The global food web illustrates how energy flows through different organisms. Humanity’s position within this complex system reveals insights into our dietary habits. Classifying humans is not as straightforward as labeling us carnivores or herbivores, as our diverse consumption patterns place us in a unique position.
Understanding Trophic Levels
Trophic levels describe the position an organism occupies in a food web, indicating how energy is transferred within an ecosystem. This hierarchical system begins with organisms that produce their own food. The first trophic level consists of producers, such as plants and algae, which generate energy primarily through photosynthesis.
The second trophic level comprises primary consumers, also known as herbivores, which feed directly on producers. Examples include animals like rabbits or cows that consume grass and other plant matter.
The third trophic level includes secondary consumers, which are either carnivores or omnivores that prey on primary consumers. Organisms like foxes, which might eat rabbits, or frogs, which consume grasshoppers, fit into this category.
Tertiary consumers occupy the fourth trophic level, feeding on secondary consumers. An eagle that preys on a snake, which in turn ate a frog, exemplifies this level. Some food webs also include apex predators, positioned at the highest trophic level, with no natural predators of their own.
The Human Trophic Level Calculation
Humans are omnivores, meaning our diet includes both plants and animals, making it challenging to assign us a simple integer trophic level. Instead, our position is represented by a calculated, non-integer value known as the Human Trophic Level (HTL). A scientific study published in the Proceedings of the National Academy of Sciences (PNAS) in 2013 calculated the global average HTL to be approximately 2.21. This value positions humans closer to the level of an anchovy or a pig in the global food web.
The HTL is determined by a weighted average of food items in the human diet. Plants, as primary producers, are assigned a trophic level of 1. When a human consumes a plant, their contribution from that item is 2.0 (1 + the plant’s trophic level of 1). If a human consumes an herbivore, like a cow (trophic level 2), the human’s contribution from that meat would be 3.0 (1 + the cow’s trophic level of 2).
The HTL calculation accounts for the varying trophic levels of all consumed food items, including different types of meat and fish, weighted by their quantity. For instance, eating fish that prey on other fish contributes a higher value than eating a herbivorous animal. This level has also shown an increase over time, aligning with a global trend towards diets with more meat.
Dietary Impact on Trophic Position
The human trophic level is not a fixed number for every individual; it varies considerably based on dietary choices. A person adhering to a 100% plant-based, or vegan, diet would have a trophic level of exactly 2.0, as they consume only primary producers. Conversely, an individual whose diet consists solely of herbivores, such as beef or lamb, would exhibit a trophic level closer to 3.0. Most human diets fall somewhere between these two extremes, reflecting a mix of plant-based foods and various animal products.
Globally, this variability is evident across different nations, as highlighted by the 2013 PNAS study. Countries with traditionally plant-heavy diets, such as India and many nations in sub-Saharan Africa and Southeast Asia, tend to have lower HTLs. For example, China and India saw their trophic levels increase from approximately 2.05 to 2.20 over half a century, indicating a shift towards higher meat consumption with economic growth. In contrast, countries with diets rich in meat and fish, like Iceland and Mongolia, exhibit much higher HTLs. Iceland’s HTL, for example, was among the highest, though it has seen a slight decline from 2.76 to 2.57.
Ecological Significance of Human Diet
Understanding humanity’s trophic level holds ecological importance due to the principles of energy transfer within food webs. A fundamental concept known as the “10% Rule” explains that only about 10% of the energy from one trophic level is typically incorporated into the biomass of the next level. The remaining energy is lost as heat during metabolic processes or is not consumed. This means that supporting organisms at higher trophic levels requires a substantially greater amount of energy and resources from the base of the food web.
For example, producing 10 kilograms of beef (a primary consumer at trophic level 2) requires roughly 100 kilograms of grain (a producer at trophic level 1). This beef, in turn, supports a human at trophic level 3. Consequently, diets with a higher proportion of meat demand exponentially more land, water, and other resources. A higher trophic level diet directly correlates with a larger environmental footprint, contributing to issues such as deforestation, water scarcity, and greenhouse gas emissions from livestock farming.