Esther Juárez

Survival of the Fittest

Competition: A look at the Struggle and Difficulty Surviving While Sharing the Environment


In this experiment two populations of bacteria Paramecium caudatum, and Paramecium aurelia were set under similar laboratory conditions to study how they adapt and survive in a controlled environment. The experiment lasted for 35 days. The paramecium was counted each week to determine the growth rate. Hypothesis: Two species that require the same set of resources cannot exist, competition results in one species driving the other to extinction (provided by instructor in lecture). The conclusion validated the expected outcome. One species of Paramecium ended up not adapting so well to the new conditions; therefore, decreased in population.


The objective of this experiment was to investigate the outcome of competition between two species of Paramecium under the same controlled laboratory conditions. In order to understand how this experiment works it was very important to understand some concepts.

How is Ecology Different from Populations and Community Ecology

A population is all the organisms that both belong to the same group of species and live in the same geographical area. Community ecology means that two or more populations occupy and share the same habitat (Campbell and Reece 9th Edition).

Interspecific competition

In ecology, interspecific competition is a form of competition in which the individuals of different species compete for the same living resources in an ecosystem like food or living space. Interspecific competition is an interaction that occurs when individuals of different species compete for a resource that limits their growth and survival (Campbell and Reece 9th Edition).

Abiotic and Biotic Factors

In ecology, abiotic factors are those non-living chemical and other physical factors in the environment that eventually, affect the ecosystems (Abiotic Component 2012). Considers temperature, water, oxygen, salinity, sunlight, or soil and that might be limiting a species’ distribution (Campbell and Reece 9th Ed).

Competition is the interaction between two or more species in which the existence of one affects or interferes with the existence of the other. “Competition is one of many interacting biotic and abiotic factors that affect community structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is interspecific competition. Competition is not always straightforward, and can occur in both a direct and indirect way” (Competition Biology).

This experiment was helpful in understanding how individuals of different species behave in a shared environment. This experiment was done representing a small scale, but it was very helpful because the outcome for a large-scale experiment would be very similar.

A disadvantage of this experiment is due to the controlled factors that mostly are not regularly controlled in nature.

What is the outcome of competition if they both share the same resources?

Even though it was expected for the species to be sharing and interacting together with the environment, a thought that came into this experiment was that one of them, the stronger in many aspects, would be dominating over the other.

Alternative hypotheses

According to the hypothesis made by the group, for this experiment it was hypothesized that both types of bacteria will survive (none of them going extinct or dying). They will be able to adapt to the presented environment. Another very interesting hypothesis was that one species would be predating on the other one, not knowing which one. My reasoning about this experiment was that the better, would make it, because they would be competing.

Observations: For this experiment, these two species of bacteria, were sharing the same conditions, hence, the same habitat. All factors were equal, which represented the abiotic components were shared by the two populations. Competition in this experiment is represented well since both species were being tested to see how well they adapt and interact. These types of experiments can help determine and compare in real life how species do to survive and the effect of dominant populations. (There is big disadvantage in performing these types of experiments, since there are a lot of other factors that affect the survival of one population). These populations were forced to share the same environment. In real life, even though species do share environments, thousands of years have shaped the way they behave, their characteristics and how they adapt; naturally. For the purpose of this experiment it was a good example of how to differentiate between coexistence of populations and how it affects survival.

Paramecium was a good resource for this experiment because of its size, their reproductive characteristics, which makes easy to manipulate the experiment. They together generate a useful tool in this field of study.

Population dynamicsPopulation dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes. Population dynamics deals with the way populations are affected by birth and death rates, and by immigration and emigration, and studies topics such as ageing populations or population decline.

In this experiment, the population dynamics taking place was represented by the ability of the species to reproduce, hence, survive and multiply.


The methods used for this experiments were very simple and easy to follow. Two populations of different bacteria species were set into 2 flasks (control groups) at densities of 10 cells per milliliter sharing the same conditions, temperature, light, food, living environment. One flask contained 10 specimens of Paramecium Aurelia (P. Aurelia) and the other flask 10 of Paramecium caudatum (P. Caudatum) per milliliter.

A different flask was set to hold the combination of both species (experimental group) with same conditions as the flasks before, and 5 specimens of each to make a total of 10 again.

The data for this experiment consisted in counting the number of individuals per milliliter every 7 days using a microscope. The counting lasted for 35 days and a data table was created with the 4 columns for PA, PC PA mix, PC mix were established to keep track of the counting. Counting techniques: Adding a 0 to the count, # cells per mL. Each of the species held in the mix flask was counted separately. The data for this experiment were being collected from different classes performing the same experiment. Since this experiment had to be done in a large scale, about 30 groups participated collecting data for statistics purposes being our assigned number #22. There was a benefit on combining the data collected from both groups because the results would be presented more accurately. It would have been impossible for each group to determine the exact amount of individuals in the populations so, average worked very well. Small numbers do not represent the actual number of individuals in a population.

After counting the Paramecium species and writing down the numbers on a clipboard, the Paramecium species were fed, and kept in the place designated.

