Key Idea: Most marks in topic 2.1 come from measuring populations and from how populations are limited. Learn those first, then humans and carrying capacity.
1. Measuring a population (most marks)
- Plants and other organisms that do not move: count them in quadrats placed at random.
- Animals that move: capture, mark, release, recapture, then the Lincoln index.
- Lincoln index: population = (M × N) ÷ R. Write the formula first, then the numbers.
- For a change along a gradient, sample at regular intervals along a transect.
2. How populations interact and are limited
- Six interactions: herbivory, predation, parasitism, mutualism, disease and competition.
- Carrying capacity is the largest population the resources can support.
- Density-dependent factors (competition, predation, disease) grow stronger as numbers rise: negative feedback.
- Growth is a J-curve (no limits) or an S-curve that levels off at carrying capacity.
3. Humans and carrying capacity
- Humans have removed many limiting factors with farming, medicine and trade.
- So the human population has grown, which puts more pressure on ecosystems.
- Human carrying capacity is hard to assess: we import resources, use technology and consume very differently.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Calculate a population with the Lincoln index | Formula, the numbers put in, then the answer with units. |
| [2] to [4] Describe a method to estimate a population | Name the method, then each step: where, how many, how you record. |
| [2] to [4] Outline the limits of a sampling method | Link each one to an assumption, such as marks being lost. |
| [4] to [7] Explain how a population is regulated | Name the factor, say how it rises with density, and close the feedback loop. |
| [9] To what extent can human carrying capacity be known? | Why it is hard, what helps, named examples, then a judgement. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Species | organisms that can breed together and produce fertile offspring |
| Population | one species living in the same area at the same time |
| Community | all the populations living and interacting in one place |
| Niche | the conditions and resources a species depends on, and its role |
| Carrying capacity | the largest population an environment's resources can support |
| Keystone species | a species whose effect on its ecosystem is much larger than its numbers |
Last-minute check: answer, then tap to see.
40 snails are marked. Later 50 are caught, and 10 of them are marked. How many snails are there? (40 × 50) ÷ 10 = 200 snails.
Why are quadrats placed at random? To avoid bias: every part of the area has an equal chance of being sampled.
What makes the S-curve level off? Density-dependent factors, such as competition for food, at carrying capacity.
Name the interaction where both species gain. Mutualism, such as bees and the flowers they pollinate.
Give one assumption of the Lincoln index. Marks are not lost and do not harm the animal, or the population is closed.
Ecologists want to estimate the number of field voles living in a grassland nature reserve. Describe how capture-mark-release-recapture could be used to estimate the size of this population.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- In a calculation, write the formula first, then the numbers, then the answer.
- For a method, give every step: where, how many, how you record, how you repeat.
- Name the exact interaction: 'mutualism', not just 'symbiosis'.
- For regulation, close the loop: more animals, less food, fewer births, numbers fall.