Sampling techniques are used to estimate population numbers when total counts cannot be made.

**Capture Mark Recapture:**
Is one of the several ways biologists estimate population size. It is a method which involves catching a certain number of individuals of a particular species, marking or tagging them in a way that does not affect their life expectancy then releasing them into the wild and after catching another group and counting the number of tagged amongst the recaptured. A formula can then be used to estimate the total size of the population. This method is suited for mobile populations where it is impossible to count all individuals at one time i.e: birds, fish, butterflies.

**Quadrant:**
A quadrant is a defined area which measures the distribution or number in a population. The size of the quadrant is usually determined by the size of the organism being counted.

The quadrants can be placed randomly in the area and numbers of organisms in each quadrant is recorded.

Random quadrants are useful to estimate population size of stationary organisms, or organisms that do not rapidly move to other areas over long distances.

The more quadrants and the more you sample the more accurate the abundance. A tally of the numbers in each quadrant can be used to estimate the total number in one area, or the average number per quadrants. It can be used for the density.

A transect is a line, strip or profile for counting and mapping the number of individuals at different distances along the line. Quadrants are often placed specified distances along the transect and individuals counted in these quadrants. Transects are often used to show how the diversity of species changes across an area. Transects often include scale to show only the height of the plants but any change in topography such as valleys or mountains.

Percentage cover is a sampling technique which uses a 100 percent point guide to estimate abundance. It is used when it is extremely hard to count each individual or it is not clear how many individuals are present. The estimate is given as a percentage.

MEASURING ABIOTIC FACTORS

A data logger is a useful instrument that you can take on a field study. It will gather data that can be taken back to a computer and analysed.

Sampling techniques measuring abiotic factors:

Sampling is carried out when it is not possible to count every individual in a population. A small is area is counted in detail and then multiplied to get a estimate for the whole area.

Measuring Distribution: Transect Lines

A transect is a straight line usually a string is laid across and area and the organism along the line are recorded a transect is used to sample plant populations. Can be used for animals especially if they attached to one sport, for example larnicle on a rock platform.

The organism that lie on the transect line or string are recorded.

Continuous sampling along a transect records every organisms that touches the string. The transect can be designated width either side (for example, a 1 meter string of the line transect). Transects are particularly for studying the changing distribution as the abiotic factors change so does the vegetation. A transect is a good way of monitoring the change.

Measuring Abundance:

Plants:

The quadrant method is used for population that does not move.

It can be made using 4 wooden stakes and string.

Quadrant (squares of a fixed area) are replaced randomly in an area.

The abundance of the organism in that area is counted.

This is then repeated several times and an average is taken

Then the size of the whole area is measured and multiplied by the average from the quadrant results. The quadrants are placed at random.

You can count the actual numbers of each plant or work out a percentage cover for each species. It is necessary to take enough samples to have a reasonable estimate of the population.

Sample size:

When studying an area you have to make a decision on the size of your quadrant.

The number of quadrants that you taken

Count the organisms that are completely within the quadrant

Count an organism if any part of it lies within the quadrant.

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**ESTIMATING USING THE RANDOM QUADRANT METHOD**
you will need a four meter length of string and four wooden pegs or a quadrant.

if you have a garden avaliable, use the lawn for the following practical.

1. select a random spot in lawn. you can do this by throwing a stick over your shoulder and starting one of your corners of the quadrant where the stick lands.

2. select a plant that you can recognise, such as dandelion, bindi eye, clover it does not matter if you don't know the name just as long as you can recognise it.

3. make your quadrant using the four pegs + the string. a good size is a meter X a meter but a smaller quadrant is acceptable. whatever size you use, make sure you can work out the size of your quadrant.

4. count the number of your chosen plants in the square.

5. repeat so that you have 10 measurements.

6. work out the average number of your plant in your samples.

you can now work out the total number of plants in the lawn by multiplying the density by the total area. measure or estimate the size of you are (multiply by the length of the breadth).

length of area:

width of area:

area in square meters:

multiply the area by the average number of plants in a quadrant.

this will give you an estimate of your chosen plant.

chosen plant per square meter.

Percentage cover:

in some cases when you are investigating an ecosystem it is more useful to record the percentage cover value than the actual number or organisms. a very small but dense plant is very difficult to count.

again you can use a quadrant to estimate the abundance of this type of plant of animal. this time the quadrant is divided into a grid using string.

to work out the percentage cover you have to count the number of squares that are covered by the plant. if the plants dosen't cover an entire square, you have to estimate the percentage of squares covered.

work out the percentage cover of the small plant from the following plan diagram.

rough estimate:

total number of squares: 10 X 10 = 100

number of squares covered: 3 + 2+ 2.4 + 4 11.5

percentage cover: number of squares covered / total number of squares x 100

11/5 /100 X 100 = 11.5%

**Transects--plan sketches and profile sketches:**
Rope or measure tape marks the line that is drawn to scale.

The area is selected at random across the ecosystem.

Species are plotted along the line, in surface view for a plan sketch or in side on view for a profile sketch.

Advantages: proves a quick, easy and inexpensive method for measuring species occurance.

Minimal disturbance to the environment.

Disadvantages: only suitable for plants or slow-moving animals. species occuring in low numbers may be missed.

**Quadrant sampling:**
Measuring tape, metre rulers or quadrants are used to randomly place the 1m X 1 m square areas.

The occurance of organisms in the quadrant is recorded and repeated a number of times.

Individual species can be counted if in small numbers or percentage cover can be calculated for larger numbers by estimating the percentage cover fro each quadrant and then finding an average of the of the quadrants taken.

Advantages: quadrants can also be used for determining the distribution of species along a transect. easy and inexpensive method for measuring abundance in large populations. minimal disturbance to the environment.

Disadvantages: only suited for plants and slow moving animals.

**Mark-Release-Recapture:**
Animals are captured, tagged or marked and then released.

After a suitable time to mix with others, a sample is recaptured.

The number of tagged or marked animals recaptured is counted.

Numbers are then entered into the formula.

Abundance = number captured x number recaptured / number marked in recapture

Advantages: a simple method that provides an estimate of abundance for animals in large populations that are difficult to count.

Disadvantages: only suitable for mobile animals.

Can be time consuming depending on the type of species captured, method of tagging, and time Suitable for waiting while tagged group mix with others.

Can be disturbing to the environment.

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