Energy Flow: Trophic Levels, Food Webs and Ecological Pyramids

Remember from Grade 7

In Grade 7 you learned about producers, consumers and decomposers, drew food chains, and met the 10% rule with an energy pyramid in kilojoules. You also saw how removing one species can cause a ripple effect. In Grade 9 you will calculate transfer efficiencies, explain exactly where the missing energy goes, compare three kinds of ecological pyramids, and connect energy flow to the dynamic equilibrium of ecosystems.

Let's start with a fishing trip

An angler on Lake Nipissing catches a walleye. Where did the walleye's energy come from? It ate yellow perch. The perch ate zooplankton and insect larvae. Those ate algae. And the algae captured sunlight.

So the energy that powers a walleye (and the person who eats it for dinner) started as light from the Sun, about 150 million kilometres away. In this lesson you will find out why it takes a huge amount of algae to grow one walleye.

Energy enters, flows and leaves

1. Energy enters when producers (plants, algae, some bacteria) capture light energy and store it as chemical energy in sugar during photosynthesis. Only a small fraction of the sunlight that reaches an ecosystem is captured this way; most is reflected or turned into heat.
2. Energy flows from one trophic level to the next when organisms eat:

• Level 1: producers
• Level 2: primary consumers (herbivores such as zooplankton, snowshoe hares, deer)
• Level 3: secondary consumers (eat herbivores, such as yellow perch, red foxes)
• Level 4: tertiary consumers (eat secondary consumers, such as walleye)

1. Energy leaves as heat. Every organism uses energy for living (moving, keeping warm, building molecules). Cellular respiration releases that energy, and some always escapes as heat, which eventually radiates to space.

Decomposers and detritivores (such as earthworms) feed on dead matter and wastes from every level, so they get energy from all levels.

The key idea: energy flows in one direction. It cannot be recycled, so ecosystems need a constant new supply from the Sun.

Why is so much energy lost?

On average, only about 10% of the energy at one trophic level ends up stored in the bodies of the next level. (Real values vary, often from about 5% to 20%.) The rest is lost because:

• Not everything is eaten. Roots, bark, bones and fur are often left behind.
• Not everything eaten is digested. Some leaves the body as waste (feces). Plant material is especially hard to digest.
• Most digested energy is used for living. Cellular respiration powers movement, body heat, growth and repair, and releases heat. Warm-blooded animals (birds and mammals) use a lot of energy just to stay warm, especially in a Canadian winter.

The energy that is left, stored in new body tissue, is all that is available to the next level.

Food webs and why food chains are short

Real ecosystems are food webs, not single chains. Most animals eat more than one kind of food, and most are eaten by more than one predator. An omnivore such as a black bear can feed at several trophic levels (berries at level 2, fish at level 3 or 4).

Because about 90% of energy is lost at each step, there is rarely enough energy to support more than four or five trophic levels.

Three kinds of ecological pyramids

• Pyramid of energy: energy stored at each level (kJ per m² per year). It is always upright, because energy is lost at every step.
• Pyramid of numbers: the number of individual organisms at each level. It is usually upright, but can be inverted: one large sugar maple can feed thousands of caterpillars, which feed a few warblers.
• Pyramid of biomass: the total dry mass of living things at each level (g per m²). It is usually upright, but in some lakes and oceans it can be inverted at a single moment, because tiny algae reproduce so quickly that a small biomass of algae can feed a larger biomass of zooplankton over time.

Only the energy pyramid is always the right way up.

Energy flow and dynamic equilibrium

Energy flow is a naturally occurring phenomenon that keeps ecosystems in dynamic equilibrium. For example, if hares become very common, lynx have more food and their numbers rise. More lynx eat more hares, so hares decline; then lynx decline too. The numbers keep changing, but they swing around a fairly steady average. That is a dynamic equilibrium: constant change, overall balance. You will look at this more closely in Lesson 8.