It's a platitude that we've all heard dozens of times, whether to justify our treatment of other species or simply to celebrate a carnivorous lifestyle: humans are the top of the food chain.
Ecologists, though, have a statistical way of calculating a species' trophic level—its level, or rank, in a food chain. And interestingly enough, no one ever tried to rigorously apply this method to see exactly where humans fall.
Until, that is, a group of French researchers recently decided to use food supply data from the U.N Food and Agricultural Organization (FAO) to calculate human tropic level (HTL) for the first time. Their findings, published today in the Proceedings of the Natural Academy of Sciences, might be a bit deflating for anyone who's taken pride in occupying the top position.
On a scale of 1 to 5, with 1 being the score of a primary producer (a plant) and 5 being a pure apex predator (a animal that only eats meat and has few or no predators of its own, like a tiger, crocodile or boa constrictor), they found that based on diet, humans score a 2.21—roughly equal to an anchovy or pig. Their findings confirm common sense: We're omnivores, eating a mix of plants and animals, rather than top-level predators that only consume meat.
To be clear, this doesn't imply that we're middle-level in that we routinely get eaten by higher-level predators—in modern society, at least, that isn't a common concern—but that to be truly at the "top of the food chain," in scientific terms, you have to strictly consume the meat of animals that are predators themselves. Obviously, as frequent consumers of rice, salad, bread, broccoli and cranberry sauce, among other plant products, we don't fit that description.
The researchers, led by Sylvain Bonhommeau of the French Research Institute for Exploitation of the Sea, used FAO data to construct models of peoples' diets in different countries over time, and used this to calculate HTL in 176 countries from 1961 to 2009. Calculating HTL is fairly straightforward: If a person diet is made up of half plant products and half meat, his or her trophic level will be 2.5. More meat, and the score increases; more plants, and it decreases.
With the FAO data, they found that while the worldwide HTL is 2.21, this varies widely: The country with the lowest score (Burundi) was 2.04, representing a diet that was 96.7 percent plant-based, while the country with the highest (Iceland) was 2.54, reflecting a diet that contained slightly more meats than plants.
On the whole, since 1961, our species' overall HTL has increased just slightly—from 2.15 to 2.21—but this averaged number obscures several important regional trends.
HTL trends in five different countries with similar characteristics. Click to enlarge. Image via PNAS/Bonhommeau et. al.
A group of 30 developing nations in Southeast Asia and Sub-Saharan Africa (shown in red)—including Indonesia, Bangladesh and Nigeria, for example—have had HTLs below 2.1 during the entire period. But a second group of developing countries that includes India and China (shown in blue) has slightly higher HTL measures that have consistently risen over time, going from around 2.18 to over 2.2. The HTLs of a third group, shown in green (including Brazil, Chile, South Africa and several countries in Southern Europe), have risen further, from around 2.28 to 2.33.
By contrast, HTL in the world's wealthiest countries (shown in purple)—including those in North America, Northern Europe and Australia—was extremely high for most of the study period but decreased slightly starting during the 1990s, going from around 2.42 to 2.4. A fifth group of small, mostly island countries with limited access to agricultural products (shown in yellow, including Iceland and Mauritania) has seen more dramatic declines, from over 2.6 to less than 2.5.
These trends closely correlate, it turns out, with a number of World Bank development indicators, such as gross domestic product, urbanization and education level. The basic trend, in other words, is that as people become wealthier, they eat more meat and fewer vegetable products.
That has translated to massive increases in meat consumption in many developing countries, including China, India, Brazil and South Africa. It also explains why meat consumption has leveled off in the world's richest countries, as gains in wealth leveled off as well. Interestingly, these trends in meat consumption also correlate with observed and projected trends in trash production—data indicate that more wealth means more meat consumption and more garbage.
But the environmental impacts of eating meat go far beyond the trash thrown away afterward. Because of the quantities of water used, the greenhouse gases emitted and the pollution generated during the meat production process, it's not a big leap to speculate that the transition of huge proportions of the world's population from a plant-based diet to a meat-centric one could have dire consequences for the environment.
