What made humans into global super predators?

In the 1970s, Thomas Reimchen sat near a remote Canadian lake. The biologist soon realized that a wide variety of animals – loons, trout, grebes, and otters – all fed on the same prey species, stickleback fish. And the predators overwhelmingly targeted the juvenile and sub-adult fish, or what Reimchen calls the “reproductive interest.” Only about 5% of kills came from the sticklebacks’ “reproductive capital,” the adults. But nearby on the ocean, Reimchen witnessed something different. There, salmon and herring fisheries were largely built upon sexually mature, adult fish. The thought was that the juveniles should be spared for conservation purposes, left to grow up, reproduce, and then be harvested later.

You can probably tell where this is going. Despite the heavy pressure on the sticklebacks from the threat of predation, the fish managed to sustain a strong, stable population year after year. Meanwhile, global fisheries are in many places collapsing.

Humans love their burgers and steaks and sushi, but we don’t usually think of ourselves as predators. Perhaps that’s because most of us are so far removed from the actual act of predation that by the time a slice of tuna or rack of ribs winds up on our plates it’s easier to forget that those hunks of flesh were once a part of actual animals. It turns out that even if you discount the massive quantities of domesticated animals we consume – cows, pigs, chickens, and the rest – humans still kill a lot of animals.

We’re among the most widely distributed species on the planet, so there’s hardly an ecosystem unaffected by our predatory behavior. We have sophisticated killing technology (including, yes, thermal imaging systems), we cooperate with other species (dogs) to hunt, and we easily exploit naïve prey that have not yet evolved anti-human defenses. Despite some efforts at moving toward sustainability, our species can drive prey declines and degrade ecosystems almost without even thinking about it.

But it turns out that humanity’s massive effects on wildlife and ecosystems don’t come about just because there are so many of us or because we’re so skilled at killing. It’s because we kill differently. There’s a lot that wolves and sharks and bears and lions have in common. Humans, meanwhile, remain in a category all their own, thanks in part to our big brains and to our complex culture. That’s the conclusion reached by Reimchen and a team of University of Victoria researchers led by Chris T. Darimont.

The researchers combed the scientific literature for information on how predators predate, combining information on humans and 117 other terrestrial predatory mammals from every continent but Antarctica, along with information on 282 marine predator species from every ocean. The human data included information about hunting (including for trophies), poaching, bushmeat, and fisheries.

At first glance, it seemed as if humans and other carnivores exploited their prey at similar rates when geography and trophic levels were combined. But when the researchers took a closer look at the data, differences began to emerge. Within individual communities, for example, hunters tended to kill more animals than other terrestrial predators did.

In some cases, that’s because human activity has already driven off predators. In parts of the US, for example, hunting is an important tool for managing deer populations, because they are no longer adequately suppressed. Their natural predators, like wolves, bears, and mountain lions, have a tough time surviving thanks to us. But humans are also exploiting mesopredators and apex predators far more than non-human predators do. In fact, hunters kill large carnivores 3.7 times more often on average than herbivores, ostensibly for trophies. And unlike most natural predators, we go after the biggest, most impressive animals, not the easiest ones to kill.

There are similar patterns in the seas as well. Human fisheries target adult prey, for example, at higher rates than any other marine or terrestrial predator. Most predators tend to go after juveniles, while humans, it seems, tend to go after adults.

How can one species exert such an impact on the natural world? “Whereas sociopolitical factors can explain why humans repeatedly overexploit, cultural and technological dimensions can explain how,” writes Darimont. He and his colleagues say that global trade and division of labor, along with specialized killing technologies, have combined to allow humans the unprecedented ability to exploit wildlife. In addition, fossil fuel subsidies allow people to move around the planet with such ease that it’s now quite cheap to search for, pursue, and capture animals. Since our species relies so heavily on agriculture and domestic livestock, our ability to eat well is increasingly “decoupled from dwindling prey.” That, in part, is why dwindling wildlife can drive exploitation even higher. Rare resources are simply worth more.

Combined, all of this makes humans into global “super predators” for four main reasons.

First, our over-reliance on adult prey is unique among predators. Our preference for large body size and large trophies “has fundamentally altered the selective landscape for many vertebrates.” That doesn’t just modify the reproductive dynamics of those species, but also the food webs in which they are situated. The focus on sexually mature adults makes it quite difficult for prey populations to sustain the intense pressures of human exploitation.

Second, despite the use of hunting in some ecosystems as a means of suppressing herbivore populations, human hunters are not perfect analogues for the mammalian predators we’ve extirpated because we don’t provide the same ecosystem services. We don’t regulate disease or wildlife, and we don’t provide control over mesopredators in many cases.

Third, we remove a considerable amount of biomass – and therefore, nutrients – from some ecosystems, re-depositing it in others via landfills and sewage systems. When a wolf or lion takes down an animal, that carcass becomes a brief, temporary ecosystem unto itself. The same dynamics do not typically play out when humans take down those prey species.

Finally, we gobble up a wider taxonomic range of species than any other predator on the planet.

Is there anything that can be done to be more sustainable in our predation of wildlife? The researchers argue that sustainable exploitation means imposing limits that are as sweeping as those that allow humans leave such a deep footprint on the planet in the first place. Central to that, they say, is a focus not just on limiting the scope of hunting, but in mimicking other predators’ behavior. It would be tough to alter our hunting and fishing strategies to target juveniles in many cases, but it is at least feasible to target adults at rates similar to other predators. – Jason G. Goldman | 21 August 2015

C.T. Darimont, C.H. Fox, H.M. Bryan, & T.E. Reimchen (2015). The unique ecology of human predators. Science 349(6250), 858-859. DOI: 10.1126/science.aac4249.

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