7.2 Abiotic and biotic factors

Two kinds of factor

Abiotic factors

• Light intensity: plants need light for photosynthesis, so fewer plants grow under dense trees.
• Temperature: affects enzymes and so growth and activity.
• Moisture levels: plants and animals need water.
• Soil pH and mineral content: some plants only grow in acidic or alkaline soils, and all need mineral ions such as nitrate.
• Wind intensity and direction: affects transpiration and where seeds land.
• Carbon dioxide levels for plants (photosynthesis).
• Oxygen levels for aquatic animals: some invertebrates only survive in water with a lot of dissolved oxygen.

Biotic factors

• Availability of food.
• New predators arriving.
• New pathogens, to which a species has no resistance.
• Competition: one species may outcompete another so its numbers are no longer high enough to breed successfully.

How factors change populations

A change in one factor can change the size of a population, and that affects others through interdependence. For example, a drought (abiotic) reduces grass, so rabbit numbers fall, so fox numbers fall. A new disease (biotic) can wipe out a species, such as ash dieback killing ash trees.