3.2.1Unit 3 — Life on Earth
Factors Affecting Distribution
5 min read 5 parts Lesson 7 of 31
Section 1 of 5Biotic Factors
The distribution of a species is determined by a combination of living (biotic) and non-living (abiotic) factors.
Competition
- If two populations compete for the same resources, the better-adapted one outcompetes the other, which then declines.
Disease
- Populations without immunity to a new pathogen may decline sharply or be wiped out locally.
Food availability
- More food means that organisms are more likely to survive and reproduce, so the population can increase.
Grazing
- Herbivores feeding on plants reduce plant numbers and affect which plants can survive in an area.
Predation
- Predators normally take enough prey to survive without destroying the prey population.
Section 2 of 5Predator-Prey Cycles
1
When prey are plentiful, the predator population has more food and increases in number.
2
More predators eat more prey, so the prey population then decreases.
3
With less prey available, the predator population decreases because there is not enough food.
4
Fewer predators means fewer prey are eaten, so the prey population increases again and the cycle repeats.
5
Real food webs are more complex, because other predators, prey and factors also affect population sizes.
Section 3 of 5Abiotic Factors
Abiotic Factors
Light intensity
- Plants need light for photosynthesis, so more light increases the rate of photosynthesis and growth.
Moisture
- Plants and animals require water, so moisture levels influence where they can survive.
pH
- Species are adapted to particular soil or water pH values, so pH affects where they are found.
Temperature
- Temperature affects the rate of enzyme-controlled reactions, and warming towards the optimum increases growth rates in plants.
| Biotic factors | Abiotic factors |
|---|---|
| Competition | Light intensity |
| Predation | Moisture |
| Disease | pH |
| Food availability | Temperature |
Section 4 of 5Using Evidence
Using Evidence
Combined effect
- The distribution of a species depends on several factors acting together, not on a single cause.
Graphs
- In predator-prey graphs the predator peak follows the prey peak with a delay.
Exam answers
- Always name the factor, state the effect on the population and explain why it happens.
Wording
- Write that populations increase or decrease and link the change to the available resources.
Section 5 of 5Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Biotic factor A living factor that affects a population, such as predation or competition.
- Abiotic factor A non-living factor that affects a population, such as temperature or pH.
- Distribution Where in a habitat or region a species is found.
- Predator-prey cycle The repeating pattern in which predator and prey numbers rise and fall in turn.
Common Mistakes
- Saying that predators drive prey extinct; in balanced ecosystems prey numbers recover.
- Placing the prey peak after the predator peak on a cycle graph, when the prey peak comes first.
- Listing temperature as a biotic factor, when it is abiotic.
- Explaining a population change without naming the specific factor that caused it.