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

Part II—Your First Year Plans

One of the most fundamental questions population ecologists want to answer is: How big is the population? You decide to answer this question for the population of Opuntia fragilis in the new quarry park. You want to know how many different plants there are as well as how many individual pads there are. You plan to survey the entire rock outcrop in the park, building a map of the locations of prickly pear, the rock outcrop segments, trees, and grassy vegetation. You will record the location of each individual and count the number of pads that the individual has produced.

A second question population ecologists want to have answered is: How is the population structured? This question involves answering several different questions, including: How many individuals are there of each sex? How big are they? How old are they? For your prickly pear, the questions are a bit simpler, because you have no way to determine how old an individual is and because prickly pears produce perfect flowers, with both male and female parts, which means that you will not have to keep track of separate sexes. Thus, you will be able to explore the population structure question by figuring out how big the plants are, which you can tell by counting the number of pads an individual has.

A third fundamental question in population ecology is: How much does the population change from year to year? You know that the number of individuals next year will be the number of individuals present this year, plus the number of new individuals produced, minus the number of individuals that die. Because the population is isolated, you don’t have to worry about emigration and immigration. This question will take at least two years’ worth of field work to answer. The first year you will need to gather initial population size data, and then develop a second year’s census to provide a basis for comparison. You know that you will need to mark and carefully map the location of individual plants so that you can return the following year to determine whether each individual is a new plant, has died, or how much it has grown in size.

You realize that each Opuntia fragilis pad could be thought of as a separate individual, because you know that the pads can break off the parent plant and form a new plant on their own. This method of reproduction is asexual, producing genetically identical individuals. Therefore, you hope to be able to calculate population growth both in terms of the number of individual plants and the number of individual pads.

In addition, you know that Opuntia fragilis also can reproduce sexually by producing fruit. You decide that you want to watch for flowers, record how many flowers each plant produces, and revisit the site later in the year to determine which flowers have produced fruit. You will dissect some fruits to determine how many seeds each flower produces.

Finally, of course you plan to spend more time reading about this plant in the library, and you want to explore the region around the new park to see if there are other populations of Opuntia fragilis on nearby rock outcrops.

Questions

1. Did your research plan cover each of the three fundamental questions of population ecology?
2. One of the challenges of being a scientist is deciding which questions are the most important and keeping your project small enough that you can actually finish it. What else could you be doing that is not included in the research plan described above?
3. What kinds of tools and supplies will be needed? Who is going to pay for them?
4. Does it matter that buffalo used to be found in Stearns County but have been eliminated long ago by hunting? What other animals might be important to the ecology of prickly pear? Should your research plan consider these questions?

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