AQA GCSE Biology: Abiotic and Biotic Factors

The size of a population is affected by the conditions around it and by the other organisms sharing its habitat. This short guide covers the two kinds of factor, abiotic (non-living) and biotic (living), and how each affects a community. Knowing which is which is a common exam point.

Abiotic factors

Abiotic factors are the non-living conditions of an environment. Examples that affect a community include:

• light intensity, which affects photosynthesis in plants;
• temperature, which affects photosynthesis in plants;
• moisture levels, which affect the survival of different species;
• soil pH and mineral content, which affect plant growth;
• wind intensity and direction, which affect transpiration in plants;
• carbon dioxide levels, which affect photosynthesis;
• oxygen levels, which affect respiration in aquatic animals.

The quickest way to sort a factor is to ask whether it would still be there if every living thing vanished. Rainfall, temperature, pH and light would all still be there, so they are abiotic. Food, predators and disease would not, so they are not.

Notice that most of the factors in that list do their work indirectly. Light intensity does not act on a woodland bird at all. It acts on the plants, which photosynthesise faster, which gives the insects more to eat, which gives the bird more insects. An answer that traces that chain scores; one that jumps straight from more light to more birds does not.

Bluebells make the point cleanly. They flower in early spring, before the oaks above them are in full leaf, because for those few weeks the light reaching the woodland floor is high. Once the canopy closes the light falls away and the bluebell dies back. Nothing about the bluebell has changed. The abiotic factor has.

Biotic factors

Biotic factors are the living parts of an environment. Examples that affect a community include:

• the availability of food, which affects whether animals can survive and breed;
• new predators, which kill prey;
• new pathogens, which kill organisms;
• competition, where one species may outcompete another until its numbers are too low to breed.

The same test sorts these: every one of them exists only because some other organism does.

Competition is the one worth watching, because of where it ends. A new species can outcompete a resident so thoroughly that the resident's numbers fall below the level at which it can breed successfully, and from there the population does not recover even if the competitor later disappears. That is the mechanism behind most of the displacement questions on this topic.

The two kinds of factor are not independent of one another. A dry summer is abiotic, but it kills plants, and less plant material is a change in food availability, which is biotic. When a question asks you to name a factor, name the one it is asking about and say how it acts. Trying to claim both usually loses the thread.

Which factor is limiting

At any moment a population is held back by one factor more than the others, and that is the one worth naming. Adding more of anything else makes no difference until the limiting factor is relieved.

A plant in deep shade does not grow faster if you water it, because water was never what was stopping it. Change the light and it responds at once. This is the same reasoning as limiting factors in photosynthesis, applied to a whole population rather than to one reaction.

It is also why answers that list every factor score poorly. The question is nearly always which factor changed, and what happened as a result.

How this fits together

Organisms survive these factors because they are adapted to their environment, which is the subject of the adaptations guide.