Energy Transfer In Food Chains

Noadswood Science, 2011

Energy Transfer In Food Chains

• To understand how energy is transferred in food chains

Tuesday, April 18, 2023

Food Chains

• A food chain shows what eats what in a particular habitat

• For example, grass seed is eaten by a vole, which is eaten by a barn owl – the arrows between each item in the chain always point in the direction of energy flow (from the food to the feeder)

Energy Origins

• Where does the energy for a food chain originate from?

• The Sun is the ultimate source of energy for most communities of living things – green plants absorb some of the Sun’s light energy to make their own food by photosynthesis

• The other organisms in a food chain are consumers, because they all get their energy and biomass by consuming (eating) other organisms

Limited

• Energy is transferred along food chains from one stage to the next, but not all of the energy available to organisms at one stage can be absorbed by organisms at the next one

• The amount of available energy decreases from one stage to the next – some of the available energy goes into growth and the production of offspring, and this energy becomes available to the next stage, but most of the available energy is used up in other ways: -• Energy released by respiration is used for movement and other

life processes, and is eventually lost as heat to the surroundings• Energy is lost in waste materials, such as faeces

• All of the energy used in these ways returns to the environment, and is not available to the next stage

Food Chain

• Food chains are never very long (usually only 4/5 stages at most) – why is this?

• Remember the arrow shows the energy being transferred from one organism to the next - between each step energy is lost in a variety of ways, including: -• Growth of the organism• Respiration• Reproductive costs• Lost through waste products (poo)• Lost through heat

• This is why food chains are never that long - as lots of energy is lost from one stage to the next

Food Chain

Food Web

• In most habitats organisms normally eat / are eaten by more than one other organism

• To represent this we use food webs (like food chains but they interlink with one another), e.g. a pond

Here the producers are the pondweed and the microscopic algae

Mayfly nymphs eat the pondweed and microscopic algae, and freshwater shrimp eat the microscopic algae

Dragonfly nymphs and brown trout eat the mayfly nymphs and freshwater shrimp

Brown trout also eat the dragonfly nymph!

Key Words

• Producers – green plants make food by photosynthesis

• Primary consumers – usually eat plant material (they are herbivores)

• Secondary consumers – usually eat animal material (they are carnivores)

• Predators – kill for food (they are either secondary or tertiary consumers)

• Prey – the animals that predators feed on

• Scavengers – feed on dead animals

• Decomposers – feed on dead and decaying organisms, and on the undigested parts of plant and animal matter in faeces

Balance• Populations in a food chain are dependent upon one another –

there is a balance between the producers and consumers in the food chain

• For example if there are lots of giraffes they will eat a large quantity of the trees and shrubs until there may be insufficient amounts to support them. If this is the case many will begin to starve and die

• Fewer giraffes means less food for lions, which will eventually see a population decrease

• Less giraffes will allow more shrubs and tress to survive, so the giraffe population recovers, leading to the lion population recovering, until the process repeats itself…

Efficiency

• This animal has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas

• The energy stored in its body tissues is 4 kJ – how much has been used up in respiration?

• How much energy is passed on (energy efficiency percentage)

Efficiency

• This animal has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas

• The energy stored in its body tissues is 4 kJ – how much has been used up in respiration?

• Energy taken in = 100KJ• Energy transferred to tissues + faeces etc… = 63KJ + 4KJ = 67KJ• Energy released by respiration = 100KJ – 67KJ = 33KJ

• Only 4 kJ of the original energy available to the animal is available to the next stage – the efficiency of this energy transfer is: -

Efficiency = (4KJ ÷ 100KJ) × 100 = 4%

Energy Transfer Question

1. What do plants use a lot of their energy for?

2. Do all organisms loose the same amount of heat energy?

3. Why doesn’t the whole of an organisms body get passed to the next stage of the food chain?

4. Food chains are rarely more than 5 trophic levels, why?

5. What happens to the energy lost in animal waste?

6. Why is food production more efficient with less stages in the food chain?

7. Explain using pyramids of biomass why it would be better to eat less meat and more plants with an increasing population

Energy Transfer Answers

1. What do plants use a lot of their energy for?

To make new cells

2. Do all organisms loose the same amount of heat energy?

No warm blooded animals loose more as their body has to be kept at a constant temperature

3. Why doesn’t the whole of an organisms body get passed to the next stage of the food chain?

Some of it is inedible or can’t be digested e.g. bones and teeth, fibre and cell walls

4. Food chains are rarely more than 5 trophic levels, why?

So much energy is lost at each stage that there is not enough left to support the organisms after 4 or 5 stages

Energy Transfer Answers

5. What happens to the energy lost in animal waste?

It is recycled in the soil and eventually new plants will use it

6. Why is food production more efficient with less stages in the food chain?

Less material and less energy is wasted this means more food can be produced

7. Why is food production more efficient with less stages in the food chain?

More food would be made as less energy is lost, so in the limited land space enough food would be made to go around