Practical Ecology

Why Do We Need To Sample??

If we want to know what kind of plants and animals are in a particular habitat, and how many there are of each species, it is usually impossible to go and count each and every one present.

It would be like trying to count different sizes and colors of grains of sand on the beach.

Solution: Taking a number of samples from around the habitat & making the necessary assumption that these samples are representative of the habitat in general.

In order to be reasonably sure that the results from the samples do represent the habitat as closely as possible, careful planning beforehand is essential.

Sampling TypesThere are 3 main ways of taking samples:

Random samplingSystematic samplingStratified sampling (Not covered in this unit)

What is a Quadrat?

Area in which you search or sample.

Does not necessarily have to be a square.

The apparatus, held by Jo Ann, Anthea & Jessica in the photo on the right, is an example of a quadrat.

The picture above shows other examples of quadrats.

Important Point to Note: Quadrat size in a particular study must be standardized to serve as a control so that it is comparable.

Use of Quadrats

Quadrat SamplingMethod by which organisms in a sample of a habitat are counted directly.

May be used without a transect when studying a relatively uniform habitat. In this case, the quadrat positions are chosen randomly.

Used to estimate population parameters when the organisms present are too numerous to count in total.

Used to estimate:

population abundance (number),

density frequency of occurance &

distribution.

Quadrat Sampling

Random SamplingThis is usually carried out when the area under study is fairly uniform, very large or when there is a limited time available.

A quadrat frame is most often used for this type of sampling.

The frame is placed on the ground (or on whatever is being investigated) & the animals &/ or plants inside it is counted, measured, or collected.

This is done many times at different points within the habitat to give a large number of samples.

ACFOR ScaleWhen working with quadrats like the one on the right, you have to decide on the extent to which each organism type appeared, and make a note according to how abundant it was; an abundance scale.

This enables the investigator to make quick estimates.

This could be done by working out rough percentages cover of species based on a scale whose name is an acronym of the various levels (ACFOR).

A = Abundant (Greater than or equal to 30%)

C = Common (20% - 29%)

F = Frequent (10% - 19%)

O = Occasional (5% - 9%)

R = Rare (1% - 4%)

Systematic Sampling

Samples are taken at fixed intervals, usually along a line known as a transect.

E.g. grid sampling, line transects, belt transects.

This normally involves doing transects, where a sampling line is set up across areas to where there are clear environment gradients.

E.g. you might use a transect line to show changes in flora and fauna of the seashore from the top of the beach down to the sea.

Grid Quadrats

The quadrat on the right is a grid quadrat and it is more accurate than the ACFOR scale.

Systematic SamplingThere are two types of systematic sampling:

Line transectsBelt transects

Systematic Sampling

Line Transects Vs Belt Transects

Line transects can reveal species zonation patterns along the line by showing where particular species occur along the line.

Belt transects show not only where species occur but also how much of it is present at any point along the line.

Mark & Recapture

‘Mark & Recapture’Method used to estimate the population size of animal populations where individuals are highly mobile.

The population is sampled by capturing as many of the individuals as possible & practical.

Each individual is marked in a way to distinguish it from the unmarked animals.

The marked animals are returned to their habitat & left for long enough a period for complete mixing with the rest of the population to take place.

Another sample of the population is then taken.

The numbers of marked to unmarked animals in the second sample is determined using the Lincoln Index.

‘Mark & Recapture’

Mark & Recapture

Sandhoppers in an exposed rocky shore being captured.

Sandhoppers being ‘marked’ before release.

Mark & Recapture -Assumptions

Size of the population doesn’t change during the experiment i.e. births, deaths, immigration & emigration do not occur.

The ‘mark’ should last the duration of the experiment.

The marking procedure & paint should not harm the animals.

This method is only applicable if the population is discrete, inhabits a defined area & whose individuals mix at random.

There should be no “trap-happy” or “trap-shy” individuals in the population.

How Big Must The Sample Size Be?Always go for as large a sample size as you can get, even if each of your samples is ‘sloppy’ & ‘noisy.’

A large, unbiased set of samples will average over the ‘noise.’

A large sample size can rescue imprecise measurements, but it cannot rescue biased measurements.

If the sample size is very large, two populations can be statistically different with means that are quite close.

On the other hand, if the sample size is small, two populations that are actually quite different will not appear significantly different.

Questions??