B2600 Sampling lab Fall 2023

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Memorial University of Newfoundland *

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2600

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Electrical Engineering

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Jan 9, 2024

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Lab 5: Introduction to Transect Sampling As you learned in the random sampling lab, it is important to document and monitor communities so changes in abundance or diversity of species can be recognized. In this lab we will look at other field sampling methodologies that can help researchers gain insight about populations or answer questions about species ranges or tolerances. We will look at three methods of sampling and discuss when we would use one method over another. Have you ever seen a large measuring tape or rope stretched out across a field or forest? This is a transect line. Researchers use transects to either search for signs of animals over a given area or sample plant communities at regular intervals. Transects are often laid down parallel to each other or in zig zag patterns. The number of transects, their position and the number of sampling locations along the transect line depend on the size of sampling area and what is being sampled. While we are using transects to sample plant communities, transects are perhaps more widely used to survey animal populations. The number of animal sightings or indicators such as calls, nests etc. found along a transect line (usually in a fixed distance from the line) can give an idea of abundance. In order to do a line survey for animals, you would need to be trained to recognize the animal or its indicators. The simplest type of line transect surveying for animals includes doing a pre-survey during which you record sightings of the animal or indicator and record the distance perpendicular to the transect line where the animal was spotted. Detectability of the animal decreases with distance from the line, so sightings can be plotted against distance from line and an appropriate searching width can be decided upon. To find the density of the animal the number of sightings can be divided by the area searched. Remember area = length (length of transect line) x width (2x the detectable width (because you search on either side of the transect line)) This following link provides a good example of basic density calculation from line survey data https://www.webpages.uidaho.edu/wlf448/lab5line.htm . While this is a basic method of calculating density, there are many ways in which the equations can be improved to more accurately measure or predict density. New or species specific models are constantly being developed; the methodology and math of which are beyond the scope of this lab. Transect line counts are one of the major ways in which animal populations are estimated. Below are links to articles that discuss the use of line transects to survey animals (optional further reading)  Kral-O’Brien, K.C. et al. 2021. Getting the Most From Surveys: How Method Selection and Method Modification Impact Butterfly Survey Data. Annals of the Entomological Society of America, Vol. 114. Issue 6. Pgs 719–726. https://doi.org/10.1093/aesa/saab004  Martin, M. et al. 2020. First Abundance Estimates of Heaviside’s (Cephalorhynchus heavisidii) and Dusky (Lagenorhynchus obscurus) Dolphins off Namibia Using a Novel Visual and Acoustic Line Transect Survey. Frontiers of Marine Science. Vol. 7. https://doi.org/10.3389/fmars.2020.555659 When sampling plant communities, transects are often used to gain insight into how communities change in response to a changing landscape gradient, for example increasing elevation or increased wetness.

Continuous line transects are done by setting out a transect and recording every plant that touches the transect line – this can show which species are found in a given area (and the number of times a given species is recorded relative to other species), or the range over which a species is found (when looking at a changing gradient). You can note if species are becoming more or less frequent or observe when a species appears or disappears and relate it to surrounding environmental conditions. Some downsides of this method are that you don’t necessarily get a good picture of the density of species and it can also be easy to miss species. Figure 1 shows one way to present the data from a continuous line transect. When looking at areas across a gradient (such as the wetness gradient below) you can see which species are found over a wider range of conditions and which are restricted to a certain area. What does this say about individual species tolerances? Figure 1. Diagram showing location of species in relation to areas of differing wetness conditions. Source: http://www.countrysideinfo.co.uk/wetland_survey/trans1a.htm Interrupted line transects involve recording only the species that touch the line at given intervals (1m, 2m etc.). Figure 2 shows a basic profile diagram indicating the species found along a transect. Counting every species in a line transect can be tedious; interrupted line transects allow a greater area to be covered, but will also result in more species being missed Figure 2 . Profile diagram using symbols to represent plants encountered on an interrupted line transect Belt transects – A belt transect is basically a wider line transect. Belt transects can be done by placing parallel strings on either side of a transect line or by using a quadrat and sampling successive quadrats along a transect line. Usually either the number or percent cover of each species that appears in the quadrat is recorded and this gives a clearer picture of the relative density of different species in an area. The results of sampling with belt transects can be displayed as stacked bar graphs, with a bar for each quadrat showing the relative density (plants/area 2 ) or percent cover of the species found within. If the belt transect is longer, sometimes subsampling with point fall can take place. In this case a large comb or matrix might be dropped systematically and a record made only of the plant touched by the point/crosshairs.

Point quarter method The point centered quarter method is a plotless method that can be used to measure tree density/dominance/frequency in forests. In this method you lay a transect line and sample at predetermined points (randomly or systematically chosen). In this method the area around the sampling point is divided into four quadrants (note: a quadrant is one of four quarters formed by two perpendicular lines crossing and a quadrat is a plot in which samples are taken, often square but may be rectangular or circular depending on design). In each quadrant you measure the distance to the nearest tree, the circumference of the tree trunk at breast height (~1.3 m) and the species of tree. See Figure 3. The point quarter method gives a better estimation of density when trees are randomly distributed. When trees are clumped, the method tends to underestimate density (Krebs, 2014). If the distance from the sampling points to the trees sampled is similar, the forest is more homogenous. If the measured distances show a lot of variation, the forest is more clumpy (USDA forest service, 2003). The clumpiness of a forest could be ecologically significant in several ways, for example it could influence the type of wildlife habitat provided or the speed at which a fire might move through an area. Figure 3. Example of which trees to sample using the point quarter method. Red shows the transect line, the dashed line is the imaginary line perpendicular to the transect and the blue lines show the closest trees to each point (the distance recorded as PQ distance) Source: Krebs, C.J. 2014. Ecological Methodology, 3rd ed. The simplest method of calculating tree density is to average the distances to the trees from the points sampled (P-Q distance) and square this distance to get an average area per tree and extrapolate to a given area. Average P-Q distance = Σ (P-Q distances) / number of trees sampled Density = 1/ (Average P-Q distance) 2 Ex. 1/ (5m) 2 = 1/25 =0.04 trees/m 2

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