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SYEFEST Activity: Dandelion Determinations
Institute of Ecosystem Studies
Millbrook, New York
Contributor: Alan Berkowitz
Starter Question:What is the population of dandelions in the schoolyard lawn?
Overview: Estimate the number of dandelions by sub-sampling a study area using ½ square meter circular plots at randomly identified locations
Background: All of the organisms of a species that live in a given place are considered a population. We might be express the size of a population as the total number in an area (e.g., the number of people in Poughkeepsie) or in the population density (e.g., the number of people per city block in downtown Poughkeepsie). Measuring population size is a necessary step in addressing on of the BIG QUESTIONS in ecology: What factors determine the population size of a species in a given place?
One of the big challenges in learning about populations is that it usually is too hard to count all of the individuals that make them up. Dandelions are no exception. While dandelion flowers are quite conspicuous in lawns, the plants themselves are harder to see. Most lawn areas have far too many dandelions - much to the chagrin of grassy lawn purists - to count. In this study you will use a point-centered circular plot technique to estimate dandelion plant density. By examining a number of plots located randomly within the study area, you will determine a mean density which, multiplied by the area, will give you an estimate of the total number of dandelions.
Materials:
- Measuring tapes at 15m long for marking the study plot
- 2 metal stakes, one for the study plot and one for marking the center of the sampling plot
- 1 1m measuring tape with "eyelet" at the end
- Field guides for dandelions
- Data sheet on clipboard or piece of firm cardboard
- Random numbers table
Procedure:
- Layout a 10m x 10m study plot
- Choose the part of the schoolyard lawn you want to study.
- Mark the lower left corner with one of the metal stakes, running it through the metal tab of the first meter tape.
- Run the tape along one of the sides of the plot to just beyond 10m.
- Run the second meter tape out in a roughly perpendicular direction and use the second stake to mark the 10m point along this side.
- Square the plot corners by using the second meter tape and measuring 14m 14cm from the 10m point on first tape to this stake and adjusting the stake accordingly.
- Go back and lay the second tape out from the first corner stake to this point, and you have two sides of a perfectly square 10m x 10m study plot.
- Identify random locations for sampling
- Use the random numbers table to pick random coordinates for 10 sample points. Record these coordinates on the data sheet.
- Go to each sample point by proceeding to the appropriate point along one of the measuring tapes, then moving into the plot until you're adjacent to the appropriate point on the other tape.
- Sample dandelions
- Place the metal stake at the randomly-located point in the study area.
- Make a 0.5 m2 circular plot by placing the eyelet of the 1 m tape over the stake and holding the tape at the 40 cm mark. NOTE: a circle with a radius of 40 cm is almost exactly 0.5 m2 in area!
- Carefully examine the entire area within the circular plot for dandelions. Only count plants whose center is inside the circular plot.
- Record the total number of dandelion plants you see.
- Repeat these procedures at all 10 spots.
- Making sense of the dandelion determinations
- What is your best estimate of dandelion density? Calculate the average number of dandelions per circular plot and multiply this by 2 to get the average density of dandelions per m2. How reliable is your estimate?
- Did dandelion density vary from place to place within the study plot? What might account for these differences?
- Calculate the total number of dandelions in the entire study plot by multiplying your estimate of average density by the area of the plot (i.e., 100 m2). Impressed or surprised?
Please post any questions or comments in our Dandelion Determinations forum. We'd love to hear from you!
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