It was noticed that from this results the outcome was very inaccurate and the results could not have been read from this graphs.

Results (Final)

The following data were obtained during 35 days. The numbers are the cells found per milliliter.Esther Juarez

From the final results on the first two graphs it is clear that both of the populations grew normally in size without any factors affecting. It was noticed that P. Aurelia grew faster than the P. Caudatum; therefore, the number of cells are very different in the two graphs.

On the other hand, for the third graph with the PC and PC Mix the results vary in many aspects. One of them is that the numbers of cells per milliliter are different from those of the normal growth. At some point the numbers in the first counting were sort of similar. After that, the P. Aurelia multiplied gaining size in their population and leaving behind the P. Caudatum.

After 30 days past the population of P. Aurelia starts decaying and then P. Caudatum ended up having more individuals in their population.

These two species of Paramecium share the same ecological necessities, better known as niche.

The sum of species’ use of the biotic and abiotic resources in its environment is called its ecological niche (Campbell and Reece 9th Ed).  When they are put together to coexist, naturally one will lead the place, and eventually will survive better than the other. In this case, Paramecium aurelia has one advantage; it grows at a fast rate. As represented in the table during the first days. When all factors are the same, then Paramecium aurelia starts dominating the space, leaving behind Paramecium caudatum. Because both species look for the same environment to exist, they cannot be together. They need to have difference environmental necessities, in order for them to adapt well to the environment and surroundings. “Two species cannot coexist in the same habitat if they have the same niche”  (Ecology).

When this experiment is compared to the real Gause’s experiment it is noticed that similar results were obtained. Our results differ in the time elapsed during this experiment. There is a special pattern found in the curves from this experiment. The curves in the mix cultures represent the competition that the mixed species faced while trying to survive in the flasks. Contrary to how they normally grown by themselves being part of the same population.

Another aspect that could have affected the way the Mix graph looked the way it did, was that, P. caudatum adapted well to the simulated ecosystem and finding their way for a better survival.

The hypothesis for this experiment was completely different from the results and therefore rejected. In the hypothesis was expected for both populations to share and adapt to the environment perfectly. Based on the results P. caudatum took better advantage and adapted better than P. aurelia. In comparison to the Gause’s experiment P. Aurelia is mentioned to dominate over all only if the conditions are kept constant. This experiment shows the opposite. The possible explanation is the time where the cells were tested.

There are other possible wrong outcomes while performing this experiment. Human experimental error is the most important factor when determining the results. It was impossible to know exactly how many cells of each population were present; however, representing that number from this experiment was enough just to grasp the whole concept of competitive exclusion.

Also, counting techniques differed from student to others. Identifying the types of right cells, since both may look the same for a person it was also quite important. If one group did not do something, like feeding the cells on time, swirling the flasks in the counting, or any other important factor for a successful and accurate approach, then the results were affected by that. It was clear that the mixed flask contained competitive populations because the densities of the cells were very low compared to the normal ones. In the mix flask, the populations competed for survival; they had less amount individuals/mL than in normal conditions. It was clear that species struggled for survival, and that led them to decrease in size.

What are implications for evolutions of natural populations and the resulting community structure/diversity? Adaptation is one of the relevant things to take into account. The species that adapted better increased the chance of survival. Those communities that do not have/ share the same characteristics have also better chance since they do not represent competition for each other.

What might we expect to see in nature, given what happened in this Paramecium experiment?

Nature might be expected to behave the same way, species no matter what kind, coexist in the ecosystem. If one represents or threatens the other then, that dominant species will lead in the ecosystem. Better adaptation represents better chance of survival. If species are competing against each other because they both exist under similar conditions, the one that adapts the best will survive.

Competitive exclusion

A slight reproductive advantage will eventually lead to local elimination of the inferior competitor (Campbell and Reece 9th Edition).

An example of competitive exclusion in nature, will be the experiment that showed competition between two barnacle species that kept one of them occupying part of its fundamental niche. The ecologist Joseph Connell studied two barnacle species (Chthamalus stellatus and Balanus balanoides) Chthamalus is usually found higher on the rocks than Balanus. To determine whether the distribution of Chthamalus is the result of interspecific competition with Balanus  from the rocks several sites. The realized niche of Chthamalus much smaller than its fundamental niche (Campbell and Reece 9th Ed. 1195).

“Abiotic Component.” Wikipedia. Wikimedia Foundation, 04 Sept. 2012. Web. 09 Apr.
Campbell, Nail A., Reece, Jain B. 2008. Campbell Biology. 9th ed. San Francisco (CA):
Pearson/Benjamin Cummings. 1946-2004. 1195 p. 9 Apr. 2012. Print.
“Competition (biology).” Wikipedia. Wikimedia Foundation, 04 Sept. 2012. Web. 10 Apr. 2012
“Ecology [c].” Upload & Share PowerPoint Presentations and Documents. Web.
“Master Frameset.” BiologyMad A-Level Biology. Web. 09 Apr. 2012.
“The Struggle for Existence by G. F. Gause.” Gause’s Russian Intelligentsia: Science, Arts, Poetry, Medicine. Web. 10 Apr. 2012.

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