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.
60 fish are tagged. Later 80 are caught, and 12 are tagged. Estimate the population. (60 × 80) ÷ 12 = 400 fish.
What is the difference between a fundamental and a realized niche? The fundamental niche is where a species could live; the realized niche is where it does live, with competitors.
Which kind of species has many young and a short life? An r-strategist, such as a mouse; K-strategists have few young and a long life.
Why does predation act as negative feedback? More prey means more predators, which then cut prey numbers back down.
Give one reason human carrying capacity is hard to assess. Humans import resources and use technology, so the local environment does not set the limit.
A report claims that no one can say how many people the Earth can support. Explain why the carrying capacity for the human population is difficult to assess.
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.
- For regulation, close the loop: more animals, less food, fewer births, numbers fall.
- In a [9] on humans, name real examples and weigh both sides before you judge.