Explain how energy flows through food chains and webs and why energy is lost at each trophic level.

What this dot point is asking

Producers, consumers, and decomposers

Organisms are grouped by how they obtain energy:

• Producers (autotrophs) make their own food, usually by photosynthesis. They are the entry point for energy into the ecosystem.
• Consumers (heterotrophs) obtain energy by eating other organisms. Primary consumers eat producers, secondary consumers eat primary consumers, and so on.
• Decomposers (such as fungi and bacteria) break down dead organisms and waste, releasing energy for themselves and returning nutrients to the environment.

Energy enters almost all ecosystems as light, is converted to chemical energy in producers, and flows onward as one organism eats another.

Food chains and food webs

A food chain shows a single line of who eats whom, for example grass to grasshopper to frog to snake. Arrows point in the direction of energy flow, from the organism being eaten to the one eating it.

In reality most organisms eat several foods and are eaten by several predators, so feeding relationships form a food web, a network of interconnected food chains. Food webs are more realistic and show how the loss of one species can affect many others.

Energy loss between levels

Energy does not pass perfectly from one trophic level to the next. At each level, most of the energy is:

• Used in respiration and lost as heat.
• Lost in undigested material (faeces) and waste.
• Locked in parts not eaten by the next consumer.

As a rough rule, only about ten percent of the energy in one trophic level is passed on to the next. The rest is lost, mostly as heat that cannot be reused by the ecosystem.

Ecological pyramids

The structure of energy flow can be drawn as a pyramid, with producers at the base and top predators at the tip:

• A pyramid of energy always narrows upward, because energy is lost at each level. It is the most reliable type.
• A pyramid of numbers counts individuals at each level and is usually pyramid-shaped, but can be inverted, for example when one large tree supports many insects.
• A pyramid of biomass measures the mass of living material at each level and is usually pyramid-shaped.

The pyramid of energy can never be inverted, because energy is always lost moving up.

Why energy flow matters

The one-way loss of energy explains many ecological patterns: why predators are rare, why ecosystems need constant energy input from the Sun, and why a long food chain is inefficient. It also has practical meaning: eating producers (plants) directly captures more of the original energy than eating animals that ate the plants, which is why plant-based food production can feed more people from the same land.

Gross and net primary productivity

When you describe how much energy producers fix, two figures matter. Gross primary productivity (GPP) is the total energy producers capture in photosynthesis. Net primary productivity (NPP) is what is left after the producers use some of that energy in their own respiration, so $NPP = GPP - R$, where $R$ is respiration. Only the NPP is actually available to the herbivores that eat the producers, which is one reason the energy passed to the next level is so much smaller than the energy that first entered the plant. Knowing the difference helps explain why a productive ecosystem (such as a tropical rainforest or an estuary) can support far more consumers than a low-productivity one (such as a desert), even when both receive plenty of sunlight, because water and nutrient supply also limit how much energy producers can fix.