(1) Introduction to investigating distribution and abundance for biodiversity

Some definitions

The distribution of animal and plant species is important to scientists to understand the ecology of a particular habitat.

The distribution of an organism is where you find it in its habitat e.g. part or the whole of a river, stream, field, heathland etc.

The abundance or population size of an organism is how many individuals are present in a given area.

Some general points

Where an organism is found depends on several environmental factors e.g. dry sandy soil or damp marshy ground, brighter light in the open or shaded by trees or bushes.

Each species of plant or animal is adapted to live in its particular habitat, but one patch of ground might be better suited than another.

This means the distribution of any species can vary even within the same habitat area.

Methodology

You need to know the methods of how to investigate the distribution and abundance of organisms in a given habitat.

Most habitats are relatively large areas and it would be too time consuming to count all the numbers of individual animals/plants of every species over the whole area.

Therefore you have to adopt a sampling strategy, and from the data, scale up the numbers to estimate the whole population of selected animal or plant species.

Abundances can be estimated by counting the number of individuals (e.g. identified plant/animal) or the percentage cover (e.g. lichen on a stone wall) for selected small areas chosen at random.

From these 'counts' you can then scale up to allow for the total area of the habitat.

You can survey a habitat in two ways: Using (1) a quadrat or using (2) a transect. - both methods described in detail below, but there other points to make before looking at them.

You can measure the number of an organism in two or more sample areas of a habitat using a quadrat (e.g. counting within a 1 m x 1m square frame) and compare the results.

You might choose quite different locations, but within the same habitat.

You can study how a distribution changes across a wider area by surveying with quadrats along a transect - basically following a linear path across a habitat.

You can lay out a long line or tape measure and systematically lay the quadrat down every one or more metres, but keeping the sampling intervals the same distance apart.

There are also capture-recapture techniques to estimate the size of a population.

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