A transect is a line across a habitat or part of a habitat. It can be as simple as a string or rope placed in a line on the ground. The number of organisms of each species along a transect can be observed and recorded at regular intervals.
The distribution of organisms in a habitat is affected by the presence of other living organisms - such as herbivores or carnivores - that might eat them. It is also affected by abiotic factors, such as availability of light or water.
A kite diagram shows the number of animals (or percentage cover for plants) against distance along a transect.
In the example above, the distribution of dandelion plants gradually changes from five metres to 20 metres along the transect. A quadrat has been placed at regular intervals of a metre (or a few metres) along the transect.
A gradual change in the distribution of species across a habitat is called zonation. It can happen because of a gradual change in an abiotic factor.
A transect is usually used to investigate a gradual change in a habitat rather than to simply estimate the number of organisms within it.