Humans are 'unique super-predator'

Jonathan Amos BBC 21 Aug 15;

Humans should focus on the "interest", not the "capital" in the system - say the authors

Humans' status as a unique super-predator is laid bare in a new study published in Science magazine.

The analysis of global data details the ruthlessness of our hunting practices and the impacts we have on prey.

It shows how humans typically take out adult fish populations at 14 times the rate that marine animals do themselves.

And on land, we kill top carnivores, such as bears, wolves and lions, at nine times their own self-predation rate.

But perhaps the most striking observation, say authors Chris Darimont and colleagues, is the way human beings focus so heavily on taking down adult prey.

This is quite different from the rest of the animal kingdom, for which the juveniles of a species tend to be the most exploited.

Part of this is explained by the tools that human hunters exclusively can deploy.

We can tackle adult prey at minimal cost, and so gain maximum, short-term reward, explained Prof Darimont from the University of Victoria (UoV), Canada.

"Advanced killing technology mostly excuses humans from the formerly dangerous act of predation," he told reporters.

"Hunters 'capture' mammals with bullets, and fishes with hooks and nets. They assume minimal risk compared with non-human predators, especially terrestrial carnivores, which are often injured while living what amounts to a dangerous lifestyle."

Predator 'financials'

This concentration on large adult prey is triggering extinctions, as well as driving an evolutionary shift towards smaller fish sizes and disrupting global food chains, say the authors.

Prof Tom Reimchen, a UoV co-author on the study, uses a financial analogy to explain the damaging consequences of hitting adult populations hardest.

He calls the adults the system's "reproductive capital" - the equivalent of the capital held in a bank account or a pension fund. And he says we are eating into this capital when we should really be living off the interest - the juveniles, which many species will produce in colossal numbers, expecting a good fraction to be doomed from the moment they are born via predation, starvation, disease, accidents and more.

The heavily biased preference for adults was not a sustainable strategy long-term, which ought to be clear from fundamental biology, argued Prof Darimont: "In the overwhelming number of cases as fishes age, they become more fecund. That is to say, they produce more eggs, have more babies, and, in fact, in many cases, many of those babies are more likely to survive and reproduce themselves.

"So when a predator targets that reproductive age class and especially the larger more fecund animals in those populations, we are dialling back the reproductive capacity of populations."

Wolf and salmon

However, much of the standard conservation management today is based on the notion that it is the "tiddlers" that should be let go, to ensure robust numbers for the next generation. Trawl nets are often designed specifically to support this approach.

Doing it the other way would be challenging, but the technical solutions were available, said Prof Reimchen.

"There are traps that can define the entrance to a net, which then very easily allows you to exclude fish above a certain size - in other words, the reproductive capital. Once the motivation is in place, clever people will work out how this transition from the reproductive capital to the interest could be brought about."

As for quotas, these should more closely align with the numbers taken out by natural predators, the team suggests.

Human population

Dr Chris Carbone studies predator-prey relationships for the Zoological Society of London, UK.

He described the research as an interesting study that aimed to put some actual numbers on phenomena that many in the field would recognize.

But he cautioned that as broad as the investigation was, the data was still sparse, especially in the marine environment.

And as for refocusing the age class to take more juveniles, Dr Carbone argued that it would very much depend on the species in question. Not all species would react in the same way. But he said there was perhaps an even more fundamental problem, which was the density of human predators versus their prey.

"We exist at vastly higher densities than natural predators," he told BBC News.

"It might be that 100 zebras could support a lion, but in the case of humans we can outnumber our prey in many instances, and that throws the system. So even if we didn't have the efficient hunting technology, we'd still have problems with sustainability."

Human 'super-predators' should change hunting, fishing habits
Kerry Sheridan AFP Yahoo News 21 Aug 15;

Miami (AFP) - Humans are super-predators that upset the natural balance on Earth by killing far too many adult animals and fish, scientists said Thursday, urging a focus on catching fewer and smaller creatures.

People tend to kill adult fish at 14 times the rate of marine predators, said the findings in the journal Science.

And humans slaughter large land carnivores such as bears and lions at nine times the rate of predatory animals in the wild.

Based on a survey of 2,125 predators around the world on both land and in the water, scientists found that people cause "extreme outcomes that non-human predators seldom impose," said co-author Chris Darimont, professor of geography at the University of Victoria in British Columbia, Canada.

These include extinctions, shrinking numbers of fish, smaller sized fish, and disruptions to global food chains.

These impacts are made possible by humans' unique approach to hunting -- using weapons and external energy sources like fuel to power our hunts, searching for the biggest catch possible and acting as suppliers for other hungry mouths in faraway places, he said.

This is in sharp contrast to the way the marine world operates, with most predators focusing on juvenile prey taking only about one percent of adults.

"Our impacts are as extreme as our behavior and the planet bears the burden of our predatory dominance," said Darimont.

- New approach -

If humans want to continue to see large beasts like rhinos, elephants and lions in the wild, as well as ensure the health of ocean life, scientists said big changes are needed.

"We're suggesting a new and perhaps transformative way to consider what sustainable exploitation could be," said Darimont.

He said the recent outrage over the killing of Cecil the Lion may be an indicator that societies are ready to at least cut back, if not stop all together trophy hunting of large beasts.

"If future generations of people are to see these magnificent animals, then this requires cultivating new tolerance for living with them," he told reporters.

"This might include increasing revenues to local communities derived not from hunting, but from non-consumptive uses such as eco-tourism, shooting carnivores with cameras, not guns."

- Smaller fish -

When it comes to fishing, Darimont and co-author Tom Reimchen urged a focus on younger, smaller fish.

Currently, humans tend to focus on catching the biggest fish, because they provide more food and they are easier to process than smaller fish, which are often thrown back.

But these adult fish are valuable when it comes to reproduction, and should be spared so that they can release more eggs over their adult lifespan, the authors argued.

Reimchen's research showed that predatory fish and diving birds overwhelmingly kill juvenile forms of freshwater fish and generally take no more than two percent of the adult fish.

Salmon fisheries run by people harvest about half of all adult fish.

"It is not simply the issue of shifting the extraction rate to juveniles," said Reimchen, a biology professor, adding that humans would have to drastically cut their fishing quotas in order to better resemble the behavior of other predators.

"If you use natural predator-prey quotas as some type of sustainable guide, we would be talking perhaps close to an 80 or 90 percent reduction in our global take," said Reimchen.

Such sweeping changes may be hard to make, but an accompanying editorial by Boris Worm of Dalhousie University argued that humans may be unique in that regard, too.

"We have the unusual ability to analyze and consciously adjust our behavior to minimize deleterious consequences," Worm wrote.

"This final point, I believe, will prove critical for our continued coexistence with viable wildlife population on land and in the sea."