Free Measuring Abundance OCR A Level Biology revision notes – covering specification points 6.3.1(ei) and 6.3.1(eii).
Measuring Abundance
Abundance is the number of individuals of a species in a defined area.
Abundance can be determined using random, opportunistic or stratified sampling to decide where quadrats can be placed for counting or estimating percentage coverage.
Opportunistic Sampling
Opportunistic sampling is used to collect data quickly, so that a preliminary judgement can be made before proceeding with a more valid investigation, such as establishing that a species is present in an area of interest.
Opportunistic sampling is inherently biased because the sampling locations are chosen for their accessibility, convenience, or prior knowledge of the likely presence of organisms of interest. This produces unrepresentative experimental results.
The table below outlines examples of investigations that would use opportunistic sampling:
| Investigation | Approach | Observation | Results |
|---|---|---|---|
| Confirming the presence of bluebells in a woodland | Use opportunistic sampling by walking accessible paths and recording locations where bluebells are observed. | Locations containing bluebells are identified. | Bluebells are confirmed as being present in certain areas, informing where more valid investigations can be conducted. |
| Investigating red squirrels in a woodland | Use opportunistic sampling by placing camera traps in accessible locations where red squirrels are likely to occur. | Images of red squirrels are captured at some locations. | Red squirrels are confirmed as being present in the woodland, informing the relevance of a future investigation into their abundance. |
Random Sampling
Random sampling is used to estimate the abundance of plants or slow-moving animals in a habitat.
The table below outlines examples of investigations that would use random sampling:
| Investigation | Approach | Observation | Results |
|---|---|---|---|
| Comparing clover cover between two lawns | Conduct random sampling in each field by placing 1 m2 quadrats at random coordinates, estimating the percentage cover of clover in each. | One lawn shows a consistently higher percentage cover of clover than the other. | Calculate a mean for each lawn’s percentage cover and compare to determine if one lawn has a greater abundance of clover. |
| Estimating plantain cover in a field | Use random sampling by placing a 50 cm point frame at random coordinates, recording how many pins touch a plantain. | Plantains are recorded by approximately half of all pins. | The number of pins that touched a plantain is divided by the total number of pins deployed, then multiplied by 100 to give the estimated percentage cover. |
Stratified Sampling
Stratified sampling is used to estimate the abundance of species across a habitat that contains clearly different areas (strata), such as the layers of a forest canopy.
The table below outlines examples of investigations that would use stratified sampling:
| Investigation | Approach | Observation | Results |
|---|---|---|---|
| Estimating marram grass abundance across a sand dune system | Use stratified sampling by dividing the dune system into different strata, and placing 1 m2 quadrats at random coordinates within each stratum. | Marram grass abundance varies within different strata. | A mean number of marram grass plants per 1 m2 is calculated for each stratum, multiplied by the area of each stratum, and combined to estimate the total abundance across the dune system. |
| Surveying the heather cover on a heath with wet and dry areas | Use stratified sampling by dividing the reserve into wet and dry strata, and placing 1 m2 quadrats at random coordinates within each stratum. | Heather percentage cover is higher in the dry stratum than in the wet stratum. | A mean percentage cover is calculated for each stratum, and strata are compared across the heath. |