Unfortunately, like the garbage problem, the meat problem doesn't hint at an obvious solution. Billions of people getting wealthier and having more choice over the diet they eat, on a basic level, is a good thing. In an ideal world, we'd figure out ways to make that transition less damaging while still feeding huge populations. For example, some researchers have advocated for offbeat food sources like meal worms as a sustainable meat, while others are trying to develop lab-grown cultured meat as an environmentally-friendly alternative. Meanwhile, some in Sweden are proposing a tax on meat to curb its environmental cost while government officials in the UK are urging consumers to cut back on their demand for meat to increase global food security and to improve health. Time will tell which approaches stick.
In the meantime, simply keeping track of the amount of meat we're eating as a society via HTL could provide a host of useful baseline information. As the authors write, "HTL can be used by educators to illustrate the ecological position of humans in the food web, by policy makers to monitor the nutrition transition at global and national scales and to analyze the effects of development on dietary trends, and by resource managers to assess the impacts of human diets on resource use."
In other words, monitoring the intricacies of our middling position on the food chain may yield scientific fodder to tackle problems like food security, obesity, malnutrition and environmental costs of the agricultural industry. A heavy caseload for a number that ranks us on the same trophic level as anchovies.
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The food chain describes who eats whom in the wild. Every living thing—from one-celled algae to giant blue whales—needs food to survive. Each food chain is a possible pathway that energy and nutrients can follow through the ecosystem.
For example, grass produces its own food from sunlight. A rabbit eats the grass. A fox eats the rabbit. When the fox dies, bacteria break down its body, returning it to the soil where it provides nutrients for plants like grass.
Of course, many different animals eat grass, and rabbits can eat other plants besides grass. Foxes, in turn, can eat many types of animals and plants. Each of these living things can be a part of multiple food chains. All of the interconnected and overlapping food chains in an ecosystem make up a food web.
Trophic Levels
Organisms in food chains are grouped into categories called trophic levels. Roughly speaking, these levels are divided into producers (first trophic level), consumers (second, third, and fourth trophic levels), and decomposers.
Producers, also known as autotrophs, make their own food. They make up the first level of every food chain. Autotrophs are usually plants or one-celled organisms. Nearly all autotrophs use a process called photosynthesis to create “food” (a nutrient called glucose) from sunlight, carbon dioxide, and water.
Plants are the most familiar type of autotroph, but there are many other kinds. Algae, whose larger forms are known as seaweed, are autotrophic. Phytoplankton, tiny organisms that live in the ocean, are also autotrophs. Some types of bacteria are autotrophs. For example, bacteria living in active volcanoes use sulfur compounds to produce their own food. This process is called chemosynthesis.
The second trophic level consists of organisms that eat the producers. These are called primary consumers, or herbivores. Deer, turtles, and many types of birds are herbivores. Secondary consumers eat the herbivores. Tertiary consumers eat the secondary consumers. There may be more levels of consumers before a chain finally reaches its top predator. Top predators, also called apex predators, eat other consumers.
Consumers can be carnivores (animals that eat other animals) or omnivores (animals that eat both plants and animals). Omnivores, like people, consume many types of foods. People eat plants, such as vegetables and fruits. We also eat animals and animal products, such as meat, milk, and eggs. We eat fungi, such as mushrooms. We also eat algae, in edible seaweeds like nori (used to wrap sushi rolls) and sea lettuce (used in salads).
Detritivores and decomposers are the final part of food chains. Detritivores are organisms that eat nonliving plant and animal remains. For example, scavengers such as vultures eat dead animals. Dung beetles eat animal feces.
Decomposers like fungi and bacteria complete the food chain. They turn organic wastes, such as decaying plants, into inorganic materials, such as nutrient-rich soil. Decomposers complete the cycle of life, returning nutrients to the soil or oceans for use by autotrophs. This starts a whole new food chain.
Food Chains
Different habitats and ecosystems provide many possible food chains that make up a food web.
In one marine food chain, single-celled organisms called phytoplankton provide food for tiny shrimp called krill. Krill provide the main food source for the blue whale, an animal on the third trophic level.
In a grassland ecosystem, a grasshopper might eat grass, a producer. The grasshopper might get eaten by a rat, which in turn is consumed by a snake. Finally, a hawk—an apex predator—swoops down and snatches up the snake.
In a pond, the autotroph might be algae. A mosquito larva eats the algae, and then perhaps a dragonfly larva eats the young mosquito. The dragonfly larva becomes food for a fish, which provides a tasty meal for a raccoon.