Lab Experiment – Competition in Protest Populations

Background: Competition is a fundamental biotic interaction between individuals of the same species (intraspecific competition) or different species (interspecific competition) in which each participant is harmed by the shared use of a resource that limits their ability to survive, grow, or reproduce. In general (see Levin 2009, Cain et al. 2014, or another general Ecology textbook for more detail): - competing organisms reduce the availability of resources; - competition can increase in intensity when resources are scarce; - competition is often asymmetrical (one individual or species is harmed more than the other); - competitors that use limiting resources in the same way cannot coexist indefinitely but may coexist if they use resources differently.
Consider the impact of limited resources on single-species cultures of Paramecium as well as mixed cultures. Over the next several weeks you will perform an experiment similar to the one shown above. Your responses to the questions above will help you form a hypothesis and predictions for your own work. Experimental Procedure: We will use two species for our class experiment: Paramecium caudatum and Blepharisma spp. Both are large, ciliated, heterotrophic protozoans. You can learn more about each at the links below.
Paramecium – eol.org/pages/484358/overview
Blepharisma – eol.org/pages/62044/overview
The protists will be grown within 75 cm2 tissue culture flasks. Boiled wheat seeds or protist pellets will serve as the food resource, for those cultures that receive food. There will be six experimental treatments that will be maintained over the three weeks of the experiment: 1.) Paramecium grown in single-species culture; unlimited resources 2.) Paramecium grown in single-species culture; limited resources 3.) Blepharisma grown in single-species culture; unlimited resources 4.) Blepharisma grown in single-species culture; limited resources 5.) Paramecium and Blepharisma grown in mixed-species culture; unlimited resources 6.) Paramecium and Blepharisma grown in mixed-species culture; limited resources. Data collection will be a collaborative effort. There will be six groups of students and each group will be responsible for one experimental treatment. At each class meeting during the three weeks of the experiment, and once per week outside of class, each group will make population estimates of the protists in their culture. Therefore, each culture will be counted twice per week. This will allow us to more accurately quantify population size for each species within each culture. Counting: Counting protists will be done using scanning power of the compound light microscope. The procedure is as follows: - Place two drops of Protoslo on a depression slide (Note: each group will be assigned three depression slides. Do not discard them. Clean them after each count, keep them in your lab drawer for the duration of the project, and return them at the end of the experiment.) - Stir culture and draw 50 μl fluid using a micropipetor with a clean pipette tip; - Place the fluid into the ProtoSlow of the depression slide and allow two minutes for protists to settle; - Scan the slide by moving across and down the drop of fluid tallying each individual as you proceed until you have counted the entire sample (see Fig. 2); - Repeat this procedure three times with a new sample of culture and average the population count of the three attempts. - Scale your count up to get an estimate of total population size in your culture. We will record population by volume using # of individuals per ml. (50 μl x 20 = 1000 μl = 1 ml) After each count, data will be entered into a common Google Docs datasheet, the link for which will be shared via Blackboard. Figure 2. Procedure for scanning samples of protozoan cultures. The large circle represents the volume of water within the depression slide. Continue slowly across and down the sample until all areas on the slide have been viewed and each individual Paramecium or Blepharisma have been counted. Consider: What are the possibilities for the experiment? If the two species are strong competitors, what Results, and Discussion section. More detail on lab report sections will be provided in a separate document. (See also Parts I and II at www.southernfriedscience.com/?p=18236).