A Study On Population Biology

Martha Eliza Lauderdale A Study on Population Biology INTRODUCTION In 1934, Russian microbiologist Georgyi Gause studied competition between three protist species: Paramecium Aurelia, Paremecium bursaria, and Paramecium caudatum (Brooker et al.,2008). The three species all fed on bacteria and yeast; the bacteria and yeast fed on an oatmeal medium in a culture tube in the laboratory (Brooker et al.,2008). The bacteria was found more in the oxygen-rich, top part of the tube, where as the yeast mainly occupied the bottom, oxygen-poor portion of the tube (Brooker et al.,2008). Since each species varied in size, Gause calculated population growth by using the number of individuals per milliliter of solution multiplied by their unit volume to give a population volume for each of the three species (Brooker et al.,2008). When grown in separate tubes, population volume of all three Paramecium species followed a logistic growth pattern, in which the growth of a population slows as it approaches its carrying capacity (Brooker et al.,2008). When Gause cultured the P. caudatum and P. aurelia together in the same tube, the P. caudatum went extinct; both species relied on bacteria for food, however, P.aurelia had a much faster growth rate and could better convert the food into offspring (Brooker et al.,2008). When Gause put the P. caudatum and P. bursaria together, neither species went extinct; however, due to some competition, the population volumes of each was much less than when grown alone (Brooker et al.,2008). From Gause’s experiments, came forth the competitive exclusion hypothesis, which proposes that two species with the same resource requirements cannot occupy the same niche (Brooker et al.,2008). A similar experiment to Gause’s was conducted using only P. aurelia and P. caudatum. The objectives of the experiment include demonstrating how competition for natural resources in the environment can affect population growth, as well as how the availability of resources, such as food, can be limiting for populations. Based on the information provided by Gause’s experiments and the competitive exclusion hypothesis, I hypothesized that the P.aurelia would indeed outgrow the P. caudatum when grown together, forcing P. caudatum to extinction. METHODS

The experiment was conducted in the laboratory. In the laboratory, two flasks containing the stock cultures of P. caudatum and P. aurelia, , each with 4 cells/mL, along with three test tubes with rice at the bottom , a microscope and microscope slides, and a calendar to manage time were used. The rice provided food for the bacteria; while, the bacteria served as food for the Paramecium. First, a pipette was filled with 5 mL of the stock culture of P.caudatum and distributed into Test Tube 1. The pipette was then filled with 5 mL of P.aurelia and distributed into Test Tube 2. Test Tube 3 was filled with 5m:L of P.aurelia and 5 mL of P. caudatum. Next, wet mounts of the samples in test tubes were made. In this process, three clean microscope slides were placed on the laboratory bench. The well in the microscope slide holds 0.5 mL; thus, 0.5 mL of sample from each test tube was dispensed separately onto each microscope slide. Each slide was placed under the microscope, the grid on the microscope was turned on, and the number of individuals found on the slide were counted and recorded in a table. The number of individuals was multiplied by two in order to obtain the concentration per mL. In Test Tube 3, which contained both P. caudatum and P. aurelia, the number of each species present was counted separately, multiplied by two, and recorded. The samples were left in the test tubes and reobserved under the microscope on clean slides at 2, 4, 6, 8, 10, 12, 14, and 16 days respectively. At each observance, the number of individuals of each species was counted, multiplied by two, and recorded in the table. RESULTS The population of P. caudatum in Test Tube 1 began with 2 cells/mL. The P. caudatum population grew rapidly until Day 10, when it reached its carrying capacity of about 60 individuals. The population of P. aurelia in Test Tube 2 also began with 2 cells/mL. This population grew more rapidly than the P. caudatum, reaching its carrying capacity at Day 8, with about 98 individuals present. Test Tube 3 consisted of 2 cells/mL of P. caudatum and 2 cells/mL of P. aurelia at the beginning of the experiment. The P. caudatum in Test Tube 3 experienced slow growth until Day 6, when the growth rate began to decline, and then finally reached extinction on Day 14. The P. aurelia in Test Tube 3, on the other hand, experienced rapid growth and began to reach carrying capacity around days 12-16. The data specifics are displayed below in Figures 1 and 2. Growth of P. caudatum and P. aurelia in Test Tubes 1, 2, and 3

