Behold, the super predator
Humans are killing predators on land and sea at rates many times higher than is natural—with grim consequences
For most of my adult life, I’ve battled fishmongers who tried to persuade me to buy black pomfret that weighed more than 2kg. Sometime this decade, these conversations ended. These days, a 1kg black pomfret is enough to excite my neighbourhood fishmonger.
“Machchi aisish ati aaj kal,” he says in Dakhni. This is how the fish are these days. Except for vague explanations of exports and fewer fish in the sea, he has no idea why the fish are shrinking. Overfishing has previously been implicated, and science predicts that fish will likely become smaller because warming seas have lesser oxygen.
Now, a new study implicates humans, calculating that humans kill fish at their reproductive prime at rates up to 14 times higher than other predators in the sea; on land we kill carnivores at nine times the rate that they—animals—kill one another. I put my fish-size observations to the study’s lead author, Chris Darimont, and he concurred.
“We see this too on this side of the world,” Darimont, Hakai-Raincoast professor at the University of Victoria, Canada, told me in an email interview. “Size-at-age (length at a given age) in many fish has declined over the years, because the faster growing individuals are more likely to be killed by fisheries before they breed (and pass on the genes that say, ‘grow fast’).”
With humans functioning as “super predators”, evolution is selecting for smaller individuals, Darimont and his colleagues in the Canadian province of British Columbia suggest in the paper published in the journal Science. They surveyed 2,125 species of predators on land and at sea to study the effects of such super-predation.
“Human predatory behavior evolved much faster than competing predators and the defensive adaptations of prey,” write Darimont and his colleagues in Science . “Indeed, division of labor, global trade systems, and dedicated recreational pursuit have equipped highly specialized individuals with advanced killing technology and fossil fuel subsidy that essentially obviate energetically expensive and formerly dangerous search, pursuit, and capture.”
When adult prey are targeted by a predator as much as humans do, individuals who breed earlier tend to gain an evolutionary advantage. “Over rapid time scales (for evolution, anyway, only a handful of generations), many fish populations in particular have shifted their breeding schedules to breeding at earlier ages and smaller sizes,” said Darimont. In a 2009 paper that reviewed a clutch of associated studies, Darimont found that human predation was causing evolutionary changes in other species at rates 300% faster than they would in the wild; such changes were, in some cases, evident within two decades.
A prominent example on land is how the horns of North America’s bighorn sheep shrank by a quarter over 30 years, although there is some disagreement if human hunters—who target sheep with the biggest horns—are forcing these changes or if there are demographic changes and environmental factors at work. Smaller specimens may indeed be a result of demographic change, said Darimont. But when humans act as predators, they specifically target the biggest, whether fish or sheep, leaving smaller individuals with an evolutionary advantage. This downsizing could also work at a species level, with larger species being—in the case of fish—removed before smaller ones.
In the oceans, Darimont offers the example of the Atlantic cod, which now breeds at four-and-a-half years of age instead of six. “The problem with this in fish is that younger/smaller breeders have far fewer eggs, and eggs that are less likely to survive,” he said.
The effects of human predation are evident on plants too, although this may not strictly be considered predation. For instance, the dwarfing of one species of an “intensely collected” flower called the Himalayan snow lotus over a century was recorded in a 2014 study in the US journal Proceedings of the National Academy of Sciences.
So, what are the implications of humans as super predators?
From an ecological perspective, human super predation is modifying the reproductive potential of many species and altering food chains and ecosystem. As the Earth loses large carnivores, it is humans who feel the loss, nutritionally and economically. Carnivores control herbivores, herbivore disease and even wildfires (by indirectly controlling the length and spread of vegetation and hence its combustibility). More obvious is the loss of food, such as fish. Less obvious are what Darimont calls “nasty social processes” that occur when wildlife and fisheries are overexploited. These include slavery on fishing boats, poaching and piracy. These implications, the paper said, “add new urgency to reconsidering the concept of sustainable exploitation”. It calls for a new definition of sustainable exploitation “that focuses not on yields to humanity but rather emulates the behaviour of other predators”, by trying to match predation rates of humans and competing predators. In other words, we should mimic sharks or leopards—or, at least, try.
Samar Halarnkar is editor of IndiaSpend.org, a data-driven, public-interest journalism, non-profit organization. He also writes the column Our Daily Bread in Mint Lounge.
Comments are welcome at firstname.lastname@example.org. To read Samar Halarnkar’s previous columns, go to www.livemint.com/frontiermail
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