4.1 Essential ideas

4.1.2 Energy flow

The supply of inorganic nutrients is relatively constant in a stable ecosystem, since nutrients are cycled repeatedly through the non-living and living components. Energy, on the other hand, must constantly be supplied.

  • Most ecosystems rely on sunlight as an energy source. There is seasonal variation in quantity, but light energy is continuously available.
  • Photosynthetic autotrophs, like plants and some algae, convert light energy from the sun into chemical energy in the form of carbon compounds. The rate at which they do this is related to how productive an ecosystem is.
  • Chemical energy is transferred through the food chain in various forms by means of feeding. The energy stored in chemical bonds is converted into different forms, in order to do work like movement and digestion.
  • In general, less than 10% of total available energy is transferred to the next trophic level. Most of the stored chemical energy is lost as heat, a by-product of respiration.
  • The number of trophic levels in a food chain and the total biomass at the higher trophic levels is limited by energy losses.

food chainFigure 4.1.2a – A simple food chain showing the flow of energy through an ecosystem
Normally, the sun is not included in a food chain, but is shown here for illustrative purposes. This food chain has three trophic levels: a producer, a primary consumer and a secondary consumer.

Energetics of food chains and pyramids of energy

Most food chains have a maximum of four trophic levels. This makes sense when the energetics of a food chain is considered. As heat energy cannot be converted back into chemical energy, it dissipates into the surroundings and is lost.

To illustrate this point, let’s imagine a food chain with four trophic levels:

Food chain: Moss → Caterpillar → Sparrow → Hawk
Tropic level Producer Primary consumer Secondary consumer Tertiary consumer
Energy available
(% of first
trophic level)
100 10 1 0.1

Assuming a 10% transfer rate, which is very efficient, four trophic levels leaves only 0.1% of the energy stored in plants available to secondary consumers.

In addition to energy lost as heat, biomass is also lost in the form of urea, faeces, carbon dioxide and water. This explains why biomass of the large carnivores is small compared with that of the producers.

The energetics of food chains can be represented as a pyramid of energy.

Pyramid of energyFigure 4.1.2b – A pyramid of energy is a scaled diagram that shows how much energy is in each trophic level of a food chain.

Important points about energy pyramids:

  • The pyramid should be stepped and not triangular. The lowest step represents the producers, and each successive step upwards represents a consumer: primary, secondary, tertiary (and quaternary, when appropriate).
  • A pyramid of energy is a scaled diagram. This means that the area of the box in each step is proportional to the amount of energy represented by that step.
  • The units used in a pyramid of energy are kilojoules (kJ) or, more appropriately, a unit of energy per unit area per unit of time, kilojoules per square metre per year (kJ m-2 year-1).
  • Pyramids of energy will never appear inverted, as some of the energy stored in one source is always lost when transferred to the next source.

This is an application of the second law of thermodynamics.

Activity

Use 1mm graph paper to draw a pyramid of energy to represent the following food chain. (Hint: Make sure your pyramid is scaled.)

Trophic level Energy
(
kJ m-2 year 1)
Producers 8805
Primary consumers 1453
Secondary consumers 79
Tertiary consumers 12

Study tip

Repeat this mantra: ‘Nutrients cycle, but energy flows. Nutrients cycle, but energy flows …’

Consider this

Archaea are a group of ancient bacteria that are able to live in conditions that resemble the very ancient earth, such as volcanic vents. Some of these bacteria feed on heavy metals and gases produced in the Earth’s core. These ecosystems would survive the death of the Sun since they don’t rely on its energy!

CyanobacteriaFigure 4.1.2c – Cyanobacteria
The poorly named ‘blue-green algae’ is actually an entire phylum of photosynthetic bacteria. They are found in almost every terrestrial and aquatic ecosystem.

International mindedness

The productivity of different ecosystems is an important factor to consider when making judgements on which environments need the most protection. For example, the productivity of tropical rainforests (37,000 kJ m-2 year-1) is about three times more than temperate grasslands (12,500 kJ m-2 year-1), and more than ten times more than Arctic tundras (2,500 kJ m-2 year-1) (all values approximate).

rainforestFigure 4.1.2d – Tropical rainforest biomes are the most productive in the world

tundraFigure 4.1.2e – Arctic tundra is one of the least productive biomes in the world, especially in winter

temperate grasslandFigure 4.1.2f – Temperate grassland productivity is very high in the summer and very low in the winter

Nature of Science

Theories explain phenomena. Energy flow is a theoretical concept that explains why food chains are limited in length.