These notes use information from two textbooks, the internet and extra information from my lessons, and cover each specification point for this topic. They are fully comprehensive and include diagrams etc. Helped me achieve an A* in Biology A level.
Sampling: taking measurements of a limited no. of individual organisms present in a particular area
Can use to...
- estimate no. of organisms in an area (don’t have to count them all!)
[__ + abundance (no. of individuals of a species in an area)
- measure a particular characteristic of an organism
[__ use results of the sample to make generalisations or estimates
How sampling is used in measuring the biodiversity of a habitat and the importance of sampling
There are 2 methods of sampling: random + non-random
Random sampling
→ each member of the population is equally likely to be included - selecting individuals by chance
→ used to select a sample that is unbiased
→ can use random number tables or computers (random number generator) to decide which organism to study
[__ taking random samples of an area → use pairs of numbers as x and y coordinates
Could use meter ruler markings or grid references
Non-random sampling
→ sample is NOT chosen at random
3 main non-random sampling techniques:
1. Opportunistic
- Uses organisms that are conveniently available (‘convenience sampling’)
- May not be representative of the population - weakest/least accurate form of sampling
2. Stratified
- Some populations can be divided into strata (sub-groups) based on a particular characteristic eg. male/female
- sex
- Take random sample from each strata, proportional to the strata size
3. Systematic
- Identify different areas w/in an overall habitat + then sample these areas separately
- Used where study includes an environmental gradient - often carried out using a transect (line or belt)
The importance of sampling the range of organisms in a habitat
Reliability of samples
- A sample is never entirely representative of the organisms present in a habitat - TIB:
- Sampling bias - selection process may be biased (accidentally or deliberately). Can reduce effects by using
random sampling → removes human involvement in choosing samples
- Chance - organisms selected may, by chance, not be representative of the whole population. Cannot ever
completely remove chance from the sampling process but can minimise its effect by using a large sample
size (greater no. of individuals studied → lower probability chance will influence the result → more reliable
result)
, Practical investigations collecting random and non-random samples in the field
Techniques to include: use of sweeping nets, pitfall traps, pooters, Tullgren funnel and kick sampling for collecting
different samples.
Sampling animals - techniques for collecting
1. A pooter
→ Used to catch small insects
2. Sweep nets
- Used to catch insects in areas of long grass
3. Pitfall traps
→ Used to catch small, crawling invertebrates eg. beetles, spiders, slugs - fall into hole
- Hole dug in ground deep enough so insects that fall in cannot crawl out
- Hole is covered w/ roof-structure (eg. glass perspex cover) propped above ground so tha insects can crawl in
but the trap doesn’t fill w/ rainwater
- Traps usually left overnight - nocturnal species are sampled too
4. Tree beating
→ Used to catch invertebrates living in a tree / bush
- Large white cloth stretched out under tree → tree is shaken / beaten to dislodge the invertebrates → fall onto
the sheet → collected + studied
5. Kick sampling
→ Used to catch organisms living in a river
- River bank + bed is ‘kicked’ for period of time to disturb the substrate → net is held just downstream for
period of time → captures any organisms released into the flowing water
with due ethical considerations (e.g. pooters, traps, mark-recapture, the ACFOR scale)
Sampling plants
- Normally use a quadrat
[__ should be used after a random sampling technique in order to collect the mpst valid representative sample of an
area
(Quadrats can also be used to pinpoint area where the sample should be collected)
2 main types of quadrat:
1. Point quadrat
- Frame containing a horizontal bar. At (10) set intervals along the bar, long pins can be pushed through the
bar to reach the ground → record each plant species the pin touches
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