By the end of this lesson
- Classify factors as abiotic or biotic.
- Explain how a named factor affects population size or distribution.
- Interpret population data involving predators, prey or competition.
BIG QUESTION
Why do populations rarely keep growing for ever?
Limiting abiotic conditions and interactions with other organisms set limits on survival and reproduction.
Abiotic factors
abiotic factorA non-living factor in an ecosystem, such as temperature, light intensity, water or soil pH, that affects the organisms living there. are non-living conditions. They include light intensity, temperature, water availability, soil or water pH, mineral ions, oxygen concentration in water, wind and salinity.
A factor affects populations by changing survival, growth or reproduction. For example, low light reduces photosynthesis, so fewer plants of a light-demanding species survive in shade.
Biotic factors
biotic factorA living factor in an ecosystem, such as competition, predation, disease or food supply, that affects the organisms there. are caused by living organisms. They include food availability, predationOne animal (the predator) killing and eating another (the prey)., disease, parasites and competitionOrganisms trying to get the same limited resource, such as light, water, food or mates. for resources such as light, space, water, food or mates.
Competition can be between individuals of the same species (intraspecificBetween individuals of the same species, as in intraspecific competition.) or between different species (interspecificBetween individuals of different species, as in interspecific competition.).
Predator–prey changes are linked
If prey numbers rise, predators have more food, so more predators survive and reproduce. Predator numbers then rise, prey numbers fall, and predator numbers later fall. The predator peak usually lags behind the prey peak.
Real populations are also affected by weather, disease and other food sources, so cycles are rarely perfectly regular.
Distribution as well as size
Factors also affect distributionWhere a species is found in an area, and how its numbers change from place to place.. Limpets may be restricted to parts of a shore where they avoid drying out, while bluebells grow in woodlands where they flower before the tree canopy blocks light.
Usually several factors act at once. The factor in shortest supply relative to need often has the strongest effect.
Factors affecting populations
Name the factor and explain its effect on survival or reproduction.
| Factor | Type | Possible effect |
|---|---|---|
| Light intensity | Abiotic | Limits photosynthesis and plant growth in shade. |
| Temperature | Abiotic | Affects enzyme activity, growth and breeding season. |
| Water availability | Abiotic | Drought reduces plant survival and food for herbivores. |
| Predation | Biotic | Reduces prey numbers; predator numbers follow prey. |
| Competition | Biotic | Limits share of resources, reducing growth or reproduction. |
| Disease | Biotic | Can reduce dense populations sharply. |
Language that earns precision
- abiotic factor
- A non-living condition that affects organisms.
- biotic factor
- A living factor, such as predation or competition, that affects organisms.
- competition
- Organisms needing the same limited resource.
- predation
- One organism (the predator) killing and eating another (the prey).
Common misconceptions
Claim: Temperature is a biotic factor because it affects living things.
Correction: It is abiotic because it is non-living. Biotic factors come from living organisms.
Claim: Predators eat all the prey and then die out.
Correction: As prey becomes scarce, predators decline, allowing prey numbers to recover.
Claim: Only one factor controls a population.
Correction: Several abiotic and biotic factors usually act together.
Match the answer to the command word
Link the factor to a process (photosynthesis, food supply, breeding) and then to survival or reproduction.
For predator–prey graphs, describe the rise and fall of each and the lag between peaks.
Recall, apply, then justify
Give two abiotic factors and two biotic factors that could affect the population of a plant species.
Hint
Non-living conditions and living interactions.
Suggested marking points
- First abiotic factor, for example light intensity.
- Second abiotic factor, for example soil water, temperature, pH or minerals.
- First biotic factor, for example grazing by herbivores.
- Second biotic factor, for example competition, disease or pollinators.
Explain why the number of predators often rises after the number of prey rises.
Hint
Food, survival, reproduction, delay.
Suggested marking points
- More prey means more food for predators.
- More predators survive and reproduce.
- There is a delay because reproduction and growth take time.
A non-native squirrel species spreads into a woodland and the native squirrel population declines. Suggest and explain biotic factors that could cause the decline.
Planning hint
Competition and disease are both possible; use conditional language.
What a strong design includes
- Interspecific competition for food could reduce the food available to native squirrels.
- The non-native species may feed more efficiently on the same food.
- Fewer native squirrels may survive winter or breed.
- The non-native species may carry a disease to which the native species is more vulnerable.
- Competition for nest sites could also reduce breeding.
- Data from several woodlands would be needed to identify the main cause.