Biotic factors
Definition
Biotic factor
A living influence in an ecosystem, such as food availability, new predators, new pathogens or competition, that affects the organisms living there.
- Biotic factors are the living influences on a community, caused by other organisms.
- A change in a biotic factor can raise or lower the sizes of the populations in a community.
- Because species are linked by feeding and competition, a change in one species affects the others.
Key Idea
Abiotic factors are non-living conditions, but biotic factors come from other living organisms.
The four biotic factors
Definition
Predator
An animal that hunts, catches and eats other animals, which are its prey.
- Availability of food matters because, when food becomes scarce, animals compete for it and fewer survive and breed, so the population falls.
- New predators arriving in an area hunt prey that have no defence against them, so the prey population drops.
- New pathogens can spread disease through a population that has no resistance, killing many organisms.
- One species outcompeting another can leave the weaker species with too few resources, until its numbers are no longer enough to breed.
Example
When the disease myxomatosis spread through rabbits, rabbit numbers crashed, so foxes that fed on them had less food and also declined, while the grass the rabbits once grazed grew tall and shaded out smaller plants.
Exam technique
- Exam questions often give you data or a context and ask how a change in a biotic factor affects the whole community.
- State which biotic factor has changed, then follow the feeding and competition links to the other species.
- When a species loses its food, or is hit by a new predator or pathogen, its numbers fall, and the species that depend on it are affected in turn.
Outcompeting and breeding
Definition
Competition
When organisms of the same or different species struggle against each other for the same limited resources in a habitat.
- When one species outcompetes another for the same resources, the weaker species can be pushed out of the habitat.
- If its numbers drop too low, there are not enough individuals left to breed, and it can disappear from the area.
- In much of Britain, grey squirrels have outcompeted red squirrels, so red squirrels now survive in only a few areas.
Common Mistake
- Do not stop at saying a factor changed.
- Marks come from tracing the effect through the community, species by species.
Putting it together
- A biotic factor changes the numbers of one species first, then the change ripples out to the species linked to it.
- The same reasoning works for food loss, a new predator, a new pathogen or one species outcompeting another.
Self review
- Name the four biotic factors that can affect a community.
- Explain how a new pathogen could reduce a population.
- Why can a new predator make its prey population fall quickly?
- What does it mean to say one species outcompetes another?