What you'll learn

• How to use the terms population, community, habitat and ecosystem correctly.
• How to estimate population size using quadrats in two different areas.
• What biodiversity means and how quadrats can be used to measure it (Paper 2 only).
• How abiotic and biotic factors affect where organisms live and how many survive.

Ecology starts with “where organisms live”

Ecology is the study of how organisms interact with each other and with their environment. In this topic, you are thinking like a field biologist: you collect data from real habitats, then use that data to describe patterns and suggest explanations.

A species is a group of organisms that can reproduce together to produce fertile offspring. For example, all common daisies belong to the same species.

Think of these terms as building up in scale: organism → population → community → ecosystem.

Sampling organisms using quadrats

In ecology, it is usually impossible to count every individual organism in a habitat. Instead, scientists use sampling: collecting data from smaller parts of the habitat and using it to estimate the whole.

A quadrat is a square frame of known area, often 1 m by 1 m, placed on the ground to count organisms in that area. Quadrats are especially useful for plants or slow-moving animals.

The diagram shows two important sampling methods: random quadrats for estimating population size, and quadrats along a transect for investigating distribution.

Random sampling for population size

Random sampling means choosing quadrat positions without bias. This matters because you must not only sample the “interesting” or “easy” parts of the habitat.

A typical method is:

1. Mark out the area using tape measures.
2. Use random numbers to generate coordinates.
3. Place the quadrat at each coordinate.
4. Count the number of individuals of the target species in each quadrat.
5. Repeat many times.
6. Calculate a mean, then estimate the population size.

Population density means the number of individuals per unit area.

$$population\; density=\frac{number\; of\; individuals\; counted}{area\; sampled}$$

Then:

$$estimated\; population\; size=population\; density\times total\; habitat\; area$$

Practical: comparing population size in two areas

For specification point 4.2, you need to understand how to investigate the population size of an organism in two different areas using quadrats.

For example, you might compare the number of daisies on a mown lawn and an unmown lawn.

Method

1. Choose the organism you are counting and define the two areas clearly.
2. Use the same size quadrat in both areas.
3. Use random coordinates to place quadrats in the first area.
4. Count the number of the organism in each quadrat.
5. Repeat with the same number of quadrats in the second area.
6. Calculate the mean number per quadrat, or calculate population density.
7. Compare the two areas.

The independent variable is the area or habitat being compared. The dependent variable is the number of organisms per quadrat, population density or estimated population size. Control variables include quadrat size, number of quadrats, counting method, time of day or season, and the edge rule used.

The expected result is not one fixed answer. You are looking for a valid comparison: one area may have a higher population density because conditions are more suitable.

Biodiversity

Biodiversity is a Paper 2 only idea in this section, but it is still worth learning carefully.

Two useful terms are:

• Species richness: the number of different species present.
• Abundance: how many individuals of each species are present.

A habitat with five species is usually more biodiverse than a habitat with only one species. However, biodiversity is also affected by whether one species completely dominates the area.

Practical: distribution and biodiversity using quadrats

For specification point 4.4B, you need to understand how quadrats can be used to investigate the distribution of organisms in their habitats and measure biodiversity. This is Paper 2 only.

Distribution means where organisms are found within a habitat. Organisms are often not spread evenly. For example, moss may be common in damp shade but rare on a dry path.

A transect is a line placed across a habitat. In systematic sampling, quadrats are placed at regular intervals along this line, such as every 1 m. This is useful when there is an environmental gradient, such as from dry to wet, or from shaded to sunny.

Method for distribution

1. Lay a tape measure across the habitat.
2. Place a quadrat at regular intervals along the tape.
3. Count the organisms, or estimate percentage cover if counting individuals is difficult.
4. Record an abiotic factor at each point, such as light intensity or soil moisture.
5. Look for a pattern between the organism’s abundance and the environmental conditions.

Percentage cover is an estimate of how much of the quadrat area is covered by a species. It is useful for grasses, mosses or spreading plants that are hard to count as separate individuals.

Percentage frequency is the percentage of quadrats in which a species is found.

$$percentage\; frequency=\frac{number\; of\; quadrats\; containing\; the\; species}{total\; number\; of\; quadrats}\times 100$$

Method for biodiversity

To measure biodiversity with quadrats:

1. Place quadrats randomly, or systematically if you are investigating a gradient.
2. Identify all the species in each quadrat.
3. Count individuals or estimate percentage cover.
4. Record species richness and abundance.
5. Compare areas using the same sampling method.

Sources of error include misidentifying species, using too few quadrats, choosing quadrat positions non-randomly when random sampling is needed, and making subjective percentage cover estimates.

Abiotic and biotic factors

An abiotic factor is a non-living environmental factor. Examples include:

• light intensity
• temperature
• water availability or soil moisture
• soil pH
• mineral ion concentration
• oxygen or carbon dioxide concentration
• wind exposure

A biotic factor is a living factor or something caused by living organisms. Examples include:

• food availability
• competition
• predation
• disease
• grazing
• availability of mates

These factors affect both population size and distribution. A population increases when conditions allow more organisms to survive and reproduce. A population decreases when death rate is higher than birth rate, or when organisms move away.