5.2.4Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Factors Affecting Populations — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 19 of 36 Unit quiz
Section 1 of 5Biotic Factors
Living factors arise from interactions between organisms.
Interspecific competition
- Different species compete for the same resource, which can reduce the numbers of one or both.
Intraspecific competition
- Members of one species compete for food, space or mates, and this limits population growth.
Predation
- Predators reduce prey numbers, and prey numbers affect the food supply for predators.
Disease and parasitism
- Parasites and pathogens spread more easily at high density and reduce survival.
Cooperation
- Some organisms gain benefit from each other, such as animals living in groups that avoid predators.
Section 2 of 5Abiotic Factors
Abiotic Factors
Temperature
- Affects metabolic rate, enzyme activity and survival of the species.
Light
- Limits photosynthesis and therefore the food supply for the community.
pH and salinity
- Affect water balance and enzyme function, so some species tolerate only narrow ranges.
Water and oxygen
- Availability affects where organisms can live, especially in soil and water.
Tolerance
- Each species has a range of conditions it can tolerate, which limits where it can live.
Section 3 of 5Population Growth
A population introduced to a new habitat often follows an S-shaped curve.
1
In the lag phase growth is slow because few individuals are reproducing.
2
In the exponential phase resources are plentiful and the population grows rapidly.
3
In the stationary phase birth and death rates are equal, and the population stays near the carrying capacity.
4
Limiting factors such as food, space and disease stop growth continuing.
Section 4 of 5Predator and Prey
Cycles
- Predator numbers often rise and fall a little after prey numbers.
Reason
- More prey allows more predators to survive, then the predators reduce the prey again.
Caution
- Other factors such as disease and food supply for prey also affect the cycle.
Density
- Factors such as competition depend on density, whereas frost affects all densities similarly.
| Type | Example | Effect |
|---|---|---|
| Biotic | Predation | Reduces prey population |
| Biotic | Competition | Limits growth |
| Abiotic | Temperature | Affects survival and growth |
Section 5 of 5Exam Checklist & Tips
5.2.4 – Key Points for the Exam
- Carrying capacity The maximum population size that an environment can support.
- Competition The struggle between organisms for a limited resource.
- Predation Feeding by one organism on another, the prey.
- Limiting factor A factor that restricts population size.
Common Mistakes
- Saying that predators alone control prey numbers; food, disease and abiotic factors matter too.
- Confusing intraspecific competition, within a species, with interspecific.
- Describing carrying capacity as a fixed value; it varies with conditions.
- Stating that abiotic factors are living factors.
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