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This experiment tested the hypothesis that the pure algae populations of Ankistrodesmus and Chorella will show growth in the number of organisms, and the mixed population will establish a dominant species. Each population should also reach its carrying capacity. All of the populations had the same resources available.

The sampling of 0.5ml of each population was used to count the number of organisms. A Neubauer slide and microscope will be needed to do this. From this count, determine the number of organisms in 1ml. After doing this procedure for three weeks the organisms in the pure samples should have grown in size and reached a carrying capacity. The mixed population of the two algae species should show that one species is dominant over the other. The results of the experiment support this hypothesis.

Both the pure samples of Ankistrodesmus and Chorella increased dramatically in size and then reached their carrying capacity. The mixed population showed that Chorella is the dominant species over the Ankistrodesmus.


This experiment was conducted by using the green algae: Ankistrodesmus and Chorella. Most species of green algae live in shallow freshwater environments. The can live on moist rocks, trees, and soil; some can inhabit shallow ocean waters. Green algae usually occur as single cells or as multi-cellular, threadlike filaments, hollow balls, or flat sheets (Postlethwait and Hopson).

This experiment was used to study the growth of the algae and the effects of competition when grown in a limited resource. Competition is a contest between rivals for the same resource and habitat. The experiment was concerned with counting the number of algae in 1ml. There was a pure sample of both Ankistrodesmus and Chorella. There was also a population that contained a mixture of the two algae.

Two types of competition were observed. Inter-specific competition occurs within a population of different species. Intra-specific competition occurs within a population of the same species (Starr and Taggert). If a resource is abundant then there is no competition. Gauses Law states that one species will always have an advantage over the other when competing for the same resource (Marshall and Walton). The species that has the advantage over the other will establish itself as the dominant species.

During the experiment a carrying capacity will be observed for all populations of algae pure and mixed. After a period of rapid growth the population will start to level off. This leveling off marks the carrying capacity of the environment. Carrying capacity can be affected by several factors. Birth rate and immigration can increase the size of population. Death rate, natural disasters, and emigration can decrease the size of a population. The amount of available resources can also affect the carrying capacity (Marshall and Walton).

The biological question was the effect of growth of algae and the competitions in the populations. The justification of this experiment is to study how competition affects the different populations.

Methods and Materials

The independent variable tested was the time used for the observation, which covered three weeks. The dependent variable was the number of algae counted. There are three side-arm flasks containing the populations of algae. There is one population of Ankistrodesmus and one population of Chorella each in a separate side-arm flask. The third side-arm flask contains a mixture of equal parts of the two types of algae previously mentioned. The control group was the pure samples, while the experimental group was the mixed population (Marshall and Walton).

After the Neubauer slide and cover slip were sterilized with ninety five percent ethanol, the materials are to be placed on a clean paper towel. Using a sterile 1ml syringe, remove 0.5ml of the population from the side-arm flask. Samples were taken from the other two flasks when this population has been counted. Make sure to swirl the side-arm flasks before removing the population to insure that the population is evenly distributed (Marshall and Walton).

After removing the sample, place the tip of the needle into the groove of the Neubauer slide and inject a small portion under the cover slip so it fills the grid. The sample will move by capillary action to fill the groove. Then place the slide on the microscope stage and count the number of organisms in the grid. The count was taken from each of the two grids called Levy Counting Chambers (Marshall and Walton).

Add together the number of organisms in the two grids, this equals the number of organisms per 1 mm3. To calculate the number of organisms in 1 ml., multiply the number of organisms in 1mm3 by 1,000. This is because 1000 mm3 = 1 ml. (Marshall and Walton).

The same procedures were followed and the same materials were used for each of the other weeks. Data was collected and recorded on the tables.


The estimated number of species changed from week to week. The populations of the algae increased in size from Week 1 to Week 2. The pure population of Ankistrodesmus increased from 32,000 to 656,000 from Week 1 to Week 2. From Week 2 to Week 3 the Ankistrodesmus population decreased from 656,000 to 562,000. This showed an increase followed by a decrease or leveling off period.

The pure Chorella population showed about the same results as the pure Ankistrodesmus population did. The Chorella population increased rapidly from 702,000 in Week 1 to 1,071,000 in Week 2. The population decreased from 1,071,000 to 968,000 in Week 3. This population also reached its carrying capacity.

The mixture of the Ankistrodesmus and Chorella populations did not show the same results as the pure samples did. The Ankistrodesmus population slightly increased from 122,000 in Week 1 to 146,000 in Week 2. Instead of the population leveling off or decreasing slightly as the pure populations did, this population increased from 146,000 to 178,000 in Week 3. The Chorella population more than doubled from 241,000 in Week 1 to 517,000 in Week 2, and then increased again to 746,000 in Week 3. Even though the population kept increasing it seemed to be leveling of. The Ankistrodesmus population was the dominant species in the mixture.

Table 1 shows how the populations of the different samples changed from week to week.

Graph 1 shows how the populations of the pure samples of Ankistrodesmus and Chorella changed from Week 1 to Week 3.

Graph 2 shows how the Ankistrodesmus population established its dominance over the Chorella population.

Table 1: Summary of all Population Growth over 3 Weeks

Sample# per ml. During Week 1# per ml. During Week 2# per ml. During Week 3







This experiment evaluated competition among algae species. The following hypothesis was formed and tested. The pure algae populations Ankistrodesmus and Chorella will show growth of the population. The mixed population of the two species will compete and one species will become a dominant species.

In the pure samples of algae the competition was intra-specific. Each algae competed with one another for the space and light available (Starr and Taggart). In population H, the pure sample of Ankistrodesmus and in population G, the pure sample Chorella, there was a dramatic increase followed by a decrease or leveling off period. This confirmed that the population had reached their carrying capacity. The Ankistrodesmus population had less organisms than the Chorella population did at the end of the experiment. This could allow another hypothesis: The Chorella population will increase faster than the Ankistrodesmus population. A different experiment would be needed to test this hypothesis.

In the mixed population F, the Chorella population behaved about the same as it did in the pure sample. It increased quickly and then started to level off. However, the Ankistrodesmus population did not start to level off, it kept increasing. The Chorella population established itself as the dominant species in the mixture.

The pure populations of Ankistrodesmus and Chorella showed population growth. The mixed population showed which species would become the dominant species. A carrying capacity was also found in the population growth.

Some errors of the experiment could be that there was not an evenly distributed sample because the side-arm flask was not swirled. Estimating the population to a large number could also increase the error of the experiment.

In conclusion, competition is a part of everyday life. A species will compete with members of the same species as well as members of different species. Many factors can determine which particular type of organisms will have an advantage when competing. The number of the organisms that do survive and reproduce will reach a carrying capacity when the environment runs out of resources.


Marshall, Joseph A., and Walton, Cheryl L., 1999, Micro-sampling 1-4, Competition 1-3, Explorations in Biology.

Postlethwait, John H. and Hopson, Janet L., The Nature of Life, 3rd Edition, p. 481.

Starr, Cecie and Taggart, Ralph, 1998, 8th Edition, Population Ecology, Biology: The Unity and Diversity of Life, p. 793-803.

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