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Quadrats and Transects

What you need to know

Reflections and Exam tips

 

Distribution of living things

Where do we find living things? Good question!

Living things are found where the requirements that they need are vailable. Distribution is about where we find living things (their spread).

Plants and animals need:

  • A good temperature
  • Water
  • Nutrients and food
  • Light
  • Carbon Dioxide and Oxygen
  • A suitable pH

Exams require you to be aware that some of these environmental/physical factors (abiotic factors) can be measured using Datalogging sensors e.g. Oxygen sensors, light sensors, pH meters

Sometimes it is necessary to counts living things in order to find out about their distribution. At times its is difficult to count individual organisms because:

  • the numbers are just too many and counting will be time and energy consuming
  • they move around

In such circumstances, we may have to take a sample.

Some key definitions first:

A sample-
is a representative portion of the whole population.
Quadrat
a square wooden or metal frame
Transect-
A line pulled across a habitat (line transect), or a narrow area/ belt (belt transect)
Mean (average)-
after counting the organisms, you add up the totals for each quadrat then divide by how many throws you made.
Mode-
the most common value/number
Median-
The number in-between
Range-
the stretch from the lowest to the highest value.
 
 

There are many ways of sampling, including: Quadrats and transects

A quadrat is a square frame, usually 1m x 1m.

  • The fixed size of the square makes sure that the sample area is always the same.
  • It makes sure that the results are both valid and reproducible.
  • A square quadrat makes it easy to estimate the overall size of the population as it can fit esily into the whole area without leaving some gaps unaccounted for.

How to use a quadrat

Randon sampling- is done where there are no obvious variations in the habitat

  1. The quadrat is thrown at random acorss an area- this reduces Bias and resulting error
  2. As many quadrats are thrown so that the sample of throws is representative
  3. The species of interest are counted inside each quadrat throw
  4. A mean can then be found
  5. The mean is multiplied by how many times the quadrat fits in the whole area to get an estimate of the size of the population.

 

Systematic sampling- is done where there are obvious variations in the habitat

  1. A line/Transect is drawn (usually a string or tape measure is used)
  2. Quadrats are placed at regular intervals along the transect.
  3. Species of interest are counted inside each quadrat

Validity and reproducibility

The more throws, the more valid and reproducible the results are

Random sampling and the fixed size of quadrats also make the results more valid and reproducible

 

Leaf size estimation

A common investigation concerns the variations in leaf size depending on how exposed they are to sunlight.

Exposed leaves tend to have a smaller surface area than those hidden behind.

Estimating leaf area

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1.Place the leaf on square paper .............›2. Trace the Leaf...................................› 3. Count squares which are half full or mor, ignore the rest (x).