Abiotic and Biotic Factors

Factors affecting a community can be abiotic (non-living) or biotic (arising from living organisms). The distinction is about the factor itself: temperature is abiotic, whereas predation is biotic.

Abiotic factors: conditions and toxic chemicals

Environmental conditions include light, temperature, water availability, soil conditions and oxygen availability. Plants need light for photosynthesis, so low light can limit their growth. A shortage of water can also limit plant growth; animals that depend on those plants may then have less food. Temperature and oxygen availability can affect which organisms survive in a habitat. Different species tolerate different conditions, so a change may reduce some populations while favouring others.

Toxic chemicals are abiotic factors too, even if living organisms released them. If a chemical contaminates soil or water, it may harm or kill organisms exposed to it. A fall in their numbers can alter food supplies and other interactions across the community. The effect depends on the chemical, its concentration and which organisms are exposed.

Biotic factors: resources, predators and pathogens

Organisms need resources to survive and reproduce. Animals may compete for food or space; plants may compete for light, water and minerals. When a resource is scarce, some organisms obtain too little of it, so their growth or reproduction may fall. Classify the resource carefully: food supplied by other organisms is biotic, while light and water are abiotic. Competition between organisms is a biotic interaction.

Predators eat prey. More predators may reduce a prey population; if prey then becomes scarce, the predators may have less food and their numbers may also fall. These effects are not necessarily immediate or identical in every ecosystem.

Pathogens cause disease. They are biotic factors because they are living organisms or disease-causing agents associated with living systems. Disease can reduce the survival or reproduction of affected organisms, changing their population size. Other species in the community may then be affected through feeding relationships or competition.

Abiotic and biotic factors often act together. For instance, a dry period may reduce plant growth, intensifying competition for food among herbivores. To explain a change in a community, trace a clear chain from the factor to one population and then to any other populations it affects.