Density dependence at Higher Level: The same ideas as SL. At HL, practise explaining each factor as part of a feedback loop, step by step.
Practise this as you read
- Sort each factor: does its effect grow with crowding?
- Trace the loop: change, response, and the change reversed.
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Does crowding matter?: A density-dependent factor gets stronger as a population gets more crowded. A density-independent factor does not care how crowded it is.
The points to remember
- Density-dependent factors grow stronger as a population gets denser: competition, predation, disease, parasites.
- In a crowded population, predators find prey more easily and pathogens pass from one to another faster.
- Density-independent factors hit a population whatever its density: storms, floods, fires, droughts, hard frosts.
- Density-independent factors can kill many, but they do not hold a population near its carrying capacity.
- It is the density-dependent factors that regulate a population around its carrying capacity.
Remember it as: Crowding makes it worse? Density-dependent. Weather that hits everyone? Density-independent.
Real example: in 2022, bird flu spread fast through the tightly packed gannets on Bass Rock, Scotland, killing thousands: density-dependent.
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Back to balance: Density-dependent factors work as negative feedback: they push a population back towards its carrying capacity whenever it moves away.
The points to remember
- The population rises above its carrying capacity.
- Competition, predation and disease all increase.
- Fewer births and more deaths, so the population falls back.
- Below the carrying capacity these pressures ease, so the population rises again.
- This is negative feedback: it returns the population to equilibrium around the carrying capacity.
Remember it as: Too many: more squeeze, fewer survive. Too few: less squeeze, more survive.
Real example: in Wytham Woods, Oxford, great tits have been studied since 1947; in years with more breeding pairs, each pair raises fewer chicks.
Two loops, one curve: An S-shaped growth curve shows both kinds of feedback: positive feedback drives the early growth, and negative feedback slows it and holds it at the carrying capacity.
The points to remember
- At the start, more individuals breed, making more young, who breed in turn: positive feedback.
- Positive feedback is a change that leads to more of the same change: fast, exponential growth.
- Near the carrying capacity, density-dependent factors (food, water, predation, disease) grow stronger.
- They slow the growth rate and hold numbers near the carrying capacity: negative feedback.
- Negative feedback is a change that leads to less of that change: it stabilises the population.
Remember it as: Positive: more makes more. Negative: more makes less.
Real example: collared doves spread across Europe from Turkey in the 1900s; in each new country their numbers grew fast, then levelled off.
Name both loops: Write 'positive feedback' and 'negative feedback' by name, each with its part of the curve. Describing the curve without naming the loops scores far fewer marks.
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How this comes up: Paper 2, Section B (b): explain how positive and negative feedback produce an S-curve [7]. Short questions ask you to tell density-dependent factors from density-independent ones [2].
A few yeast cells are added to a flask of sugar solution kept at a steady, warm temperature. Their numbers are counted every hour.
Explain how both positive and negative feedback mechanisms may play a role in producing a typical S population growth curve for a species.
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