Sampling and Comparing Biodiversity
Author: Nicole VanderSal
Overview: This lesson compares the density and distribution differences of four species of spiders in two hypothetical habitats. Students "sample" two habitats and then do calculations such as average density and relative distributions to compare the data from their two habitats.
Grade Span: 912
Materials (all groups need one of each):
Time: 50 minutes
Grouping: Works best with two to three students to ensure everyone stays involved in calculations.
Begin by giving each group a Habitat "A" and Habitat "B" packet, one Quadrat sampling sheet and a worksheet. Be sure to tell them not to flip to the bottom page of the packet (the one with spiders on it) until told to do so by the directions.
Explain that these packets represent two different habitats where spiders may be found. Have each student make a hypothesis about which habitat will have the highest biodiversity of spiders and why they think so (question 1). Go over the different hypotheses and write them on the board. You can say that we will now be testing to see which hypotheses are supported by doing three quadrat samples in each habitat.
Have the students take the quadrat sampling sheet (with the squares already cut out) and place it in between the top and bottom sheets of Habitat "A" packet without looking at the bottom sheet. Once the sampling paper is in place, have the students lift up the top habitat sheet and record the number of spiders, and the species, found in each quadrat in Table 1 of the worksheet. Be sure the students only count spiders that are at least halfway in the quadrat (they can see at least five legs). This is to ensure that the sample estimate is not overly inflated, and you can say that this is realistic in biological sampling where people only want to know what is directly inside the quadrat, rather than at its borders.
Repeat this procedure for Habitat "B".
Using the data that they just collected in Tables 1 and 2, have the students answer questions 2 and 3 (fractions are easiest for question 3). This data can be written on the board so students can see differences between groups. As a class, go over the estimate for each habitat (just multiply the average quadrat number by twelve), then have the students look at the bottom sheet to count the actual number of spiders in both habitats (questions 4 and 5). You can change question 4 to have more calculations by giving the students the dimensions of the quadrats (7 x 7 cm) and the dimensions of the paper (28 x 21 cm) and having them figure out the estimation calculation.
Are the numbers for the estimate and actual count close, or are they off (question 6)? Why would the numbers be different (question 7)? (your samples fell in areas where spiders were clumped, or where there were no spiders). Here you can talk about the limitations of our sampling technique. Would you have gotten more accurate results if you had done more quadrats or fewer? (more is better: increasing accuracy up to the point where you can actually sample everything in the area). What are some reasons researchers cannot do a very large number of quadrats when they sample an area? (time, money and manpower limitations).
Now we are going to introduce another way to compare the biodiversity of two areas by comparing the relative distribution of all four species in the two habitats. Explain that knowing how the species are distributed in a habitat relative to each other can also be a good way to compare two different habitats. In "Area 1" the area has four species, but the environment doesn't support all of the species equally well, and one species dominates. If you did only a few samples, you might not even see the three other species because they are rare. On the other hand, "Area 2" supports all four species equally well and would be considered a better habitat for all of the species to live together.
Using the information from the last column in Tables 1 and 2, have the students fill out Tables 3 and 4. For the proportions table it is easier to leave the numbers in decimals, but you can convert them to percentages if this has recently been covered. Based upon the information in Table 4, have the students compare the distributions within the two habitats (question 8). Now, see if they think these are distributions that would be seen in nature and why. There are no right or wrong answers here: just be sure that the students come up with reasons why they think it would or would not be this way in nature (e.g., spiders use different habitats so they wouldn't be evenly distributed, or, all different types of spiders will congregate in areas of high prey density, so you would see even distributions: this is a factor of the scale at which you are sampling).
And finally, have the students reevaluate their original hypotheses. Have them consider if it was supported or refuted equally by all the different ways we measured biodiversity: number of spiders, number of species, relative distribution of species. Usually Habitat "B" has a higher number of spiders, but fewer species and not as evenly distributed. Emphasize that it is important to see if our hypotheses are supported or refuted at the end of a study and then modify the hypothesis for future studies, including this new information.
This lesson worked really well. The students were in groups of three or four and most of them helped to complete the whole worksheet (with the larger groups there were some free-riders that just copied answers). I had to define mathematical terms like proportion and average on the board to be sure everyone did the right things, but then they did it by themselves. The students were going along at a good pace, so we had much more time than I expected to tie things together. As a class we did the population estimate of the entire area, and I have since added that onto the worksheet. Joni Grisham was very happy with the lesson and requested copies of everything so that she could use it in her other classes. So overall, this lesson should work very well in other classes.