3.8.3Additional lessons
Limiting Population Size — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 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 5Limiting Factors
Definition
- Definition A limiting factor is any resource or condition that restricts the size of a population, such as food, space or temperature.
Carrying capacity
- Carrying capacity The carrying capacity is the largest population an environment can support over a long period with the resources available.
Reproductive potential
- Reproductive potential Most species could produce enough offspring to grow exponentially, but limiting factors prevent this.
Negative feedback
- Negative feedback As the population increases, limiting factors become stronger,
- which slows growth and pulls numbers back toward the carrying capacity.
Section 2 of 5Density-Dependent Factors
The effect of these factors becomes stronger as the population density increases.
Density-Dependent Factors
Competition
- Competition As numbers rise, individuals compete more strongly for limited food, water, light or mates, lowering survival and births.
Predation
- Predation Predators often take more prey when the prey population is large, because prey are easy to find and catch.
Disease
- Disease Parasites and pathogens spread faster when individuals are close together, so disease kills a larger proportion in crowded populations.
Waste
- Waste Accumulation of waste products in a dense population can become toxic and reduce survival.
Section 3 of 5Density-Independent Factors
1
A density-independent factor affects the population in the same proportion whatever its size or density.
2
Weather events such as drought, frost, floods and storms kill a similar fraction of organisms in sparse or crowded populations.
3
Fire and volcanic eruptions can wipe out large parts of a population regardless of its density.
4
Such events may cause sudden crashes, after which the population may recover if the habitat remains suitable.
| Feature | Density-dependent | Density-independent |
|---|---|---|
| Effect varies with population density | Yes | No |
| Biotic or abiotic | Mostly biotic | Mostly abiotic |
| Examples | Competition, predation, disease | Fire, drought, hurricane |
| Regulates population | Yes, through negative feedback | No, only causes fluctuations |
Section 4 of 5Applying the Ideas
Stable populations
- Stable populations Over long periods many populations stay close to the carrying capacity because density-dependent factors balance births and deaths.
Raising the limit
- Raising the limit A change such as new food sources or removal of a predator can increase the carrying capacity and allow growth.
Lowering the limit
- Lowering the limit Habitat destruction or pollution reduces the available resources, so the carrying capacity falls.
Exam approach
- Exam approach When asked to explain a decline, state the factor, say whether it depends on density, and link it to birth or death rate.
Section 5 of 5Exam Checklist & Tips
3.8.3 – Key Points for the Exam
- Limiting factor A condition or resource that restricts population growth.
- Carrying capacity The maximum stable population an environment can support.
- Density-dependent Having an effect that changes with population density.
- Density-independent Having an effect unrelated to population density.
- Negative feedback A process that opposes change and stabilises a system.
Common Mistakes
- Writing that all abiotic factors are density-independent, when the supply of space or water can be density-dependent.
- Saying carrying capacity is a fixed number that never changes, although it varies with conditions.
- Forgetting that density-dependent factors act through negative feedback.
- Describing the carrying capacity as the point where birth rate is zero rather than equal to death rate.
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