Ecosystems and Food Chains: Quick Revision

A brief note on Ecosystems and Food Chains. Read it before a test. Try each question at the end before you open its answer.

The ecosystem

• An ecosystem is the living things of an area with the non-living parts around them.
• Biotic: plants, animals, microorganisms. Abiotic: soil, water, air, light, temperature.
• Natural: pond, forest, grassland. Artificial: garden, crop field, aquarium.
• Producers make food by photosynthesis. Consumers are herbivores, carnivores, omnivores and parasites.
• Decomposers, bacteria and fungi, break down dead matter and return nutrients to the soil.

Chains and webs

• A food chain shows who eats whom. It always starts with a producer.
• Each step is a trophic level: producers, herbivores, carnivores.
• Chains link into a food web, which is steadier than one chain.
• Higher levels usually hold fewer animals.

Energy and chemicals

• All the energy comes from the Sun. Plants store about 1% of the sunlight that falls on them.
• About 10% of the energy passes from one level to the next. The rest is lost, mostly as heat.
• So energy flows one way, and chains have only three or four levels.
• Non-biodegradable chemicals build up along a chain: biological magnification. The top consumers have the most.

A worked example

Maize plants store $40\,000$ kJ. The chain is maize → grasshopper → frog → snake. How much energy reaches the snake?

1. Grasshopper. $\dfrac{40\,000}{10} = 4000$ kJ.
2. Frog. $\dfrac{4000}{10} = 400$ kJ.
3. Snake. $\dfrac{400}{10} = 40$ kJ.

So the snake gets $40$ kJ, a thousandth of the energy in the maize.

Practice with new numbers

A program to try

The program follows the energy in the worked example, level by level.

Mistakes to avoid

• Leaving out the abiotic parts. An ecosystem includes soil, water, air and light.
• Drawing the arrows the wrong way. Arrows go from the food to the eater.
• Saying energy goes round. Nutrients cycle; energy flows one way.
• Putting the most pesticide in the crop. It builds up most at the top of the chain.

Check yourself

1. Give one producer, one consumer and one decomposer from a forest.

2. What is the trophic level of a herbivore?

3. Why is a food web steadier than a food chain?

4. Shrubs store $3000$ kJ. How much energy reaches the animals that eat the shrub-eaters?

5. Why are top carnivores most at risk from a pesticide that is not broken down?