Title
Sampling for Abundance
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A transect is a line of known length, marked at regular intervals, that is used for sampling in the field. The length of the transect and the distance between intervals is dependent on what is being studied. A transect line can be a physical item that is purchased commercially (Fig. 1 A and B) or a line that is hand-made by marking regular intervals on a piece of rope (Fig. 1 C). A transect line can also be an invisible line between two points. For example, ships can sample along an invisible transect line across the ocean using maps and GPS.
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Fig 1. Different types of transect lines: (A) Open-reel measuring tape transect, (B) Surveyor’s rope transect, and (C) Rope with regularly marked intervals
A quadrat is a framed area of known size and dimensions. Physically, quadrats are generally square and often constructed with string that divides the area into a set number of squares. A quadrat frame can be made using PVC pipes, wire hangers bent into squares, wooden dowels, cardboard, or even a hula-hoop for a round quadrat. String or monofilament fishing line can be used to divide the quadrat into a grid (Fig. 2). Much like transects, different sized quadrats are used for different types of sampling. The size of a quadrat is dependent on the size of the area that scientists want to sample. Quadrats can be used in conjunction with transects or quadrats can be used on their own to aid in sampling an area.
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Fig 2. Quadrat in field with lines clearly marked. This quadrat has six intersecting lines of string that divide the quadrat into 16 squares and create nine intercepts. Quadrats can have more or less intercepts and squares depending on the number of string lines.
Transects and quadrats are two sampling tools that can be used to randomly or systematically survey an area. The purpose of a survey is to examine and describe some aspect of an area or environment. Both random and systematic sampling strategies are valid ways to characterize an environment; the choice of sampling strategy is dependent on the survey goal. Random sampling can involve techniques such as taking measurements at random coordinates on a map or random points along a transect line. Choosing random numbers can be as simple as picking numbers out of a hat, but sometimes scientists use computer software to make generating random numbers easier. Systematic sampling is regular. Systematic sampling generally involves focusing on transect points a set distance apart and taking measurements along the transect at those pre-chosen points.
Sampling Methodologies
Three common systematic sampling methods using transects and quadrats will be demonstrated by surveying a mock study site (Fig. 3). Although the different colored circles can represent any variable of interest in an area, such as rocks, water chemistry, or plant growth, at this mock study site the colored circles represent different species of organisms. The empty white area represents the substrate, which is the substance on the bottom of the environment. Examples of different substrates include mud, pebbles, and boulders. In this example, the white area represents sand. In this case, the environment is also the habitat where the colored circle species live. This is a benthic survey because the circles represent organisms that live close to, or attached to, the substrate.
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Fig 3. A mock study site where different organisms are represented by different circles. Different colors represent different species of organisms.
Notice how the circle species in Figure 3 appear to occur in zones from left to right. The blue circles are grouped on the left; the red circles are located on the right. Green and yellow circles, on the other hand, are located throughout the study site. Ecological zonation is the distribution of organisms into bands based on differences in biotic and abiotic factors. To capture this zonation, a transect, represented in Figure 4 by a black line, is laid perpendicularly through the zones (Fig. 4).
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Fig 4. The black line represents a transect line. White circles mark numbered points at regular intervals along the transect.
One of the simplest ecological sampling methods is transect point intercept. In this method, a transect is laid in an environment and scientists record what is directly under the transect at certain points. The sampling points are often at regularly spaced intervals, such as each meter or half meter, along the transect line (Table 1). In this mock study site, transect points are in meters. When using the point intercept method, it is important to record only the organisms or substrate that is directly beneath the transect line. For example, even though a red circle is very close to transect point #5, only “sand” is recorded on the data sheet (see Table 1).
|
Transect Point (m) |
Organism/Substrate |
|
1 |
Blue |
|
2 |
Blue |
|
3 |
Sand |
|
4 |
Yellow |
|
5 |
Sand |
The transect point intercept technique is good for sampling a large area relatively quickly, but this method can miss information in complex areas. In the mock study site (Fig. 4), we only recorded two of four species in the area. Sometimes placing more than one transect in a study site or using quadrats can help to further document and characterize the complexity of an area.
Quadrats are often placed at designated points along a transect to get more detailed information about a study site. A quadrat should always be placed the same way in relation to the transect. For example, you can place the quadrat directly over the transect point (Fig. 5 A), or you can line up a corner of the quadrat with a transect point (Fig. 5 B). Although it may be easier to remember to always place the quadrat right over the transect point, the transect tape might obscure some organisms on your field site (Fig. 5 B). Whichever method is chosen, it is important to be consistent and place quadrats the same way each time along the transect.
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Fig 5. Quadrats can be placed right over the transect point (A) or at the corner of the transect point (B). The quadrats in these diagrams have intercepting lines, but because the lines are fishing line and the quadrats are in the water they are hard to see in these photographs.
In the mock study example below (Fig. 6), the squares represent quadrats. These quadrats have nine intercepts and 16 squares. The bottom left corner of the quadrats are placed along pre-determined points on the transect line.
