The Ecology of Opuntia fragilis (Nuttall) Haworth by Eric Ribbens

Part III—Your First Year Results

You explored the region around the new park to see if there are other populations of Opuntia fragilis on nearby rock outcrops. You used topographic maps to find areas where rock outcrops could be found nearby, and you examined each rock outcrop. You found one previously undocumented population. Also helpful for this aspect of your research project was the university’s retired botanist, Nick. Nick told you that there were several small populations growing on rock outcrops along a road near the park, and one large population on a granite hill about 15 miles away. One of the populations turned out to have been destroyed by a recent road expansion, but the others were still there. You added these populations to your research, so that now you have four sites.

You surveyed the entire rock outcrop in each site and created a map of the locations of prickly pear, the rock outcrop segments, trees, and grassy vegetation (see Figure 3). You planned to record the location of each individual and count the number of pads that the individual has produced. However, you discovered that in several sites there were large clusters of prickly pear pads, one with more than 100 pads. In the clusters you were not able to distinguish individual plants. Therefore, you modified your original plans and marked the location of clusters without counting the pads within the clusters (remember a cluster may be one large or several smaller individual plants).

How big are the populations? You wanted to know both how many different plants there are and how many individual pads there are. Cold Spring Hill had the largest population of Opuntia fragilis; you found at least 230 plants. Your original site, Quarry Park, had the second largest population, with about 100 plants. A site on a small outcrop nestled into a woods, which you named the Woods Site, had only six individual plants, and a site on a small flat rock outcrop which you named the Table Site had only one large cluster covering about two square meters.

You also wanted to know how the populations were structured in terms of the size of each individual. The table in Figure 4 shows for each possible number of pads how many plants you found with that number of pads, added up over the four sites. Note that several plants had more than 10 pads, and are included in the “10” column.

Figure 3: Quarry Park. Green areas are grassy. The black lines are cracks in the granite, and the long black object is a fallen tree. O. fragilis plants are marked with black circles.

Figure 4: Your First Year Data.

In June you watched for flowers. You planned to record how many flowers each plant produced and then revisit the site later in the year to determine which flowers had produced fruit. However, you discovered that only four flowers were produced in one entire population, one flower at another site, and two sites never produced any flowers! Furthermore, no fruits were formed. Therefore, your plans to observe the flowers and dissect the fruits had to be set aside.

Questions

1. What does the map of the site tell you (Figure 3)?
2. What does the bar graph of plant sizes tell you (Figure 4)?
3. What changes did you make in your research plans? Does the fact that you had to change your plans mean that you are a poor scientist?
4. In your library research, you discovered a research article published by Frego and Stanisforth (1986), two Canadian scientists who studied Opuntia fragilis on rock outcrops in Manitoba and western Ontario. They reported that in their study sites, the brittle prickly pear only reproduces by fragmentation. Thus, no sexual reproduction is occurring in their populations, and the population expands by having individual pads break off and become established elsewhere on the site. This intrigues you, because you also found no successful sexual reproduction. What are the implications for the population if it is only reproducing asexually?
5. In another article, Frego and Staniforth (1986) point out that isolated populations of Opuntia fragilis in Ontario are always located along rivers. Citing the earlier work of Szczawinski and Turner (1980), the authors speculate that they were possibly spread by Native Americans, perhaps because they used the spines as fish hooks. Should you change your research plans to try to answer this question? If so, how would you change it? If not, why not?
6. What does it mean that no fruits were produced this year? Does it mean the population never reproduces sexually? What should you plan to do?
7. What should you do in the second year of research? Decide what your next research plan is.

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