Trophic Levels

Ecological efficiency and biomass distribution in nature.

Trophic levels are the different positions that organisms occupy in a food chain, based on what they eat and how they obtain energy.  The term trophic comes from the Greek word for "feeding," so each level represents a step in the flow of energy through an ecosystem.  At the base of the system are organisms that capture energy from sunlight, and as you move up, you find organisms that consume other living things.

Understanding trophic levels helps us see how energy flows through an ecosystem and how our food choices affect environmental health and resource use.

The Trophic Pyramid

If we think of energy distribution in nature as a pyramid, the wide bottom layer represents plants and other photosynthetic organisms that harness energy directly from the sun.  These plants, algae, and some bacteria are called primary producers, and they "eat sunlight."  Through photosynthesis, they convert solar energy into chemical energy in the form of sugars, which then serve as food for the rest of the ecosystem.

But not all sunlight captured by plants ends up as food for herbivores.  Some of the energy is used up by the plant to stay alive, to grow, and to reproduce.  What’s left is called Net Primary Productivity, which is the energy available to the next trophic level.

The next level up in the pyramid is the primary consumers, who eat the primary producers.  Insects, rabbits, deer, and other herbivores eat plants to gain the chemical energy in carbohydrates that the plants create.

Next are the secondary consumers, such as foxes and snakes, who are carnivores that eat the primary consumers.  When these animals eat other animals, they gain the stored chemical energy in their prey, primarily in the form of proteins and fats.  Tertiary consumers eat secondary consumers, such as eagles that eat foxes and snakes.  Some ecosystems even have quarternary consumers.

When organisms on all levels of the trophic pyramid die, their biomass is consumed and recycled into raw materials for the primary producers to reuse.  These recyclers are called decomposers and include organisms like fungi, bacteria, and worms.

Ecological Efficiency

The energy conversion process from one organism to another involves losses.  This transfer of energy between trophic levels is known as ecological efficiency, and it’s remarkably low.  Typically, only 10% of the energy from one level is passed on to the next. The other 90% is lost, primarily as heat, through biological processes like respiration and movement.

Photosynthesis is about 1% efficient at converting sunlight to biomass.  For every 100,000 joules of radiant energy from the sun, plants can produce 1,000 joules of chemical energy.  When a rabbit, a primary consumer, eats those plants, 100 joules of energy are incorporated into the rabbit.  When a fox eats the rabbit, 10 joules of energy are left in the fox.  And when an eagle eats the fox, only 1 joule of energy found in the original 100,000 joules of sunlight is left.

This ecological efficiency is why the trophic pyramid not only represents how energy flows through an ecosystem but also mirrors the distribution of biomass, or the weight of living things.  Most mass of living matter is found among the primary producers at the bottom, while the top predators make up only a tiny fraction of the overall biomass.

Mean and Fractional Trophic Levels

Not all organisms fit neatly into these trophic levels. For example, some carnivores may eat other carnivores and herbivores, and some plants eat insects. Some animals eat each other, such as when adult frogs eat crayfish and adult crayfish eat young frogs. There is a simple equation that calculates an organism's mean fractional trophic level based on what it eats:

where TLj  is the fractional trophic level of the prey j, and DCij represents the fraction of j in the diet of i. That is, the consumer trophic level is one plus the weighted average of how much different trophic levels contribute to its food.

A strict herbivore has a trophic level of 2, whereas a carnivore that only eats herbivores has a level of 3. So if a predator equally eats animals on level 2 and level 3, it would have a fractional trophic level of 3.5.

Thermodynamics of Animals

Metabolism is a term that means the total amount of biochemical reactions that occur in an organism. Kleiber's Law states that an animal's metabolic rate scales to the 3/4 power of its body mass. In other words, as animals get larger, their metabolism increases, but at a slower rate than their size. For example, a larger animal, like an elephant, uses more energy overall than a smaller animal, like a mouse, but it uses less energy per unit of body weight.

In endotherms, this is partly due to the role of surface-area-to-volume ratios and the requirement to maintain constant body temperatures. Respiration, the process of using oxygen and stored chemical energy to produce energy for the animal, is only about 40% efficient. 40% of the energy is used to contract muscles to move, and 60% is converted to heat.

Thermodynamics of Livestock for Meat Production

Since larger animals generally burn fewer calories per unit of body weight, we might think that it's better to raise larger animals for meat production. But this is not the case for a few reasons. Larger animals have to carry their extra mass around with them as they forage for food. Just the energy required to pump blood around, digest food, and breathe takes a large metabolic toll.

But the key factor is the big energy requirement for growth. Creating new biomass in an animal takes a lot of energy. A calf can't just maintain its small organs and skeleton and pack on edible meat. As it grows into a cow, it needs to build new bone, blood, hoof, and organ tissue.

Here's where things get interesting—where Klieber's Law runs into the Square-Cube Law. A bone's strength is proportional to its cross-sectional area, which scales with the square of its side length. But a bone's mass is proportional to its volume, which scales with the cube of its side length. Essentially, as animals grow larger, a greater percentage of their overall biomass becomes required for their basic structure and systems, and a lesser percentage is available as meat.

The results of these dynamics can be seen by looking at the Feed Conversion Ratios—the amount of calories that must be fed to an animal to produce a calorie of edible meat—of three main animals raised for meat. Chickens require about 7.7 calories to produce 1 calorie of meat, pigs require 11.6, and cows require 52.6. While not the same as ecological efficiency, this demonstrates some variation in biomass accumulation efficiencies within the same trophic level.

Human Diets

We humans occupy a fractional position in the trophic pyramid around 2.0 - 2.5, but this can vary widely depending on diet. We are omnivores by nature, and we can eat a variety of different diets to grow and be healthy. A person who eats mostly plants occupies a lower mean trophic level than someone who regularly consumes a lot of meat, especially from top predators like large fish or land animals.

The flexibility in our diet gives us the power to lower our ecological footprint by eating lower on the trophic pyramid. Eating more plants, for instance, uses less energy overall compared to eating animals, which require more energy, land, water, and other resources to raise. It pushes our trophic level more towards a 2 than a 3.

Author's note: This article is part 1. Part 2 will discuss the different environmental and resource costs of producing and consuming different foods. We will answer: How can we eat in a more ecologically friendly way? And, can it really make much of a difference?

Question for you:

• What's surprised you most about trophic levels?