TOPIC 2 DAY 4 FACTORS AFFECTING POPULATION SIZE

A population of flat-tailed horned lizards was found in a geographically isolated area. In 2002, the size of the population was about 25,500. In 2004, the size of the population was about 32,800.

Which of the following statements explains the change in the size of the lizard population from 2002 to 2004?

• The birth rate was greater than the death rate.

• The emigration rate was greater than the birth rate.

• The death rate was greater than the immigration rate.

• The emigration rate was greater than the immigration rate.

Answer explanation

The population of flat-tailed horned lizards increased from 2002 to 2004. This means there was a net addition of lizards in the population. This could only happen if the birth rate was greater than the death rate. The other options pertaining to emigration and immigration do not explain the increase in the population.

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Scientists introduce ten male pheasants and thirty female pheasants to an island on which pheasants have not previously lived. The island has a natural food source and no predators of pheasants.

Which graph best predicts the number of pheasants on the island 50 years after their introduction to the island?

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Answer explanation

This graph is the best choice because initially the population of pheasant will increase exponnetially due to the lack of limiting factors. As the amount of resources is limited the growth of the population will slow down reaching the carrying capacity of the ecosystem resulting in a logistic growth.

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Which of the following phrases best describes line K in the graph representing the growth of a rabbit population in a forest named 'Emerald Woods' over a period of 5 years?

• The rate of population growth of the rabbits in 'Emerald Woods'

• The carrying capacity of 'Emerald Woods' for the rabbit population

• The time it takes for the rabbit population in 'Emerald Woods' to double in size

• The number of rabbits in the population at the beginning of the study in 'Emerald Woods'

Answer explanation

The correct choice for this question is 'The carrying capacity of 'Emerald Woods' for the rabbit population'. Line K in the graph represents the maximum number of individuals in a species that the environment can support—known as the carrying capacity—not the rate of growth, time to double, or initial population size.

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What might cause the position of the dashed line K to move from its current position to a lower point on the Y-axis in the graph above?

• The birthrate of the rabbit population was equal to the death rate of the rabbit population.

• There was a decrease in the number of rabbit predators and an increase in the availability of plants.

• There was a decrease in the availability of food, minerals, water, and shelter.

• The birthrate of the rabbit population was greater than the death rate of the rabbit population.

Answer explanation

The position of the dashed line K on the Y-axis in the graph would move to a lower point if there was a decrease in the availability of resources. This is because line K represents the rabbit population's carrying capacity, which is directly influenced by the availability of food, minerals, water, and shelter. When these resources decrease, the carrying capacity also decreases, causing the position of line K to drop.

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A partial food web is shown above.

Which of the following will most likely occur if the grouse population decreases?

• The butterfly population will increase.

• The red-tailed hawk population will increase.

• Emigration of grizzly bears and deer will increase.

• Competition between grizzly bears and deer will increase.

Answer explanation

The question asks what would happen if the grouse population decreases. The correct answer is that the butterfly population will increase. This is because, in the food web, grouse feed on butterflies. If the grouse population decreases, there would be less predation on the butterflies, allowing the butterfly population to grow.

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Cactus was introduced to Australia and quickly spread over millions of acres of land. A parasitic moth which prayed on the cactus was then introduced.  

Which of the following best describes the population growth of the cactus? 

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Answer explanation

This graph is the best choice because initially the population of pheasant will increase exponnetially due to the lack of limiting factors. As the amount of resources is limited the growth of the population will slow down reaching the carrying capacity of the ecosystem resulting in a logistic growth.

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SC.912.L.17.5