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An ecosystem consists of biotic factors (organisms) and abiotic factors such as temperature and humidity, all depending on each other directly or indirectly. Ecosystems range from tide pools to large biomes; the five main types of biome are desert, aquatic, tundra, grassland and forest.

Energy enters almost every ecosystem as sunlight, captured by producers (autotrophs). It then passes along trophic levels: primary consumers eat producers, secondary consumers eat primary consumers, and so on up to apex predators.

On average only about 10% of the energy at one trophic level passes to the next. The rest is lost: much escapes as heat during respiration, some food is indigestible and leaves as waste, and some organisms die without being eaten and are broken down by decomposers.

Experiment: Abiotic factors and plant distribution

1. Choose two contrasting areas, for example ground under trees and open grass nearby. Write a hypothesis linking an abiotic factor to the plants.
2. In each area use two tapes at right angles and random numbers to place quadrats.
3. In each quadrat record the plant species present and estimate their abundance (number of individuals or percentage cover).
4. At the same spot measure light intensity, air or soil temperature and soil moisture. Take all readings within a short time, so the weather changes as little as possible.
5. Take at least 10 quadrats in each area (10–30 is better for statistics).
6. Calculate means for each area, draw a chart, and discuss whether the abiotic differences could explain the plant differences.

Quadrat	Area	Light	Temperature (°C)	Soil moisture	Species and abundance
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2
3
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10

Questions

1. Producers capture 20,000 kJ/m²/yr. Use the 10% rule to estimate the energy at the third trophic level (secondary consumers). Show your working.

2. Give three reasons why energy is lost between trophic levels.

3. Explain why a pyramid of energy is always upright.

4. Using the Yellowstone example, explain what a trophic cascade is and why scientists are careful about saying wolves 'caused' all the changes.

Sources for every fact on this sheet: echoesof.science/school/ecosystems/sources/

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