Organisms require energy for basic life processes, such as growth, respiration and reproduction. Therefore, in order to sustain life, energy must be available within an ecosystem. The initial source of energy for almost every ecosystem on Earth is the sun: Solar energy is converted into biomass by primary producers and is then transferred between trophic levels from one consumer to the next up the food chain.

Trophic Levels and Food Chains
Organisms are categorized into trophic levels based on how they obtain food. Producers, alternatively known as autotrophs, are organisms that produce their own food by converting solar energy. Consumers, otherwise known as heterotrophs, are incapable of producing their own food and therefore rely on consuming the biomass of other organisms to obtain energy.

The first trophic level in any ecosystem is occupied by primary producers. These organisms use photosynthesis to convert solar energy into biomass, which sustains the entire ecosystem. Primary producers are usually plants in terrestrial ecosystems or phytoplankton in marine ecosystems.

The second trophic level is occupied by primary consumers: herbivores or omnivores who consume primary producers. The third trophic level is occupied by carnivores or omnivores who predate upon primary consumers from the second trophic level. The fourth trophic level is occupied by tertiary consumers, carnivores or omnivores who predate upon animals from the third trophic level.

A food chain illustrates these trophic relationships. Each stage represents a different trophic level, and the arrows demonstrate the flow of energy between levels. Here are examples of a terrestrial and marine food chain:

Grass → Grasshopper → Snake → Hawk

Phytoplankton → Zooplankton → Cod → Killer whale

The primary producers in these examples are phytoplankton and grass, and they occupy the first trophic level at the bottom of the food chain. The second trophic levels are occupied by the zooplankton and the grasshopper who feed on the phytoplankton and the grass, respectively. They are the primary consumers. The third trophic levels are occupied by the snake and cod, and they prey on the zooplankton and the grasshopper. These are the secondary consumers. The hawk and killer whale occupy the fourth trophic level and they hunt the snake and cod. These are the tertiary consumers.

Of course, this is an excessively simplified model. In reality, primary producers would be consumed by many different species, and tertiary consumers would need to predate upon varied species to survive. Most ecosystems are more complex and would be better represented by an interlinked series of food chains known as a food web.

Energy Transfer and the 10% Rule
Not all food chains and food webs consist of four trophic levels. Five is the maximum number of trophic levels most ecosystems can support. This is because of inefficiencies in energy flow, which begin with photosynthesis.

Of all the solar energy that reaches Earth, only a small percentage lands on plants. Furthermore, of all the sunlight that reaches plants, only a small amount is the right wavelength to fuel photosynthesis. Once a primary producer has converted this solar energy, it requires some for its own life processes such as respiration and growth. Only the remaining energy, known as Net Primary Productivity, is available to consumers and can be distributed as biomass throughout the trophic levels.

At each subsequent step along the food chain, more energy is wasted. There are three main reasons for this:

Some of the biomass consumed at each level can't be digested and is excreted as waste. A substantial proportion of energy escapes as heat during respiration. Plants and animals may die a nonpredatory death and their biomass isn't passed on to the next consumer.

Waste and dead matter are broken down by decomposers, and the resulting nutrients are recycled into the soil to be reabsorbed by plants. But the majority of the energy is lost as heat during this process.

The concept of energy loss between trophic levels can be illustrated using an energy pyramid diagram. Each step of the pyramid represents a different trophic level, starting with primary producers at the bottom. The size of each step represents the rate of energy flow through each trophic level. The steps decrease in size as you travel up the pyramid because energy is lost at every level in the food chain. Eventually, the step can't get any smaller, as there is no energy available to support another trophic level.

In a difficult-to-digest nutshell, energy transfer through an ecosystem is restricted by the ability of primary producers to convert solar energy, and the ability of consumers to take in the energy they obtain as biomass. Ultimately, only 10% of energy is transferred from one trophic level to the next. This is known as the 10% rule and it limits the number of trophic levels an ecosystem can support.