14.2 Applications and skills
14.2.4 Field techniques: Using Transects
Transects are often used to sample the distribution of populations. The most common type is a line transect. A piece of tape or string of fixed length is randomly placed in a straight line in a test site. The number of organisms that fall on the line and their locations are noted. The process is repeated until a representative sample has been taken.
A belt transect involves laying two lines at a fixed distance from each other and counting the organisms within the two lines. Sometimes this is done using a quadrat, so that an estimate not only of distribution but also of relative abundance can be calculated.
Figure 14.2.4b – Belt transect
In this case, counting organisms within a 1 x 1m quadrat along a line transect is equivalent to using a belt transect with two lines 1m apart.
A point transect is useful for sampling organisms in tall trees (e.g. birds and bats) or to determine canopy cover. The researcher stands at a fixed point and measures the number of organisms detected within a certain radius of that point. Sometimes a point is painted on the researcher’s boot for accuracy.
Figure 14.2.4c – Point count
In all cases, care must be taken to ensure that transects are done in random locations through the test sites.
Application: Primary succession in sand dunes
- Sand dunes develop in coastal areas where there are high winds and little vegetation.
- High tides deposit sand, which becomes an effective wind (and wave) breaker, leading to more sand deposition.
- Grasses may eventually colonise the dunes if they provide enough wind cover.
- Sand dune ecosystems demonstrate primary succession in action because the youngest dunes are closest to the strand line (i.e. the beach) and the oldest dunes are furthest away.
Figure 14.2.4d – Primary succession in sand dune ecosystems
The diagram shows the successional gradient from the newest sand dunes (east) to the oldest sand dunes (west) with respect to various abiotic factors.
Figure 14.2.4e – Data collected from a sand dune ecosystem near Sefton, UK
Activity 1: Analysing data
Referring to the data in Figure 14.2.4e:
- Name the pioneer species.
- Describe the trends in abiotic factors that are typical of primary succession.
- Identify which data was collected using transects and describe what type of transect was used.
- Explain the trend in species number from early to late stages of primary succession.
- Suggest a reason why marram grass does not appear in sites 3, 4 or 5.
Activity 2: Investigate the effect of disturbance on an ecosystem
Disturbances affect living and non-living parts of the ecosystem, including, but not limited to:
- species diversity
- nutrient cycling
- water movement
- leaf area index
- percentage cover
- relative abundance
- edge effects (e.g. narrowing of realised niche, or defended territories)
For this task, work in groups of two or three. You can use any of the skills you have learned in this course, including quadrat samples, transects, mark-recapture (HL), Simpson’s reciprocal index of diversity, and statistical analysis such as the t-test and chi-square test. Have your plan approved by your teacher before you begin your investigation.
- Perform a transect.
- Analyse data showing primary succession.
- Investigate the effect of disturbance on an ecosystem.
If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically appropriate language, on an internet search. It’s like a reverse glossary search!
Nature of Science
Avoiding bias: Be specific in your reporting about how you decided on the length and placement of your transects. It’s best to follow an arbitrary rule or use a random number generator.
Extended Essay ideas
Get to know your local ecosystem:
Figure 14.2.4f – Sand dunes
Sand dunes on Schiermonnikoog, Netherlands (above) and Lake Huron, Canada. Sand dune ecosystems occur in both saltwater and freshwater environments.
- What constitutes a disturbance? A paved path through a park reduces connectivity for small insects (island/edge effects).
- Transects should be laid out in a way that you can collect a reasonable amount of data (minimum 40 organisms).
- It takes practice to determine how many samples is enough.
- Keep track of the total length of your transect lines and the distance from the line in order to improve precision of your method.