Ecology

Geology/Geography/Oceanography/Atmospheric Sciences

Quiz 13 :

Competition

Quiz 13 :

Competition

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Design a greenhouse (glasshouse) experiment to test for intraspecific competition within a population of herbivorous plants. Specify the species of plant, the volume (or size of pot) and source of soil, the potentially limiting resource you will focus on (e.g., Tilman and Cowan [1989] studied competition for nitrogen) and how you will manipulate it, and the measures of plant performance you will make.
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Intraspecific competition is a type of interaction in which the members of same species compete for the same resource. The resource may be nutrients, light, water, or air. In order to study the intraspecific competition, the following experiment can be conducted.
Plant to be studied in the experiment for intraspecific competition : Tiger lily ( Lilium columbianum )
Nutrient or resource to be kept as the limiting factor for competition : "Potassium." The optimal amount in soil for proper growth of plants is 20,000ppm.
Description about the plants :
• It is herbaceous in nature.
• It is a general inhabitant of the sunny areas.
• Possess well-spaced orange colored flowers
• The moderate height of the plant is about 2 to 4 feet.
• The flowers face downwards
Requirements of the experiment :
• Green house: This can be built up in an area that has abundance of sunlight. It can be covered by transparent fiber sheet or glass in such a way that ample amount of sunlight penetrates.
• Pots: A total of 300 pots filled with soil that vary in concentration of potassium as follows:
Sample A: Concentration of potassium is kept at 20,000 ppm. This is the optimal concentration.
Sample B: Concentration of potassium is kept at 10,000 ppm. This is the medium concentration.
Sample C: Concentration of potassium is kept at 5,000 ppm. This is the lowest concentration.
• Plants: Around 2700 seeds have to be planted in all.
Experiment:
1. The 30 pots of sample A that is potassium concentration of 20,000ppm have to be divided into two sets:
Sample A set 1: 15 pots have to be planted with 10 seeds per pot.
Sample A set 2: 15 pots have to be planted with 50 seeds per pot.
2. The 30 pots of sample B that is potassium concentration of 10,000ppm have to be divided into two sets:
Sample B set 1: 15 pots have to be planted with 10 seeds per pot.
Sample B set 2: 15 pots have to be planted with 50 seeds per pot.
3. The 30 pots of sample C that is potassium concentration of 5,000ppm have to be divided into two sets:
Sample C set 1: 15 pots have to be planted with 10 seeds per pot.
Sample C set 2: 15 pots have to be planted with 50 seeds per pot.
4. The seedlings and plants have to be incubated for 4-5 months till the average height is gained in the normal conditions.
5. The other sources like water, oxygen, and light must be kept in normal concentration in the green house chamber.
6. The pots must be regularly checked for the presence of any other plants or weeds.
Conclusion :
As intraspecific competition is a natural phenomenon in such a case as described above, the following observations would be seen:
• Sample A : In the pots of 20,000ppm concentration of potassium, the plants in set 1 would grow at their best. They would attain maximum height, produce maximum flowers, and would show healthy leaves.
However, the set 2 would have lesser height, lesser number of healthy plants and reduced number of flowers.
• Sample B : In pots of 10,000ppm concentration of potassium, the plants in set 1 would grow moderate height and medium number of flowers. However the plants in set 2 would have much lesser number of flowers and the plants would be much shorter in height. Due to the scarcity of potassium, the ends of leaf can show brown coloration.
• Sample C : In pots with 5,000ppm concentration of potassium, the plants in both the sets would be reduced in number and size. However, the plants in set 1 would be better than set 2.
The plants that grow at a higher density face a higher amount of intraspecific interactions and therefore show reduced growth than plants in lower density. Since potassium is an essential macroelement for plants, the pots that have reduced concentration of potassium show higher level of competition for the nutrient.
With this experiment, it can be well demonstrated that intraspecific completion occurs in nature and is well prominent in the case where the density of plants is more.

