8.5Ecology
Community Ecology — AP BIOLOGY Revision Notes & Topical Questions
5 min read 10 parts Lesson 6 of 8 1 quiz
Lesson summary
8.5 Community Ecology — Notes — AP Biology notes: Describe the structure of a community according to its species composition and diversity.Community = all populations of different species living and interacting in an area; its structure includes species composition and species di…
- Competitive exclusion
- Keystone species
- Simpson's index
- Parasitism
- Primary succession
- Secondary succession
Section 1 of 10Interactions between populations
1
Community = all populations of different species living and interacting in an area; its structure includes species composition and species diversity.
2
Dynamics — communities change over time because interactions change, and interactions can be positive or negative.
3
Predator–prey — as predators thrive they put pressure on prey populations.
4
Trophic cascades — adding or removing a top predator affects all species and the energy distribution in the web.
5
Competition — for resources drives natural selection, and the best adapted survive.
6
Niche partitioning — natural selection drives species to use slightly different niches or resources to reduce competition.
Section 2 of 10Symbiosis and interaction types
Symbiosis and interaction types
Parasitism = one organism is harmed while the other benefits (ticks on rhinos).
Commensalism = one benefits and the other is neither harmed nor helped (cattle egrets feeding on insects disturbed by cattle).
Mutualism = both benefit (oxpeckers feeding on rhino ticks, mycorrhizae and roots).
| Interaction | Effect on A / B | Example |
|---|---|---|
| Competition | − / − | Barnacle species |
| Predation | + / − | Lynx and hare |
| Herbivory | + / − | Deer and plants |
| Mutualism | + / + | Mycorrhizae and roots |
| Commensalism | + / 0 | Barnacles on a whale |
| Parasitism | + / − | Tick and dog |
Section 3 of 10Niches and competition
Niches and competition
Niche — the role of a species,
- including all biotic and abiotic resources it uses;
- the fundamental niche is potential and the realised niche is actual.
Competitive exclusion principle — two species competing for the same limiting resources cannot coexist indefinitely; one is excluded.
Resource partitioning — allows coexistence.
Section 4 of 10Diversity and Simpson's index
Diversity and Simpson's index
Species diversity — has two parts: species richness (number of species) and evenness (relative abundance); more species does not always mean higher D because evenness also matters.
Simpson's index of diversity — D = 1 − Σ(n/N)2, where n is the number of individuals of one species and N the total number of all organisms.
Diversity and Simpson's index
Calculate n/N for each species.
Square each value.
Add the squares and subtract the total from 1.
Diversity and Simpson's index
Interpretation — D ranges from 0 to 1; values near 1 mean high biodiversity and values near 0 mean low.
Uses — studying community composition and structure.
Section 5 of 10Keystone species
Keystone species
Keystone species — have a disproportionately large effect relative to their abundance (sea otters controlling urchins and protecting kelp).
Removing a keystone species changes the structure of the whole community.
Section 6 of 10Succession
1
Ecological communities change over time.
2
Primary succession — begins on bare rock or new land with no soil (e.g. a lava flow); pioneers such as lichens and mosses break rock and build soil, followed by grasses, shrubs and trees.
3
Secondary succession — occurs where soil remains after a disturbance (fire, abandoned farmland) and is much faster because seeds and nutrients remain.
4
Pioneer species — fast-growing and tolerant; later species are more competitive and the community moves towards a stable climax community.
5
Repeated disturbance — may hold a community at earlier stages.
Section 7 of 10Energy and community structure
Energy and community structure
Energy dependence — survival of organisms relies on sufficient energy in the previous trophic level,
- which sets matter and energy available to the next.
Knock-on effects — changes to community structure affect all interdependent species.
Section 8 of 10More from the specification
1Ecological niche differentiation model
- Ecological niche differentiation model competitive exclusion principles predict species coexistence patterns within a predictive mathematical ecosystem model.
2Logistic growth
- Growth slowing to a plateau at K
3Carrying capacity
- The maximum population an environment can support.
4Biotic factor
- Living factor such as a predator
5Exponential growth
- Unconstrained growth giving a J-shaped curve
6Density-independent factor
- Factor acting regardless of density
7Growth
- A permanent increase in size and dry mass.
8Density
- Number of individuals per unit area
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Community | all populations living and interacting in an area |
| Niche | the role of a species including resources used |
| Competitive exclusion | one species eliminating another competing for the same resource |
| Keystone species | species with a disproportionate effect on its community |
| Simpson's index | D = 1 − Σ(n/N)2, a measure of diversity |
| Mutualism | interaction where both species benefit |
| Parasitism | interaction where one species benefits and the other is harmed |
| Primary succession | colonisation of bare rock with no soil |
| Secondary succession | recovery after a disturbance where soil remains |
Section 10 of 10Exam technique
1
Confusing commensalism (+/0) with mutualism (+/+), or with parasitism — in commensalism the other partner is unaffected.
2
Forgetting to subtract from 1 or to square each n/N in Simpson's index.
3
Thinking a higher D means lower diversity — higher D means higher diversity.
4
Saying primary and secondary succession start the same way — primary starts without soil.
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