VIRTUAL LABS
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Population Biology
How does competition affect population growth?

Purpose
In this investigation you will conduct an experiment and grow two species of the protozoan Paramecium, alone and together. You will then compare growth curves of the populations of each species.

Objectives:

  • Demonstrate how competition for natural resources in the environment can affect population growth.

  • Explain how availability of resources, such as food, can be limiting for populations.

Procedure:

1. Click Information to read about Paramecium, population growth, and competition.

2. Begin the experiment by filling the test tubes with samples from the stock cultures in the flasks. Click the bulb at the top of the pipette to fill the pipette with culture. Then click and drag the pipette to a test tube. Fill the three test tubes with 10 mL samples of Paramecium aurelia, Paramecium caudatum, and/or a combination of both. Note: There is rice in the test tubes. The rice is food for bacteria, which in turn will be food for the Paramecium. The two species of Paramecium do not prey upon each other.

3. Click the Journal button to open the Journal and answer the first Analysis question.

4. Click the Microscope on the back shelf to go to the lab bench. Then you will make wet mounts of the samples.

5. Click the Clean microscope slides box to set up clean microscope slides.

6. Click the test tubes to prepare wet-mount slides of the samples.

7. Click and drag a wet mount to the stage of the microscope. Count or estimate the number of cells of each type of Paramecium. Click the Grid On button for help with counting.

8. Click the Table button to record your data. Note: The well in the microscope slide holds 0.5 mL. You need to double the number of cells you counted or estimated in order to obtain the concentration per mL.

9. Click the Clear Slides button. Click the Calendar to advance it by two days. Then get a new set of clean slides, place samples on them, and count or estimate the number of Paramecium you see. Record your data in the Table.

10. Open the Journal and answer the remaining Analysis Questions.

Information
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How does competition affect population growth?

The genus Paramecium consists of unicellular species of protists that live in freshwater environments. Under ideal conditions—sufficient food, water, and space—populations of these species grow rapidly and follow a pattern known as exponential growth. Exponential growth is explosive population growth in which the total number of potentially reproducing organisms increases with each generation. However, populations of organisms will not increase in size forever. Eventually, limitations on food, water, and other resources will cause the population to stop increasing.

When a population arrives at the point where its size remains stable, it has reached the carrying capacity of the environment. The carrying capacity is the greatest number of individuals a given environment can sustain. Competition for resources among members of a population (intraspecific competition) places limits on population size.

Competition for resources among members of two or more different species (interspecific competition) also affects population size. In a classic series of experiments in the 1930s, a Russian ecologist, G.F. Gause, formulated his principal of competitive exclusion. This principle states that if two species are competing for the same resource, the species with a more rapid growth rate will outcompete the other. In other words, no two species can occupy the same niche.

In competing populations of organisms, genetic variations that reduce competition are favored through natural selection. Suppose two species (A and B) compete for the same food source. Individuals of species A can also use another food source, which reduces the competition over the food source needed by species B. The individuals of species A that can use another food source survive because they do not have to compete with individuals of species B for that food. In nature, organisms frequently invade unoccupied habitats simply to avoid intense competition. Once the organism is in a new habitat, any variations that allow it to use the available resources will tend to be perpetuated through the population. In this way, the genetic makeup of the population may slowly change, and the species will become adapted to a new niche.