Lesson 5, Topic 2
Quantitative surveys and potential sources of error
- A quantitative survey measures the population size or distribution of a species
Quantitative survey of plants/flora
- Plants and flora can be quantified using either quadrats, belt transects or line transects.
A basic quadrant:
A graduated quadrant
- Quadrats can be used to quantify plants/flora and slow-moving animals (e.g. shellfish on the rocky seashore).
- Quadrats can be plastic, wooden or metal square frames of various sizes e.g. 1, 0.5, or 0.25 metres squared.
- Quadrats can be graduated (sub-divided into smaller squares) or a basic quadrat.
- Disadvantages of using quadrats is that they cannot be used to quantify trees or large bushes and cannot quantify fast-moving animals.
- Quadrats are always placed randomly in habitat by throwing a pencil over your shoulder and placing the quadrat where the pencil landed.
- Two possible types of measurement can be taken with a quadrat:
1. Percentage frequency
2. Percentage cover
- Percentage frequency: chance of finding a named species with any one throw of the quadrat.
- A basic quadrat is assigned randomly within the habitat (as described previously).
- The presence or absence of a named species is noted.
- This is repeated at least 5 times for each species.
- A table of results is drawn up as shown.
- Percentage cover: area of the ground covered by the aerial parts of the plant.
- A graduated quadrat is assigned randomly within the habitat (as described previously).
- If the graduated quadrat has 25 squares, each individual square represents 4% of the entire quadrat.
- The area of each square each species covers is estimated. In this way, it is subjective (depends on the individual’s opinion) and can be very inaccurate method.
- The totals from each square is totaled to come up with a percentage cover value for each species.
- There are two types: belt transects and line transects.
- Transects are useful for studying numbers of plants/flora that vary due to the topography of the land.
- They are also used very often to quantify flora/slow moving animals on the rocky seashore.
- Set up using two long ropes marked off at regular intervals (e.g. 1m/ 2m/ 5m etc) or one long rope and a graduated quadrat.
- If using the two ropes the number of each species is counted at each interval and recorded.
- If using the rope and the quadrat, the number of organisms of each species is counted in the usual way (see “quadrats” above).
- Consists of a long piece of rope/string marked off at regular intervals (e.g. 1m)
- Very useful for quantifying species along a gradient in a habitat.
- Line transects are carried out by recording the presence or absence of a particular species at the regular intervals along the rope.
- This is repeated at different locations.
- Line transects have the advantage that they can include and quantify large trees (unlike quadrat studies).
- In the capture-recapture technique, animals are caught using one of the methods listed earlier – e.g. mammal traps.
- For Leaving Certificate Biology it is best to use the recapture technique on easily caught animals such as snails.
- Snails live in long grass, bushes/shrubs and in trees.
- Search for snails in the habitat and mark each one with a small inconspicuous mark on the side of their shell (e.g. using Tippex)
- Record the total number found and marked.
- It is important that the mark does not harm the snail or make the snail more visible to predators.
- Place the snails back in the same position.
- Approximately one week later, return to the same habitat and search for the snails again, taking note of the total number found and the total number of these snails that had the mark.
- Use the formula below to estimate the number of snails in the habitat:
Sources of error in studying an ecosystem
- Miscalculation – e.g. when estimating percentage frequency
- Misidentification – e.g. species not identified correctly
- Sample size too small