and Elizabeth Sibert examined ocean core samples for shark teeth and scales. From Live Science, what they discovered:

[Nineteen million years ago,] about 90% of sharks disappeared from the oceans in less than 100,000 years, but it’s unknown why and whether they died off in a single day, weeks, years or even thousands of years. This extinction event significantly altered the ancient marine environment, and sharks never recovered from the die-off, according to the study, which was published Thursday (June 3) in the journal Science.

We only know this because of the core samples Rubin and Sibert recently took. More:

“Sharks have been around for 400 [or 450; give or take] million years; they’ve weathered a lot of mass extinctions,” some of which wiped out almost all life, said co-author Elizabeth Sibert, a postdoctoral fellow at Yale University’s Institute for Biospheric Studies (who was a junior fellow at Harvard University at the start of the research). Yet during the early Miocene epoch, something “clearly happened to almost wipe this group off of the face of this Earth.”

“I think what has been the most surprising is just how extreme” the decline in shark diversity and abundance truly was during this time period, Rubin, who is now an incoming doctoral student at the State University of New York College of Environmental Science and Forestry, told Live Science in an email. The “million dollar question” is, what caused it?

No clear environmental driver, such as a major change in climate, accounts for this significant decline in sharks. And predators probably didn’t drive sharks to extinction, as this die-off occurred several million years before tuna, billfish, seabirds, beaked whales and even migratory sharks exploded in numbers.

“We really, truly don’t know” what caused the extinction, Sibert said.

“We really, truly don’t know.” That is appropriately humble. It applies to so much, and not just in sharkology; it certainly applies to medicine, as SARS-COV-2 is currently teaching us. It also applies to the entire concept of “ecosystem services,” a topic I will look at later in the week. Do we really know enough about anything to assign prices to, say, carbon? I doubt it very much.

NOTES

[1] Attacks have slightly increased recently, probably due to climate change.