National 5 Biology Unit 1 to 3 Revision

Unit 1: Life on Earth



Example of a food chain. Example of a food web.


Competition between organisms is usually caused by a lack of resources (such as food or mates). Competition between organisms of different species is called interspecific competition, while competition between organisms of the same species is called intraspecific competition.

Distribution of Organisms

The distribution, or spread, of organisms describes where a specific species of organism can be found, either in the world or in a given ecosystem, and also how many individuals of that species can be found there.

The biotic factors which affect the distribution of species include:

The abiotic factors which affect the distribution of species include:

Abiotic factors can be measured using light meter(s), moisture meter(s), and pH meter(s) on a given sample site.

Paired-Statement Keys

In order to identify an organism,paired-statement keys are often used. A paired-statement key is a series of pairs of questions, which direct you towards the name of the relevant organism, based on its features.

If the factors affecting a given area are too extreme, few or no species will be able to live there. As such, the more extreme conditions are in an area, the fewer species will survive there, in general.

Indicator species are species that can tell you about the quality of the environment by their presence or the lack thereof.