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Researchers have characterized the niches of Galápagos finches by beak size (which correlates with diet) and the niches of salt marsh grasses by position in the intertidal zone. How would you characterize the niches of sympatric canid species such as red fox, coyote, and wolf in North America? Or felids such as ocelots, pumas, and jaguars in South America? What characteristics or environmental features do you think would be useful for representing the niches of desert plants? Or the plants in temperate forest or prairie?
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The coexistence of the sympatric candid species in the remote areas of North America can only be explained by the overlapping niches. The requirement of each of these species differs from each other. The difference is up to such an extent that their niches are well defined and are not similar. Thus, the overlapping supports the coexistence of the species here.
The niches of red fox, coyote, and wolf in North America vary in the following ways:
• Dietary habits : The red foxed prefer to prey upon the kangaroo rats and smaller insects whereas coyotes preferred to prey upon the rabbits, birds and vegetation. The prey for wolfs is larger animals such as deer, moose and wild boar. Their preferred prey is entirely different. Thus, the niche is different.
• Habitat : The red foxes prefer to stay in the shrub steppe habitats where as coyotes prefer grasslands and the wolves prefer the mainland. Their habitat differs and thus, they can coexist with overlapping niches.
However, in the winter seasons, it is seen that coyotes compete with wolves for their prey. The dominant competitor amongst them is the wolf.
The overlapping niches help in coexistence of the ocelots, pumas, and jaguars in the South America. Their preying habits vary so well defined that they do not compete for the same food source in majority of the cases. The pumas prefer to feed on the large rodents, porcupine and hares. On the other hand, ocelots prey upon the deer, fishes, frogs, and lizards. On contrary the coyotes feed mainly on voles, prairie dogs, squirrels, birds, and large insects.
Main characteristics of niches of desert plants are as follows:
• The rainfall and moisture content in air is very low. To manage this environment, the desert plants are adapted to have less transpiration and more radiation of heat to maintain the temperature.
• The plants are succulent in nature. This enables them to absorb large amount of water and lose less amount of water.
• Leaves are modified into spines. This reduces transpiration.
• The plants are glossy and spiny, this reflects more radiant energy.
• The plants perform CAM photosynthesis in which the process of stomatal opening occurs at night to increase moisture absorption.
• The CAM plants open less stoma and absorb more carbon dioxide but lose less amount of water.
• Lengthy roots are present to absorb water from deep water tables.
Main characteristics of plants niches of temperate forests or prairie are as follows:
• High moisture content in the atmosphere.
• More transpiration and less radiation
• Leaves can be both of thick and thin cuticles but they are capable of increased rate of photosynthesis.
• Presence of C3 plants. These are plants which need high water content and high concentration of carbon dioxide for photosynthesis.
• Roots are not very long as water is present in the top most layers of the soil for use.

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Why did Brown's research team (see p. 300) create controls by completely fencing study plots and then cutting holes in their sides to allow free passage of rodents into and out of the plot? Why not just compare the density of small rodents in the large granivore removal plots with their densities in the surrounding desert?
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The mouse proof fences created around the study plots by Brown and his colleagues was an attempt to establish a control of the condition which needed to be studied. A field experiment leads to the most reliable results if any parameter being studied is well controlled. To understand the effect of the parameters, it is important to note the effect on the study area before and after a change is being induced.
The complete fencing around the study plots that was created by the researchers ensured the absence of any movement of the rodents to any area apart from the study plot. The researchers observed the parameters at regular interval and concluded the results on the base of this controlled condition. Here, the population density of the species was being studied. This was related to the completion.
Later the cuts were made that were 6.5cm in diameter. This allowed the free movement of the large granivores in and out of the study area. The effect of this situation was studied by the researchers in the regular intervals of time.
The study gave a complete idea about the effect of both the conditions on the population. The conditions were controlled. Therefore, the interference of external disturbances was avoided.
The comparison between the study of density of small rodents in the large granivore removal plots with that of their population density in surrounding desserts would be less reliable because of the following reasons:-
• The conditions in the surrounding desserts would not be controlled as effectively as that of the study areas. Hence, the comparison wouldn't be reliable.
• Along with the small and large granivores, there would be other organisms inhabiting the dessert areas. They would influence the population of each other. Hence, the population of the smaller granivore wouldn't be solely influenced by their larger competitor as in the study area.
• The conditions of the desserts wouldn't be fully known. Hence, the response to removal of large granivores cannot be correlated to a similar response in a non comparable different environment like a desert.

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How was the amount of food that Gause (1934) provided in his experiment on competition among paramecia related to carrying capacity? In Gause's experiments on competition, P. Aurelia excluded P. caudatum faster when he provided half the amount of food than when he doubled the amount of food. Explain.
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Draw the four possible ways in which Lotka's (1932a) isoclines of zero growth (see fig. 13.14) can be oriented with respect to each other. Label the axes and the points where the isoclines intersect the horizontal and vertical axes. Explain how each situation represented by the graphs leads to either competitive exclusion of one species or the other or to stable or unstable coexistence.
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How can the results of greenhouse experiments on competition help us understand the importance of competition among natural populations? How can a researcher enhance the correspondence of results between greenhouse experiments and the field situation?
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Explain why species that overlap a great deal in their fundamental niches have a high probability of competing. Now explain why species that overlap a great deal in their realized niches and live in the same area probably do not compete significantly.
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One of the conclusions that seems justified in light of several decades of studies of interspecific competition is that competition is a common and strong force operating in nature, but not always and not everywhere. List the environmental circumstances in which you think intraspecific and interspecific competition would be most likely to occur in nature. In what circumstances do you think competition is least likely to occur? How would you go about testing your ideas?
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Discuss how mathematical theory, laboratory models, and field experiments have contributed to our understanding of the ecology of competition. List the advantages and disadvantages of each approach.
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Explain how self-thinning in field populations of plants can be used to support the hypothesis that intraspecific competition is a common occurrence among natural plant populations.
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In his experiments on competition between T. confusum and T. castaneum, Park (1954) found that one species usually excluded the other species but that the outcome depended upon physical conditions. In which circumstances did T. confusum have the competitive advantage? In which circumstances did T. castaneum have the competitive advantage? Could Park predict the outcomes of these experiments with complete certainty? What does this suggest about competition in nature?
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