Food chains, food webs and trophic levels
A food chain shows how energy passes from one living thing to the next as food. The arrows always point in the direction the energy flows, that is, from the thing being eaten to the thing eating it.
A simple example: grass → grasshopper → frog → snake. The grass is the producer because it makes its own food by photosynthesis. Everything after it is a consumer.
- Trophic level
- A feeding stage in a food chain. Producers are the first level, primary consumers (herbivores) the second, secondary consumers the third, and so on.
In real life, most animals eat more than one thing, so several food chains link together into a food web. A food web is a more honest picture, because it shows that if one species disappears, many others are affected, not just one.
Energy loss between levels
Energy enters almost every food chain as sunlight, which producers capture in photosynthesis. As you move up the levels, though, the amount of energy passed on gets smaller each time. Only about 10% of the energy at one level reaches the next.
Where does the rest go? Living things use most of their energy for:
- respiration (releasing energy for movement and warmth, much of it lost as heat),
- waste passed out as faeces and urine, and
- uneaten parts, like bones or roots, that the next animal does not eat.
This is why food chains rarely have more than four or five links: after a few steps there simply is not enough energy left to support another level. It also explains why there are usually many more producers than top predators.
Grass contains 10,000 kJ of energy. If 10% passes to each level, how much reaches the frog (the third level: grass, grasshopper, frog)?
Biotic and abiotic factors
Whether an organism can survive in a place depends on many conditions. We sort them into two groups.
| Type | Meaning | Examples |
|---|---|---|
| Biotic factors | The living parts of an environment | Predators, prey, competition for food, disease |
| Abiotic factors | The non-living, physical parts | Temperature, light, water, pH, oxygen level |
An easy way to remember it: biotic means biological, so alive. Abiotic is the opposite, the physical conditions. A change in either can push a population up or down.
Where this is assessed
Explaining food webs and energy flow is Criterion A (knowing and understanding). Planning a fair quadrat survey is Criterion B (inquiring and designing), scaling up a sample to an estimate is Criterion C (processing and evaluating), and discussing human effects on ecosystems is Criterion D (reflecting on the impacts of science).
Quadrat sampling
You cannot count every daisy in a field, so ecologists count a small sample and scale it up. A quadrat is a square frame, often 1 m by 1 m, placed on the ground so you can count what lies inside it.
To keep it fair you place quadrats at random positions (for example using coordinates from a random number generator), so you do not just pick the patches that look interesting. Then you take the mean of several quadrats and multiply up to the whole area.
A student places 5 quadrats (each 1 m²) and counts 4, 6, 3, 5 and 2 dandelions. Estimate the total number in a field of 800 m².
It is only an estimate
The more quadrats you use and the more random your placing, the more reliable your estimate. A few quadrats in one corner would give a biased, misleading answer.
Check yourself
1. In the chain grass → rabbit → fox, what is the rabbit's trophic level and role? +
The rabbit is at the second trophic level. It is a primary consumer (a herbivore), because it eats the producer.
2. Why is so little energy left for top predators? +
Only about 10% of energy passes between levels. The rest is lost through respiration (heat), waste and uneaten parts, so after several steps very little remains.
3. Six quadrats of 1 m² give a mean of 5 buttercups each. Estimate the number in a 200 m² lawn. +
Multiply the mean per m² by the area: 5 × 200 = 1,000 buttercups.
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