Ecology & Food Chains

You cannot count every individual organism in a large area, so ecologists sample: count what is in a small area, then scale up to estimate the whole population.

Quadrats

A quadrat is a square frame, often 0.5 m × 0.5 m or 1 m × 1 m, placed on the ground. You count all the individuals of the species you are interested in inside the quadrat.

Quadrats are mainly used for plants and slow-moving animals (snails, limpets). They are no use for fast animals like birds or mammals, which would run away.

Using a quadrat to estimate population size

1. Mark out the area to be sampled (e.g. a 10 m × 10 m square) using two tape measures.
2. Place the quadrat at a random position by generating two random numbers (one for each coordinate). Random placement is essential: a biased choice of position (e.g. only sunny patches) would give a biased result.
3. Count and record the number of individuals of the target species inside the quadrat.
4. Repeat at least 10 times at different random positions, and take a mean count per quadrat.
5. Calculate the total population using:

estimated total population = (mean count per quadrat) × (total area ÷ area of one quadrat)

Two ways to measure with a quadrat

• Count individuals: works well for things like dandelions or daisies that you can clearly count.
• Percentage cover: works for plants like grass, moss or clover that you cannot count individually. Estimate the percentage of the quadrat area covered by the species.

Line transects (showing how a species varies along a gradient)

If a species is not evenly distributed but instead changes along a gradient (e.g. seaweed species change from the high tide line down to the sea), a quadrat placed randomly is not the right tool. Instead use a line transect:

1. Lay a tape measure in a straight line across the area, along the gradient (from one extreme to the other).
2. At regular intervals along the line (e.g. every 1 m, or every 5 m, depending on the length), place a quadrat.
3. Count the species in each quadrat as before.
4. Plot the results as a graph of abundance vs distance along the transect.

A line transect is perfect for studying changes from a sunny field edge into a shaded woodland, or from a polluted river bank along the gradient of pollution.

Capture-recapture (for mobile animals)

Quadrats do not work for animals that move (mice, fish, butterflies). Instead use the capture-recapture method:

1. Capture a sample of the animals using a humane trap. Count them (let's say you catch n₁ individuals).
2. Mark each one (e.g. small dot of paint, plastic tag).
3. Release them back into the habitat.
4. Wait long enough for the marked animals to mix back in with the rest of the population.
5. Capture a second sample (count = n₂). Among them, count how many are marked from the first sample (m).
6. Estimate the total population using:

estimated total population = (n₁ × n₂) ÷ m

Assumptions and limitations of capture-recapture

The capture-recapture method only gives an accurate estimate if:

• The population size has not changed between the two captures (no births, deaths, or migration)
• The mark does not harm the animal or make it easier for predators to see
• Marked animals mix back in randomly (don't avoid the traps the second time)
• Sampling is random

In real ecological studies, these assumptions are never perfectly true, but with care the estimate is usually within 10–20% of the true value.