3.8.1Additional lessons
Populations in Ecosystems — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Populations, Species and Communities
Ecologists work at different levels, and each term has a precise meaning.
Species
- Species A species is a group of organisms with shared characteristics that can interbreed and produce fertile offspring.
Population
- Population A population is a group of organisms of the same species living in the same area at the same time, able to interbreed.
Community
- Community A community is all of the populations of different species that live and interact in one area.
Ecosystem
- Ecosystem An ecosystem is a community together with the non-living factors of its environment, such as light, water and soil.
Habitat
- Habitat The habitat is the place where a species normally lives, for example a rock pool or a woodland floor.
Section 2 of 5Isolation and Gene Flow
Isolation and Gene Flow
Isolated populations
- Isolated populations Two populations of one species in different places cannot exchange genes, because their members never meet to breed.
Gannet colonies
- Gannet colonies Seabirds that nest on separate cliffs form distinct breeding populations even though they belong to the same species.
Consequences
- Consequences Isolated populations may become genetically different over time, because natural selection and chance act separately on each.
Contact
- Contact If individuals move between populations they reproduce together, and gene exchange makes the populations more alike.
Section 3 of 5What Changes Population Size
1
Individuals are added to a population by births, which depend on the number of breeding adults and how many offspring each produces.
2
Individuals also join a population by immigration, which means moving in from neighbouring areas.
3
Individuals leave the population by death, caused by predation, disease, starvation or old age.
4
Individuals also leave by emigration, so the size of the population depends on the balance between all four processes.
| Process | Effect on population size |
|---|---|
| Natality (births) | Increases |
| Immigration | Increases |
| Mortality (deaths) | Decreases |
| Emigration | Decreases |
Section 4 of 5Describing Populations
Describing Populations
Population size
- Population size This is the total number of individuals and it is usually estimated by sampling rather than counted directly.
Population density
- Population density Density is the number of individuals per unit area or volume, and it allows populations in different sized areas to be compared.
Distribution
- Distribution Individuals may be spread evenly, in clumps or at random, depending on resources and behaviour.
Age structure
- Age structure A population with many young individuals is likely to grow, whereas one dominated by old individuals may decline.
Section 5 of 5Exam Checklist & Tips
3.8.1 – Key Points for the Exam
- Population Organisms of one species living in the same area at the same time.
- Community All the populations living and interacting in a habitat.
- Immigration Movement of individuals into a population.
- Emigration Movement of individuals out of a population.
- Natality The birth rate of a population.
Common Mistakes
- Saying a population can contain several species, which is the definition of a community.
- Writing that a population is all the individuals of a species in the world, ignoring its area and time.
- Forgetting immigration and emigration when explaining a change in population size.
- Using habitat and ecosystem as though they were the same term.
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