4aThe organism in the environment

The four terms — learn them word for word

How they nest inside each other

One species → population.

All the populations together → community (living things only).

Community + abiotic environment → ecosystem.

So an ecosystem is always bigger than a community, because it includes the non-living surroundings.

Wording that earns the mark

• Population: say "all the organisms of one species in a habitat at a given time". The words one species are the mark.
• Community: say "all the populations of different species" — not "all the organisms" and not "all the animals".
• Ecosystem: you must mention both the living community and the abiotic/physical/non-living factors.

Common mistake (examiners flag this every series): writing that a population is "all of the organisms in an area". That is a community. Equally common is defining an ecosystem when the question asked for a community — the community leaves out the soil, water and light.

Quick self-test

• All the dandelions in a field → population
• All the plants, insects and mice in that field → community
• The field including its soil, rainfall and sunlight → ecosystem
• Under a rotting log → habitat

Examiner tip. A common error is to refer to organisms rather than species, and to confuse population with community by saying a population is "all of the organisms". Population = one species; community = all the species; ecosystem = community + abiotic factors. Check which of the three the question actually names before writing.

The aim

Estimate the population size of one species (e.g. daisies, plantains, limpets) in two different areas — for example a mown lawn and an unmown field — and compare them.

Apparatus

• Quadrat — a square frame, usually 0.5 m × 0.5 m (area 0.25 m²)
• Two long tape measures
• Random number generator (calculator or table)
• Identification key

Method

1. Lay the two tape measures at right angles along two edges of the area to make x and y axes.
2. Use the random number generator to get pairs of coordinates. This gives random sampling, which avoids bias in where you put the quadrat.
3. Place the quadrat at those coordinates and count the number of individuals of your chosen species inside it. (For plants that are hard to count as individuals, record percentage cover instead — the % of the quadrat's area covered.)
4. Agree a rule for edges, e.g. only count a plant if more than half of it is inside the frame — and use the same rule every time.
5. Repeat for at least 10 quadrats in that area.
6. Repeat the whole procedure, using the same size quadrat and the same number of quadrats, in the second area.

The calculation

mean number per quadrat ÷ quadrat area = mean number per m²

mean per m² × total area of habitat (m²) = estimated population size

Worked example: 10 quadrats of 0.25 m² give a total of 60 daisies → mean = 6 per quadrat → 6 ÷ 0.25 = 24 per m² → in a 200 m² lawn the estimated population is 24 × 200 = 4800 daisies.

Variables

Reliability and accuracy — say it properly

• Reliable results come from taking many quadrats and calculating a mean; repeats reduce the effect of anomalies and of uneven (clumped) distribution.
• Random placement removes bias, so the sample better represents the whole area.
• A larger number of quadrats (or a larger sampling area) gives a more representative estimate.
• It is an estimate, not a count, because only part of the habitat is sampled.

Common mistake: writing "repeat to make it reliable" without saying you calculate a mean, or naming the dependent variable as "the quadrat" or "the area". The dependent variable is the thing you measure — the number counted.

Examiner tip. Examiners report that practical technique is usually good but that candidates misuse the terms 'reliable' and 'dependent variable'. Reliable = repeated many times (10+ quadrats) with a mean calculated; the dependent variable is the number of organisms counted, never the site. Also watch the command word: 'describe' the method means list the steps, while 'explain' means give reasons (e.g. why placement is random).

Biodiversity

Biodiversity is the number of different species present in an ecosystem (and how abundant each of those species is).

• A habitat with 20 plant species has higher biodiversity than one with 3.
• A woodland has higher biodiversity than a ploughed field of one crop.
• Note the word species — a habitat containing thousands of individuals of a single species has low biodiversity.

Measuring biodiversity with quadrats

Instead of counting one species, record how many different species appear in each quadrat.

1. Place quadrats randomly (random coordinates from two tape measures at right angles).
2. In each quadrat, use an identification key to name every species present, and record either the number of individuals or the percentage cover of each.
3. Take at least 10 quadrats.
4. Add up the total number of different species found across all quadrats in that habitat.
5. Repeat in the second habitat and compare. More species = greater biodiversity.

Investigating distribution — use a transect

Distribution means where organisms are found across a habitat. When the habitat changes gradually (up a beach, out from under a tree, from a path into a field), use systematic sampling, not random.

1. Lay a tape measure (the transect line) across the area, running along the gradient you are studying.
2. Place a quadrat at regular intervals along the line — e.g. every 2 m (this is a belt transect).
3. At each point record the species present and their percentage cover or numbers.
4. At the same points measure the abiotic factors — light intensity with a light meter, soil moisture with a moisture meter, temperature with a thermometer, soil pH with a pH meter.
5. Plot species abundance and the abiotic factor against distance along the transect.

Interpreting your results

• Describe = state the pattern, quoting figures: "percentage cover of moss falls from 40% at 0 m to 5% at 10 m."
• Explain = give the biological reason: "moss cover falls because light intensity increases away from the wall, and moss is out-competed by grasses in bright, drier conditions."

Examiner tip. Candidates lose marks by confusing 'describe' and 'explain'. Describe the trend using the data given; explain links it to an abiotic or biotic factor. Also define biodiversity as the number of different species — writing 'the number of organisms' scores nothing.

The split you must get right

Common mistake: listing "number of predators", "amount of food" or "competition" as abiotic factors. If it involves another living organism, it is biotic. Abiotic factors are the physical and chemical conditions only.

How abiotic factors change population size and distribution

• Light intensity — plants need light for photosynthesis. Bluebells grow and flower on a woodland floor in spring before the tree canopy shades them out; few plants grow under dense conifers.
• Temperature — enzymes work slowly in the cold, so growth and reproduction are slow. Few species live on high mountains or in polar regions.
• Water availability — desert plants are widely spaced because of low rainfall; woodlice are found in damp places under logs because they lose water easily.
• Soil pH and mineral content — nettles grow where soil nitrate levels are high; few plants grow in very acidic peat.
• Oxygen concentration — mayfly larvae are only found in fast-flowing, well-oxygenated streams; polluted, low-oxygen water contains sludgeworms instead.

How biotic factors change population size and distribution

• Predation — more predators eat more prey, so the prey population falls; the predator population then falls because there is less food, allowing prey numbers to rise again.
• Competition — organisms of the same or different species compete for limited resources. Plants compete for light, water and minerals; animals compete for food, water, territory and mates. The better-adapted competitor survives and its population grows.
• Food availability — if the food supply increases, more individuals survive and reproduce, so the population rises.
• Disease — a pathogen spreading through a dense population kills individuals and reduces population size rapidly.

Answering the question you were actually asked

Exam questions often give a food web or a graph and ask for the effect on one named organism.

• Name the actual organisms from the data — "the number of caterpillars falls" — not "the consumers decrease".
• Check whether it asks about a primary consumer (eats producers) or a secondary consumer (eats primary consumers) before you answer.
• If it asks how a change affects one population, do not write a general essay about the whole ecosystem.

Examiner tip. Two errors dominate here. First, candidates list biotic variables such as predator number when asked for abiotic factors — abiotic means non-living only. Second, candidates answer a different question from the one set (e.g. describing effects on the whole ecosystem when asked about one population) and refer vaguely to 'consumers' instead of naming the organisms in the food web. Read every word of the stem before writing.