The quadrat point intercept method utilizes the grid of lines within the quadrat (Fig. 2); scientists record what is underneath each intercept within the quadrat frame. This is similar to the transect point intercept method, but there are multiple intercepts within the frame to be counted. Since there are nine intercepts in the example quadrat, there are nine data points for each quadrat. Quadrat data sheets (see Table 2) list the categories at the study site in the first column. Each subsequent column corresponds to a point along the transect where a quadrat has been placed. Finally, columns are included for the total number across all quadrats points by category and then the average across all quadrats by category. Scientists keep track of the number of times each category occurs under an intercept for each quadrat with tick marks. In this example, the total in each column should add up to nine. Scientists complete the column corresponding to a quadrat and check their addition before moving on to the next transect point. Figure 6 shows four quadrats, however, scientists rarely place more than one quadrat along a transect line at the same time. Often data is collected with just one quadrat that scientists move along the transect line as they collect data.
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Fig 6. The large black squares represent quadrats placed along designated points along a transect.
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The quadrat point intercept method has captured three of the four species at this site. Green circles have not been recorded because they are so small that they did not occur under any of the study intercepts.
In the quadrat percent cover method, scientists capture and record the area of all of the organisms that occur in the total quadrat. In the quadrat percent cover method, scientists estimate and record the percentage that each organism takes up within the quadrat frame. Scientists do not need to pay attention to the intercepts inside the quadrat, although they can be useful reference points to help with estimation of percent cover.
A helpful tip when using the quadrat percent cover method is to imagine all the organisms of one species pushed into a corner of the quadrat and estimate what percentage of the quadrat they take up. It is often easier to start with the rare organisms and rare substrate and finish with the most common organisms and substrate. In our mock study site example (Fig. 6), we subtracted the total colored circle percentage from 100 to get the percentage of sand (Table 3). The total percent in each quadrat column should add up to 100. Note that species that occur in less than 1% of the area are not considered in the total.
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The quadrat percent cover method captured all of the species in our mock study site, but this method is prone to a large degree of error because it requires estimating. For example, one person might estimate yellow circles occur in 5% of a quadrat while another person collecting data from the same quadrat at the same time might think the yellow circles occur in 8% of the quadrat. One way to address this issue and decrease estimation error is to use categories. Example category options for percent cover might be 0%, 1-10%, 10-20%, 20-30%, etc.
Layering
In the field, organisms and substrate will often be layered on top of each other. For example, a quadrat point intercept may fall on top of a snail that is on top of an urchin that is on top of an alga that is growing on a rock. Some sampling methodologies take into account all of the different layers. However, these types of methodologies can make analysis difficult. Most scientists who sample the benthic environment in the field only record the surface layer, the top layer. In the example above, the top layer would be the snail. Only recording the top species or substrate ensures that there is only one data point for each point intercept and that each quadrat percent cover data will add up to 100 percent. Water is usually not recorded as there is always substrate at the bottom of the water.
Data Analysis
Transects and quadrats are sampling tools that are often used in ecology to collect information on relative species abundance. The relative representation of species in a location refers to how rare or common a species is relative to other species in a defined area or community. Relative species abundance is often displayed as the number of species or the percent cover of each species in a sample.
Calculating relative species abundance from transect as well as from quadrat point intercept and quadrat percent cover methods allows direct comparisons between methods. Relative species abundance of each category of organism or substrate is shown as a percentage in the grey columns of Table 5 and graphically in Fig 7.
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Fig 7. Comparison of the percentage of each species represented in the mock study site using three methods: transect point intercept, quadrat point intercept, and quadrat percent cover. Each column represents the species found using a single method. All species total add up to 100% within a column.
Data Analysis Questions
Refer to Fig 3, Table 4, and Fig 7 to answer the following questions.
- Which sampling method(s) captured all of the colored circles? Which sampling method(s) did not capture one or more colored circles?
- Compare the quadrat point intercept and quadrat percent cover data. What are the similarities and differences in category abundance?
- Compare the transect point intercept abundance estimations to the quadrat estimated abundances. Why does the transect point intercept method overestimate some category abundances while underestimating other category abundances?
- Compare the transect and quadrat data to the actual mock study site (Figure 3). Do you think the sampling methods accurately sampled this area? Why or why not?
The mock study site (Figure 3) is unusual because the objects of interest, the circle species, stand out against the white background. Every object can clearly be seen in the area of interest. In nature, however, things are often more camouflaged. With this in mind, answer questions 5 and 6.
- What are the pros and cons of each sampling method:
- Transect point intercept
- Quadrat point intercept
- Quadrat percent cover
- What are potential sources of error for each sampling method?
Sampling Strategies
The mock study site was used to demonstrate how to use three sampling methods: transect point intercept, quadrat point intercept, and quadrat percent cover. There are many other population sampling strategies that use transects and quadrats (Table 6). Scientists choose sampling techniques based on the survey site (e.g. field site conditions like weather conditions and terrain) and goals (e.g. sampling large or small organisms) Sampling strategies are also influenced by the amount of time scientists have at the site and the materials that are available. For example, both transects and quadrats are useful in surveying sessile organisms, but only transects are useful when surveying more mobile organisms, like swimming fish or flying birds.
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Fig 8. (A) Holding a T-bar while swimming along a transect (B) Taking a photoquadrat