Figure 1: # of P. caudatum, Test Tube 1 2 cells/mL 10 cells/mL 26 cells/mL 48 cells/mL 56 cells/mL 60 cells/mL 58 cells/mL 62 cells/mL 58 cells/mL

Day 0 2 4 6 8 10 12 14 16

# of P. aurelia, Test Tube 2 2 cells/mL 10 cells/mL 52 cells/mL 84 cells/mL 98 cells/mL 98 cells/mL 98 cells/mL 98 cells/mL 98 cells/mL

# of P. caudatum, Test Tube 3 2 cells/mL 10 cells/mL 22 cells/mL 18 cells/mL 16 cells/mL 10 cells/mL 4 cells/mL 0 cells/mL 0 cells/mL

# of P. aurelia, Test Tube 3 2 cells/mL 6 cells/mL 34 cells/mL 66 cells/mL 82 cells/mL 92 cells/mL 94 cells/mL 100 cells/mL 96 cells/mL

Figure 2:

Discussion and Conclusion: As seen in the table and graph above, the P.caudatum and P. aurelia cultured separately experience exponential growth until their carrying capacities are reached. Exponential growth is an explosive growth in which the total number of potentially reproducing organisms increases with each generation until limitations on resources cause the population to stop increasing (Brooker et al., 2008). When a population size becomes stable, its carrying capacity has been reached (Brooker et al., 2008). Intraspecific competition is present among the P. caudatum in Test Tube 1 and among the P. aurelia in Test Tube 2 and places limit on the population growth of each. Intraspecific competition is defined as competition between individuals of the same species (Brooker et al., 2008). Interspefic competition, defined as competition between individuals of different species, was present between the P. caudatum and P. aurelia in Test Tube 3 of the experiment, resulting in the extinction of P. caudatum in the sample. (Brooker et al.,2008). Competition occurs between species with similar needs and habits when the population size becomes too large and resources become too scarce (Crombie, 1947). When competition is present between species whose external conditions remain constant, it is said that two species with the same needs and habits cannot survive in the same place due to limiting resources (Crombie, 1947). The conditions for interspecific competition were present in the

experiement in Test Tube 3. Superiority of one species over another depends on that species adaptation to the environment, including reproduction and survival rates, as well as interspecific interference, such as consuming the same type of food (Crombie, 1947 pg 49). As previously stated, both species use bacteria for food, but P. aurelia grows at a rate six times faster than P. caudatum, and can also better convert the food it consumes into offspring (Brooker et al., 2008). “P. aurelia has two adavantages over P. caudatum: the rate of geometric increase of the biomass of the former species is higher and it accumulates a greater biomass than P. caudatum at the expense of the same amount of food (Crombie, 1947 pg 52).” As seen in the experiement, initially the two species both increased in number; however, as the population grew, it became too large for the food source to support, leading to the extinction of P. caudatum (Crombie, 1947 pg 52). According to the results of the experiment, my hypothesis that the P. aurelia would outgrow the P. caudatum, forcing it to extinction, proved correct due to the presence of intraspecific competiton between the two species. The hypothesis could be tested further by adding or changing some of the conditions of the experiment, such as the environment or the food source. Also, another Paramecium species, such as P. bursaria (as Gause conducted) could have been added to the experiement to further investigate competitive factors. LITERATURE CITED Brooker, R.J., Widmaier, E.P., Graham, L.E., and Stiling, P.D. 2008. Biology. 56: 1207-1208, 57: 1220, 1223-1224. Crombie, A.C. 1947. Interspecific Competition. Journal of Animal Ecology Volume 16: 48-49, 52.