8.4Ecology
Effect of Density on Populations — AP BIOLOGY Revision Notes & Topical Questions
5 min read 8 parts Lesson 5 of 8 1 quiz
Lesson summary
8.4 Effect of Density on Populations — AP BIOLOGY notes: Explain how the density of a population affects and is determined by resource availability.Carrying capacity — the sustainable abundance an ecosystem's available resources can support.Limiting factors — resources or conditi…
- Logistic model
- Definition
- Competition
- Disease and parasites
- Toxic waste
- Regulation
Section 1 of 8Carrying capacity and limiting factors
Carrying capacity
- Carrying capacity the sustainable abundance an ecosystem's available resources can support.
Limiting factors
- Limiting factors resources or conditions restricting growth, which determine K.
Logistic model
- Logistic model limits to growth from density-dependent and density-independent factors typically give logistic growth, dN/dt = rmax N (K − N)/K.
Section 2 of 8Density-dependent factors
Definition
- Definition their effect increases as population density increases.
Competition
- Competition for food, water, space or mates; intraspecific competition rises with density.
Predation
- Predation more prey are caught at high density.
Disease and parasites
- Disease and parasites spread faster because contact is more frequent.
Toxic waste
- Toxic waste accumulates.
- (yeast and ethanol)
Regulation
- Regulation as density rises, birth rate falls and/or death rate rises until births equal deaths near K, a negative feedback that produces the S-shaped curve.
Section 3 of 8Density-independent factors
Definition
- Definition affect a population regardless of density.
Examples
- Examples weather (drought, frost), natural disasters (fire, floods) and many human impacts.
Effect
- Effect can cause sudden crashes at any density.
Section 4 of 8Population cycles and overshoot
1Predator–prey cycles
- Predator–prey cycles prey increase, predators increase after a lag, prey decrease, then predators decrease.
- (lynx and snowshoe hare)
2Overshoot
- Overshoot some populations exceed K, then crash because of resource depletion.
Section 5 of 8Limiting factors compared
| Factor | Type | Effect at high density | Example |
|---|---|---|---|
| Competition | Density-dependent | Stronger | Deer for food |
| Predation | Density-dependent | Stronger | Lynx and hare |
| Disease | Density-dependent | Stronger | Flu in crowded areas |
| Waste build-up | Density-dependent | Stronger | Yeast and ethanol |
| Weather | Density-independent | Same | Frost kills insects |
| Fire or flood | Density-independent | Same | Wildfire |
Section 6 of 8More from the specification
Biotic factor
- Living factor such as a predator
Exponential growth
- Unconstrained growth giving a J-shaped curve
Organism
- A living thing, such as a plant or an animal.
Community
- All populations in an ecosystem.
Ecology
- The study of organisms in their environment.
Biodiversity
- The number of different species in an area.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Density-dependent factor | factor whose effect grows with population density |
| Density-independent factor | factor acting regardless of density |
| Limiting factor | resource or condition restricting growth |
| Carrying capacity | sustainable population size (K) |
| Intraspecific competition | competition between members of one species |
| Overshoot | population rising above K before crashing |
Section 8 of 8Exam technique
Avoid these mistakes
Calling disease density-independent — it is density-dependent.
Calling a flood density-dependent — it affects all densities alike.
Thinking K is fixed forever — it changes with resources (humans have raised K through technology).
Marked practiceQuizzes for this lesson
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