3.7.1Additional lessons
Populations in Ecosystems — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 82 of 142
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.
Part 1 of 6Defining a Population
Population
- Population A group of organisms of the same species living in the same area at the same time that can interbreed.
Species
- Species A group of organisms that can interbreed and produce fertile offspring.
Separate populations
- Separate populations The same species in different areas may form separate populations when geography prevents gene exchange between them.
Community
- Community All the populations of different species living and interacting in the same area.
Ecosystem
- Ecosystem A community together with the abiotic environment in which it lives and with which it interacts.
Part 2 of 6What Changes Population Size
A population grows when more individuals are added than are lost.
Natality
- Natality Births add new individuals to the population.
Mortality
- Mortality Deaths remove individuals from the population.
Immigration
- Immigration Individuals moving into the population from elsewhere increase its size.
Emigration
- Emigration Individuals leaving the population to live elsewhere decrease its size.
Population change
- Population change Change in size = (births + immigration) − (deaths + emigration).
Part 3 of 6Describing a Population
Describing a Population
Population size
- Population size The total number of individuals present at a given time.
Population density
- Population density The number of individuals per unit area or volume of habitat.
Distribution
- Distribution Individuals may be spread evenly, in clumps or at random, depending on resources and behaviour.
Abundance
- Abundance A species may be rare or common, and abundance is found by sampling, not by counting every individual.
| Term | Level of organisation |
|---|---|
| Population | One species, one area |
| Community | Several species, one area |
| Ecosystem | Community plus abiotic environment |
Part 4 of 6Why Study Populations
1
Monitoring population size shows whether a species is increasing, stable or declining.
2
Declining populations may need conservation action, such as protecting habitat.
3
Growing populations of pests or invasive species may need control measures.
4
Understanding what limits populations helps predict how ecosystems respond to change.
Part 5 of 6Worked Example
Question
- A population of 500 voles has 80 births, 55 deaths, 20 immigrants and 15 emigrants in a year.
- Calculate the new population size.
- Individuals added = births + immigration = 80 + 20 = 100.
- Individuals lost = deaths + emigration = 55 + 15 = 70.
- Change = 100 − 70 = +30, so the new size = 500 + 30.
Answer 530 voles
Part 6 of 6Exam Checklist & Tips
3.7.1 – Key Points for the Exam
- Population Individuals of the same species living in one area at the same time.
- Community All the populations of different species in an area.
- Immigration The movement of individuals into a population.
- Emigration The movement of individuals out of a population.
- Population density The number of individuals per unit area.
Common Mistakes
- Saying a population contains different species, when it contains only one.
- Forgetting immigration and emigration when explaining population change.
- Confusing population density with population size.
- Thinking an ecosystem includes only living things.
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