Are humans at the top of the food chain?

Lions, gray wolves and great white sharks have one thing in common: They're top predators. Their diets consist almost entirely of meat, and except in rare instances, these animals have no natural predators — except humans. So, if we are predators of top predators, does that mean humans are at the top of the food chain?

The answer depends on how you define "predator," that is, whether you're killing to eat or just killing other animals, as well as whether you're looking at prehistoric or modern-day humans.

In ecology, or the study of how organisms relate to one another and to their environments, humans' place in the food chain isn't based on what does or doesn't eat us, or on what we kill, said Sylvain Bonhommeau, a marine ecologist at IFREMER, a marine research institute in France. Rather, "It's completely based on what you eat," Bonhommeau told Live Science. Based on that definition, the answer is no — humans aren't top-predators because we don't eat everything we kill.

Related: What's the first species humans drove to extinction?

Bonhommeau and colleagues at IFREMER set out to determine humans' position on the food chain, also known as their trophic level. Scientists typically score trophic levels on a scale of 1 to 5. Plants and other primary producers, which obtain energy using sunlight, occupy level one, and herbivores are in level two. Meanwhile, species at the third level eat only herbivores, and species at the fourth level eat only level-three carnivores — and so on. Species that get their food from multiple trophic levels, like omnivores, are scored by the average trophic level of what they eat, plus one. For example, an animal that eats exactly 50% plants and 50% herbivores would be a level 2.5-omnivore.

Using data from the Food and Agriculture Organization of the United Nations on human food consumption around the world, the IFREMER scientists assigned a trophic level to each food we eat. They found that, on average, humans get 80% of their daily calories from plants and 20% from meat and fish, according to the team's 2013 study results, published in the journal Proceedings of the National Academy of Sciences. That puts us at an average trophic level of 2.21 — somewhere between anchovies and pigs. But humans' trophic levels vary worldwide. In Burundi, for instance, plants made up 96.7% of the local diet in 2009, giving those in that country a trophic level of 2.04. Meanwhile, those in Iceland, where the diet consisted of around 50% meat that same year, had a trophic level of 2.57.

Of course, humans pose a much larger threat to other animals than anchovies and pigs do. Some scientists argue that humans' pressure on other species makes us "super predators," a term the authors coined to refer to the rate at which humans kill other species. In a 2015 report published in the journal Science, scientists at the University of Victoria in Canada compared the activity of human hunters and fishers with that of other terrestrial and marine predators. They found that humans kill adult prey at rates up to 14 times higher than other predators. "If you take into account how wide our impact on wildlife is, it's huge," Bonhommeau said. However, Bonhommeau disagrees with the assessment that humans are super-predators, which he interprets as a conflation with the term "top-predator." (The authors of the Science paper were not available for comment.) In ecology, predator has a specific definition: they eat what they kill. "I think this article was misleading by confusing killing and predating (kill and ingest food)," he wrote in an email.

For the most part, we're not killing wildlife to eat them. For instance, the main causes of lion population declines are habitat loss and clashes with humans, who don't want lions threatening them or their livestock. Meanwhile, people fishing the oceans throw away between 10% and 20% of total catches as bycatch, according to a 2017 study in the journal Fish and Fisheries. These unintentionally caught animals often sustain injuries or die, according to the National Oceanic and Atmospheric Administration. "A predator ingests what it kills," Bonhommeau and colleagues wrote in an unpublished response to the Science article. Instead, they suggest the term "super-consumer."

Related: Humans are practically defenseless. Why don't wild animals attack us more?

This young shark was caught by mistake as industrial fishing bycatch. It was later released back into the water. (Image credit: Arrlxx via Getty Images)

Historically, there may have been less of a discrepancy between what we eat and how much we kill. Ben-Dor and colleagues reviewed studies on human physiology, genetics, archaeology and paleontology to reconstruct the trophic levels of our Pleistocene (2.6 million to 11,700 years ago) ancestors. 

They concluded that humans likely were apex predators who ate mostly meat for around 2 million years, up until 12,000 years ago, when the last ice age ended. The review, published in 2021 in the American Journal of Biological Anthropology, argued that humans have more physiological similarities to carnivores than to herbivores, such as highly acidic stomachs to break down complex proteins and kill harmful bacteria, and the high body fat capable of carrying carnivores through a period of fasting before the next big kill. 

The scientists also pointed out that an analysis of different nitrogen isotopes (variants of the element nitrogen) in ancient human remains, the ratio of which tends to increase with a meat-heavy diet, reveals consistently high ratios of nitrogen compared with the ratios of nitrogen isotopes in the fingernails and hair of people with a primarily plant-based diet. This analysis, in essence, is another line of evidence that ancient humans ate a ton of meat.

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A few changes may have caused humans to descend the food chain, Ben-Dor and colleagues write in their review article. The primary change, they suggest, was the disappearance of large animals like woolly mammoths. Around that same time, humans began to develop technology that allowed them to consume a higher number of plants, like stone tools for processing grains. (The advent of agriculture was still just around the corner.) 

But even if we were once apex predators with meat-heavy diets, that doesn't mean modern humans should ascend the trophic ladder, Ben-Dor told Live Science. "It doesn't necessarily follow that because we were carnivores in the past, we are today at the top of the food chain," he said. "However, our love for meat has everything to do with our Pleistocene carnivorous past."

Originally published on Live Science.