Ecological relationships and energy flow (CCEA)

The Sun is the source of energy for most ecosystems on the Earth.

Plants are producers. They use light energy absorbed by chlorophyll to make sugars and starches through photosynthesis, which provide food for animals.

Animals, or consumers, get this energy by feeding on plants or other animals.

Herbivores (plant eating animals) are primary consumers.

Carnivores (animals who eat other animals) are secondary consumers, and those eating secondary consumers are tertiary consumers.

The feeding sequence, showing the energy flow from producers to consumers, is called a food chain.

Each step is called a trophic level: producers are on the first trophic level 1, primary consumers are 2nd trophic level, and so on.

Specific feeding sequences (and therefore flow of energy) can be represented as a food chain.

Arrows between each organism show the direction of:

• feeding (consumption)
• energy flow
• the transfer of substances such as carbon and nitrogen

Food webs show interconnecting food chains showing feeding relationships within a community.

Most organisms will have more than one food source that can be seen in a food web but not a food chain.

Energy is lost between each trophic level.

From the Sun to the plant (producer), energy is lost when light is reflected off the leaf or passes through the leaf missing the chloroplasts.

Between each trophic level only 10-20% of the energy is transferred resulting in a loss of 80-90% of energy.

Energy is lost at every trophic level due to:

• the whole organism not being eaten (skeleton and fur left behind)

• not all the food being digested – some passes out of the animal in excretion or egestion

• energy being lost as heat in respiration and therefore not being passed onto the next level

• energy used for movement, reproduction and growth

Shorter food chains are more energy-efficient because there are fewer levels for energy to be lost through.

These show the number of individual organisms at each trophic level in a food chain.

Advantage Simple and easy to construct.

Disadvantage Can be misleading if organisms vary greatly in size, as they do not take the organism’s size into account. For example, if the producer is one tree then pyramid will look inverted (smaller on bottom).

These show the total mass of living organisms at each trophic level, which gives a better idea of the energy available.

Advantage More accurately reflects the energy stored in organisms at each level as the organism’s size is taken into account. For example, a bar representing one tree will be largest as it has more mass than primary consumers.

Disadvantage It's more difficult to create because measuring biomass requires killing the organism, drying it in an oven to remove water and reaching a constant dry mass, which is time-consuming and complex.