Medium ground finch

Behind the scenes

What causes change?

Ecologists often divide the factors influencing ecosystem and population change into two types. Biotic or intrinsic factors relate to living organisms, and include predators and competition between organisms for food, while abiotic or extrinsic factors relate to non-living aspects of the environment, such as water or places to live.

Biotic and abiotic factors interact. For example, in the 1970s a drought (an abiotic factor) caused a crash in the population of medium ground finches living on one of the Galapágos Islands. The birds were pushed to the brink of extinction as the drought wiped out their food supply, seeds (a biotic factor).

Biotic and abiotic factors typically influence population size in different ways. Abiotic factors (like temperature) may affect individuals whatever the size of the population: they are density-independent.

On the other hand, biotic factors may have different effects depending on the population size: a single predator may have a much bigger impact on a small population than a large one; and competition for territory may force some animals out of prime mating areas when populations are large. Such effects are density-dependent. However, weather alters habitat quality, so these factors almost always interact.

Lead image:

Charles Sharp/Flickr CC BY

About this resource

This resource was first published in ‘Populations’ in June 2014.

Ecology and environment
Education levels:
14–16, 16–19, Continuing professional development