To really understand food webs, consider humans

Predators rule the planet, and they exert their influence on the rest of all living creatures in two ways: they eat them and they terrify them. But if enough predators aren’t around to control the species on which they like to snack and scare, or so the theory goes, their entire food web could suffer, toppling the ecosystem.

The classic example comes from the Yellowstone ecosystem. When the wolves were driven away, the elk population, suddenly free from the threat of predation, exploded. With too many elk around, the plant communities suffered. The altered plant communities went on to change riparian ecosystems. In addition, wolves help to suppress communities of smaller predators, like coyotes and foxes. Absent wolves, those species also experience a population increase, and birds and smaller mammals wind up suffering.

Or take sea otters. When hunters on the Central Californian coast wiped them out for their fur, the sea urchins they eat suddenly took over the seafloor. And the urchins all but destroyed the kelp forests they lived in, which in turn caused lots of other critters to lose their homes.

The problem with this idea – called trophic cascades – is that most studies have been carried out in otherwise intact ecosystems. That’s despite the fact that most of the earth’s surface is intensely affected by human behavior. What Ine Dorresteijn, a graduate student at Germany’s Leuphana University, wanted to know was how these predator-prey dynamics might be different when there are humans around.

Dorresteijn, together with her team, focused on a diverse ecosystem in Transylvania, Romania. Despite human presence, there are at least two apex predators, brown bears and grey wolves, along with mesopredators like the red fox, and large herbivores like red and roe deer. In addition to the bears and wolves, both humans and free-ranging dogs (which guard livestock) can be thought of as predators. That is, they provide pressure on prey species through hunting and through fear. To model the ecosystem, the researchers used data from 138 camera traps placed on commonly used human and game trails, some of which were baited with honey (for bears) and wolf urine (for wolves).

From more than 3,000 “camera days,” they detected roe deer 2,197 times, red foxes 388 times, humans 275 times, dogs 120 times, along with 94 red deer sightings, 76 bears, and just two wolves.

As expected, wolves and bears both suppressed red foxes, and bear sightings were negatively correlated with wolf sightings. In other words, wild predators tended to keep to themselves. Wolves drove red deer away, and bears drove roe deer away. Perhaps related to the fact that predators also drive each other away, wolves were often found near roe deer, and bears were often linked with red deer.

Also as expected, humans limited the presence of all other species except for domestic dogs. In fact, the effects of humans on both predators (especially bears) and herbivores were stronger than the effects of landscape (forest versus pasture) on predators and herbivores. The human effects on red deer were nearly five times larger than those of the wild predators and were roughly equivalent to the pressure that bears place for roe deer. When you add the effects of dogs on wildlife, the humans’ footprint on the ecosystem becomes even larger. After all, without humans, there would be no guard dogs.

In part, trophic cascade theory holds up. Top predators are absolutely important in structuring the ecosystems in which they live, especially through suppressing the herbivores they eat. But, according to Dorresteijn, humans have a stronger effect on the ecosystem, both directly and indirectly, than do wolves and bears. In that sense, we really are super-predators.

It means that studies of food webs and ecosystem dynamics ought to account for the effects of human (and domestic animal) behavior. Absent our species, such studies ignore the tremendous influence we have at all levels of the food web. “Humans are perhaps unique among apex predators in their ability to influence ecosystems through simultaneously directly reducing large carnivore, mesopredator, and herbivore populations, and by impacting their behavior by creating a landscape of fear for all three trophic levels,” write the researchers.

Understanding the role that humans play in maintaining or altering food web dynamics isn’t just a purely academic exercise. Other apex predators are declining throughout the world, while human populations are rapidly expanding. Meanwhile, in many places conservation efforts are in place to aid wild predators in recovery or reintroduction. Such efforts can be better poised for success if aided by information about how humans, predators, and their prey all interact. – Jason G. Goldman | 02 September 2015

Source: Dorresteijn, I., et al. (2015). Incorporating anthropogenic effects into trophic ecology: predator–prey interactions in a human-dominated landscape. Proc. R. Soc. B. DOI: 10.1098/rspb.2015.1602.

Header image: Brown bears at Romania’s Zarnesti Bear Sanctuary via Flickr/WSPA International.