Practicals related to 3.7 Genetics, populations, evolution and ecosystems

Describe how you could investigate the effect of an environmental factor on

the distribution of a species in a habitat (random sampling in two areas)

1. Divide two areas into grids / squares eg. place 2 tape measures at right angles

2. Generate a pair of coordinates using a random number generator (eg. on a calculator)

3. Place a quadrat here and count number / frequency of [named species]

○ Standardise this eg. only count it if it is more than half in the quadrat

4. Repeat a large number of times (10 or more) and calculate a mean per quadrat for both areas

5. Measure environmental factor in each area eg. take soil moisture readings with a soil moisture meter

Suggest why percentage cover may be used rather than frequency.

● Too difficult to count individual organisms / individual organisms are too small to count

Explain why random sampling is used.

● To avoid sampling bias

Explain the importance of a large sample size.

● Minimises the effect of anomalies

● Ensures sample is representative of the population

Describe how you could decide the number of quadrats that should be used in order to collect representative data.

● Calculate a running mean

● When enough quadrats, this shows little change

● Enough to carry out a statistical test

Describe how you could investigate the effect of a factor on the distribution of a species in a habitat (systematic sampling)

1. Place a transect line (tape measure) across an area with an environmental gradient eg. tree to full sun

2. Place quadrats at regular intervals eg. 1m (until end of transect) and record the number of organisms of

[named species] and [named environmental factor] eg. light intensity using a light meter

3. Repeat in other parallel areas and calculate mean number of plants at each point along the transect

Explain the limitations of using systematic sampling to estimate the population of a

species in a field.

● Not appropriate unless an environmental gradient

● Transects run in one direction, but to cover the entire field, they would need placing in multiple directions

Which statistical test should be used to determine the relationship between abundance & an environmental factor?

● Correlation coefficient eg. Spearman’s rank