(1) Introduction to investigating distribution and abundance for monitoring biodiversity

This section will help you answer questions such as ...  What is a quadrat?   How do you do a quadrat survey?  How do you do calculations from quadrat measurements?   What is a transect? How do you do a survey using a transect? What are indicator species? How do you draw kite diagrams? How to use an identification key

The ideas - learning objectives on this page are explored more fully in sections 2. to 8.

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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Key points - surveying for measuring biodiversity

Based on the syllabus-specifications for students taking the UK AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

Keywords, phrases and learning objectives for this part on ecological surveying and biodiversity

Know and understand why ecological surveying is important to investigate the distribution and abundance of organisms as a check on biodiversity.

The ideas - learning objectives on this page are explored more fully in sections 2. to 8.

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• investigative fieldwork involving sampling techniques and the use of quadrats and transects; which might include, on a local scale, the:

  • patterns of grass growth under trees,

  • distribution of daisy and dandelion plants in a field,

  • distribution of lichens or moss on trees, walls and other surfaces,

  • e.g. distribution of the alga Pleurococcus on trees, walls and other surfaces,

  • leaf size in plants growing on or climbing against walls, including height and effect of aspect.

• analysing the measurement of specific abiotic factors in relation to the distribution of organisms

• the study of hay infusions

• the use of sensors to measure environmental conditions in a fieldwork context.

• investigations of environmental conditions and organisms in a habitat such as a pond,

• ‘hunt the cocktail stick’ using red and green cocktail sticks on a green background,

• investigating the distribution of European banded snails,

• investigating the behaviour of woodlice using choice chambers,

• investigating the effect on plant growth of varying their environmental conditions, eg degrees of shade, density of sowing, supply of nutrients,

• investigating particulate levels, eg with the use of sensors to measure environmental conditions,

• the use of maximum–minimum thermometers, rainfall gauges and oxygen meters,

• investigating the effect of phosphate on oxygen levels in water using jars with algae, water and varying numbers of drops of phosphate, then monitor oxygen using a meter,

• computer simulations to model the effect on organisms of changes to the environment.

• Know that living organisms form communities, and we need to understand the relationships within and between these communities.

• Know that these relationships are affected by external influences.

• You should be able to use your skills, knowledge and understanding to:

  • suggest reasons for the distribution of living organisms in a particular habitat,

  • evaluate methods used to collect environmental data, and consider the validity of the method and the reproducibility of the data as evidence for environmental change,

  • at the end of an investigation and analysis, can you distinguish whether differences in distributions of an organism are due to one or more environmental factors?

  • is it possible to control, or allow for, different environmental factors?

  • and you should understand:

  • the terms mean (average of all of a data set), median (middle value in a data set) and mode (the most common value in a data set - could be more than one value)

  • that sample size is related to both validity and reproducibility,

  • the larger the sample, random sampling from many locations and the more times the experiment is repeated, the more reliable will be the final analysis and conclusions,

  • reproducibility is the key to a successful valid investigation,

• Know and understand the physical factors that may affect an organism in its habitat (area where the organism lives):

  • To study the distribution of a species of animal or plant you must measure the population of the organism in different sample areas.

  • A habitat is where an organism lives (plant or animal) and its distribution is the areas where the organisms live an this may depend on environmental factors such as ...

  • ambient temperature,

  • availability of nutrients in the soil or water,

  • amount of light falling on the habitat,

  • availability of water in soil for plants,

  • availability of oxygen and carbon dioxide in the air or water.

    • Organisms will be adapted by evolution to fill a niche in a particular area of the environment,

    • but if there is a change in any of the factors above, then organism populations will be affected, some will increase and some will decrease,

    • in extreme cases, one species might die out in an area and another species may take advantage and move in.

    • An organism will be more common in an area, where environmental factors make conditions more suitable for the organism to survive and reproduce e.g.

    • shade for plants that need little sunlight, or out in the open for plants that need bright sunlight,

    • some creatures may prefer damp conditions, others adapted to dry conditions,

    • greater density/availability of the organism's specific food requirements

• Know and understand that quantitative data on the distribution of organisms can be obtained by:

  • (i) random sampling with quadrats to cover a large area without sampling all of it

    • A sampling quadrat is usually a 1m x 1m (1 m²) square frame of wood or plastic,

    • Therefore if you measure the number of organisms in a quadrat you get the density in organisms per square metre.

    • You can average the random individual quadrat results to get the mean value for a particular organism/m².

    • To work out the total population of an organism in the area you have been randomly sampling, you multiply the mean by the total area.

    • The more samples you take, the more reliable is your data, and therefore any deductions made will also be more reliable, but the data is only statistical, never completely precise, but

    • In presenting data make sure you know how to use the terms mean (average of all of a data set), median (middle value in a data set) and mode (the most common value in a data set - could be more than one value).

  • (ii) sampling with quadrats along a linear transect to look for changes across an area of land e.g. to see how a population changes across a wider area.

    • You can mark out the transect with two sticks and a long piece of string.

  • Note (iii) Whatever field work you do, the only really reliable data, are data that are consistent, i.e. always show the same pattern of organism distribution (plant or animal), and in that way the data is repeatable and reproducible.

    • This involves random sampling with samples using many quadrats and transects.

    • Does the data support the question posed about organism distribution?

    • Are differences in population due to environmental factors?

    • What are the variables?

    • Have the variables been controlled properly in your survey design?

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