It’s not news that humans are having an astonishing and unprecedented impact on global ecosystems, or that we consume other species at a voracious and unsustainable rate. But analyses of humans’ roles in ecosystems tend to look only at how much we do kill, and how much we should kill. But there are other important questions to answer. Specifically, how are we different from other predators, and what does this mean for sustainability?

According to a paper in this month's issue of Science, one of the most important differences is the age of the animals we prey on. Non-human predators tend to catch mainly juveniles, which tend to be smaller, weaker, slower, and easier to catch. Humans, in contrast, go for the grown-ups. This makes sense: they’re meatier—or, in the case of trophy hunting, more impressive mounted on a wall. But it has important repercussions for the populations being hunted.

The effect is that fewer adults survive to reproduce and replenish the population. If we think of sustainable ecosystems as being a bit like an investment, reproductively fit adults are the capital, and babies are the interest. As any wise retiree could explain, it’s safer to dip into the interest and keep the capital stable, than it is to start eating into the capital.

There’s also the problem that humans insatiably target carnivores. The land animals that we kill for food might be predominantly herbivores. But we target many others by hunting for fun and trophies, poaching for animal parts, and wiping out predators that prey on livestock or prized game like moose and elk. As a result, we kill a whopping nine times more carnivores than are killed by other predators (who usually only kill each other because of competition for resources).

A lack of sustainability is a serious problem not just for the survival of healthy ecosystems, but also for the human economies that depend on those resources. Depleting the resources too quickly is a clear concern, but there are less obvious problems, too. Wiping out predators can cause problems for the populations they would normally hunt, which can enhance disease outbreaks—it's not a great idea if you're killing the predators to protect your elk, and then the elk get sick.

Mimicking the hand of Darwin

There’s also the question of just what happens when we kill so many of the reproducing adults. By removing these adults from populations, we’re creating an environment where members of the species with unusual reproductive patterns might have an easier time of surviving and reproducing.

For instance, in “sustainable” fisheries that demand the release of juvenile fish, there might be some juvenile fish that happen to reach reproductive age when they're younger and smaller. When the population is flooded with these earlier breeders, the overall effect is a population of smaller fish, which is a bad thing for people making their livings off big catches.

How could we do better? Non-human predators tend not to destroy the populations they hunt from, and we could learn something by paying attention to how these predators operate. It's possible that we could adopt their techniques into our own hunting and fishing practices, said Thomas Reimchen, one of the authors of the paper, at a press conference about the research.

Ecosystems with predators and prey have usually had very long periods of time for adaptation to take place. Human hunting practices operate on a different timescale: our population growth, use of technology, and spread across the globe have outstripped the ability of the hunted populations to adapt to us. Our strategy of preying on adults is the opposite of the strategy that has evolved in many ecosystems, which is why it causes such complete havoc.

This could give us a new way of thinking about sustainability, the authors explain. Although we do need to reduce the sheer quantity of the animals we kill, we would also improve our sustainability if we considered “the hand of Darwin," and copied the strategies of an ecosystem’s predators to avoid disrupting the evolutionary strategies that are already in place.

The problem, as with any drive towards sustainability, is contending with the powerful economic forces in place. In the long-term, it might be better to catch fewer, smaller fish, but people working in fishing industries have families to feed and profits to be made immediately if they catch large quantities of big fish. And it’s difficult to imagine a trophy hunter who’ll be persuaded to shoot a lion cub rather than an adult lion.

Knowing what we need to do is only the beginning—the vital next step is finding pragmatic ways to implement the very desperately needed changes.

Science, 2015. DOI: 10.1126/science.aac4249 (About DOIs).