Humans eat nearly everything, so we must be at the top of the food web, right? Not so according to Bonhommeau et al. (Bonhommeau et al., 2013, PNAS 110: 20617-20620). The position of a species in a food web is called the ‘trophic level’ of the organism. By definition, plants have a trophic level of 1, such that animals that eat plants have a trophic level of 2. Animals that eat only plant-eating animals would have a trophic level of 3, and so on. Top, or apex predators (e.g. some sharks) max out at a trophic level of about 5.5. How do you get a decimal place in a trophic level? The trophic level is usually described as an average of all food consumption of the organism. So even though humans in some regions regularly eat shark, they also eat other animals and plants as well, meaning that on average they are not an apex predator.
Human trophic level
Bonhommeau et al. used almost 50 years of information on human food consumption around the world to calculate the human trophic level at a global level. They found that the human trophic level is ~2.21, meaning a diet that is primarily composed of plant foods, and is similar to that of pigs according to the authors. So what use is this information about human trophic level? Since producing food requires a considerable amount of resources (for plants it requires water, fertilizer, fossil fuels to transport fertilizer and work the fields, etc.; for animals it requires plant material, shelter, water, etc.), a measure of the human trophic level could provide one way to estimate the human impact on global resources and ecosystems, as well as human energy demand.
Trophic levels and energy
Plants use the energy contained in sunlight to produce sugars from sunlight and CO2, which can then be converted to other materials such as lipids and proteins to provide energy for almost all other non-plant organisms. As a rule of thumb (with many exceptions), only about 10% of the energy from one trophic level is transported to the next: for example, if there is enough energy from the sun to produce 100 kg of a grass, then the ecosystem might be able to support 10 kg of rabbit (I use mass in my example because sugars, proteins and fats can be considered as a form of stored energy). The reason for this relatively low level of energy transfer up the food web is because the other 90% of the energy at a given trophic level goes to reproducing, obtaining food, and maintenance of the organism. This also means that the population limit of animals that eat mainly plants tends to be higher than animals that eat other animals.
If we apply this to human diets and the human trophic level of ~2.21, this means that the planet could support an even higher global population if people switched to entirely plant-based diets. Thus we can use the human trophic level to also provide aid in estimating global population limits (where everyone would have enough to eat). *Disclaimer* – this isn’t an argument against eating meat, simply an observation. However knowing how the human trophic level changes with socio-economic factors and technological development just might give us the information we need to lessen our impact on the environment through agriculture.
Stay safe and stay informed,