Explain why populations tend not to continue to increase exponentially in an environment: 2. Explain what is meant by environmental resistance: 3. (a) Explain what is meant by carrying capacity: (b) Explain the importance of carrying capacity to the growth and maintenance of population numbers: 4. Species that expand into a new area, such as rabbits did in areas of Australia, typically show a period of rapid population growth followed by a slowing of population growth as density dependent factors become more important and the population settles around a level that can be supported by the carrying capacity of the environment. (a) Explain why a newly introduced consumer (e. g. rabbit) would initially exhibit a period of exponential population growth: (b) Describe a likely outcome for a rabbit population after the initial rapid increase had slowed: 5. Describe the effect that introduced grazing species might have on the carrying capacity

Answer and Explanation

Explain why populations tend not to continue to increase exponentially in an environment: The exponential growth model is based on unlimited resource availability which means that there is no effect depending on density. There is no competition for resources. Natality and mortality rate do not depend on density. There is a constant growth rate per capita and it is proportional to the population size. A population that exhibits exponential growth model increases in proportion to its size. In the Logistic growth model, the population growth depends on density, the natality and mortality rate depends on the population size, which means that there is no independence between population growth and population density.

When a population grows in a limited space, density rises gradually and eventually affects the multiplication rate. The population's per capita growth rate decreases as population size increases. The population reaches a maximum point delimited by available resources, such as food or space. This point is known as the carrying capacity, K.

Explain what is meant by environmental resistance: It refers to the limiting factors. There are factors and conditions that regulate the population growth, that might be biotic or abiotic, and that avoid the over-growth of a population in a certain space. It results in a natural equilibrium of its biotic potential.  

Explain what is meant by carrying capacity: The carrying capacity K is the maximum point delimited by available resources, such as food or space.

K is a constant that coincides with the size of the population at the equilibrium point when the natality rate and the mortality rate get qual to each other.

Explain the importance of carrying capacity to the growth and maintenance of population numbers: If the population size, N, is inferior to K (N<K) the population can still grow. When N approximates to K, the population´s growth speed decreases. When N=K, the population reaches equilibrium, and when N is superior to K (N>K) the population must decrease in size because there are not enough resources to maintain that size.  

Explain why a newly introduced consumer (e.g. rabbit) would initially exhibit a period of exponential population growth: The example of the rabbit is a case of invasion. Biologic Invasions refer to new species that disperse and establish in a new area far or out of their original distribution range. Once established, these species expand their distribution from the first invaded spot and overgrows. Once stablished, they expand. In the new area, they have less environmental pressure and better conditions than in their origin area -fewer predators, more resources, better nitches-, and these factors favor their overgrowth and consequently uncontrolled expansion. At this point, the population is exhibiting exponential growth. They have enough food available, they might not have predators, they do not have enough competitors for food or space, and if they do they are adapted to live under harder conditions so they are able to compete. The new species is not suffering from the effects of limiting factors yet.  

Describe a likely outcome for a rabbit population after the initial rapid increase had slowed: There are some typical steps in an invasion process:

1) Introduction or dispersion to the new area,

2) Naturalization. The new species establishes in the new area. It can grow, reproduce and make use of resources.

3) Overgrowth and uncontrolled expansion.  Exponential growth

4) Interaction with other species.  At this point, they compete for limiting factors such as food or space. Their population is big enough to be affected by resource availability and by other species that interact with them. The invasive species reach the point in the curve where they stabilize.

5) Stabilization. They reach an equilibrium point.  

Describe the effect that introduced grazing species might have on the carrying capacity. The introduced species expand fast and consume too many resources. Available resources, such as food or space, decrease.

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