National 5 Biology | Unit 3 | Key Area 2: Distribution of Organisms

Sample

A group of organisms selected to find out about the nature or number of the whole population.

Explain why samples are taken.

The numbers of individuals of a population are usually too large as the actual size of ecosystems is often very large. Samples are taken so that an overall size and nature of a population can be estimated.

Explain the purpose of calculating an average.

A more reliable and representative estimate is obtained; any atypical results have a less significant effect on the final answer.

With plants, quadrats are a common method of sampling. What assumptions are you making about a sample and the complete area, using data gathered from quadrats?

You assume that the organisms in the sample area are the same types and numbers as in the whole population, making it less accurate but much quicker than a complete count.

To ensure that an area is sampled in a way that produces a valid estimate of a population, what must be considered?

- The number of quadrats (ie. number of samples taken). This is ideally 10% of the total area sampled and ensures reliability.

- Quadrat location and position. Quadrats should be randomly placed to avoid bias, which ensures validity.

What are the limitations of using quadrats to sample plants?

- Not enough samples taken

- Deliberately ‘choosing’ good spots

When sampling plants, how can you rectify limitations caused by human error or otherwise?

- Placing a transect over the area of study and taking samples at set intervals along the line.

- Samples repeated and averages calculated to minimise error.

- Continue sampling until average values change little.

What are the advantages of using a transect to sample plants?

- Makes sure an adequate number of samples are taken, ensuring reliability.

- Samples taken in regular intervals avoid bias, ensuring validity.

- Often reveals a clear change in organisms from one end to another. Most useful when there is a clear directional change in environmental factors.

What are the disadvantages of using a transect to sample plants?

- Often time-consuming

- Organisms may be missed on either side of the transect

The pitfall trap is used to catch small invertebrates which live on or just below the surface of the soil. Describe how this trap is set up.

A pot with small holes in the bottom is placed in the ground with the top flush (rim) to ground level. It is covered and collected regularly.

What are some limitations of using a pitfall trap to sample small animals?

- Not enough samples taken.

- Occupants destroy and eat each other.

- Escapees.

- Some animals are more/less likely to fall into the trap.

When sampling animals using the pitfall trap, how can you rectify limitations caused by human error or otherwise?

- Checking traps regularly and recording information at frequent intervals, ensuring reliability.

- Set up more traps in the area, ensuring validity.

- Using different types of bait to attract different species.

- Using alcohol (typically now an unorthodox method).

What was alcohol used for in pitfall traps, and why are they not typically used in practice today?

Alcohol kills and preserves the animals as they fall into the trap, which eliminated the limitation of animals destroying one another and escaping.

This has been discontinued as it is destructive and affects the ecosystem which is being investigated.

Reliability

Concerning reproducibility and replication.

Replication: Experiments are repeated multiple times to see if the same answer (within an acceptable variation) is obtained.

The more often an experiment is replicated and the same answer is obtained, the more reliable a result is.

Validity

Concerning the quality of information given by results. To improve validity, errors of procedure should not give misleading information.

eg. ensuring a sufficient area is sampled in an ecosystem when investigating.

What is important to note about results obtained from experiments?

It is possible to have reliable but invalid results.

Commonly measured abiotic factors include:

pH, temperature, water availability, light intensity

How can light intensity be measured?

Using a light meter or a probe

Describe possible sources of error when measuring light intensity, and how may they be solved.

- User may cast shadow over the meter:

Stand so that shadow doesn’t fall on the light meter.

- Changes in cloud cover during measurements could make comparisons between different locations invalid:

Ensure minimum delay between measurements and cloud cover has not changed (take at same time in the day)

- Readout may take time to stabilise:

Allow time before reading.

- Test area does not receive same light as light meter:

Ensure meter is positioned correctly. eg. hold sensor flat at ground level for ground-living organisms.

How can temperature be measured?

Using a thermometer

Describe possible sources of error when measuring temperature, and how may they be solved.

- Readout may take time to stabilise:

Allow time before taking reading.

- Incorrect reading of thermometer:

Ensure thermometer scale is read accurately and is placed at the same depth every time.

- Test area not at same temperature as thermometer:

Ensure thermometer bulb is not warmed by the hand and is in the correct position, eg. actually in stream water.

How can soil moisture content be measured?

Using a moisture meter

Describe possible sources of error when measuring soil moisture, and how may they be solved.

- Incorrect reading of meter:

Set the meter at a fixed depth to measure each time.

- Fluctuations in meter readings:

Take the readings as soon as the needle has settled.

- Moisture left on probe from previous samplings affecting following results:

Wipe probe between readings.

How can pH be measured?

Using a pH meter or probe

Describe possible sources of error when measuring pH, and how may they be solved.

- Incorrect readings of meter:

Set the meter at a fixed depth to measure each time.

- Sediment on probe from previous samplings affecting following results:

Wipe probe clean between readings

Indicator species

An indicator species by their presence or absence will indicate environmental quality / levels of pollution in an area.

What does finding a species that is particularly sensitive to air or water pollution tell you about the environment?

This indicates that the environment is clean as the air and water that they live in must be clean. Examples include mayflies, stonefly nymphs.

What does only finding species that are particularly tolerant of air or water pollution tell you about the environment?

This indicates that the environment is polluted as no other organism can survive the high pollution levels. Examples include bloodworms, rat-tailed maggots.