Transcript Population and Community structure Species: a group of

The relationship between species, populations
community and the ecosystem
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Carrying capacity is the maximum population size a certain
environment can support for an extended period of time, for a
population of a particular species.
Under ideal conditions, a population naturally increases until it
overshoots the carrying capacity. At this point, the environment
can no longer provide for the species, due to a number of
different environmental resistances, including food, crowding,
competition, etc. The population, due to lack of resources, will
begin to die out, allowing the environment to recover. As the
environment recovers, the species population is able to flourish
once more. This leads to a fluctuation between the prosperity of
the species and the prosperity of the environment (hence the
fluctuations in the graph).
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Carrying capacity
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If a population reaches
carrying capacity it can
remain stable or move
up and down (fluctuate).
If there is more rainfall
and more food available
the carrying capacity
increases and the
population will increase
until it reaches the new
carrying capacity before
it levels off again. If
there is habitat
destruction or a draught
the carrying capacity
decreases and the
population will decrease
until it reaches the new
carrying capacity and
levels off again.
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Carrying capacity of rabbits in a specific area
1. What does the blue
line represent? What
does the purple line
represent? What does
it mean when the
purple line rises above
the blue line?
2. Which of the
following situations
might cause the purple
line to decrease below
the blue line: abundant
food sources, lack of
competition, a young
population, or plentiful
roaming space?
3. Can you think of any
events that would cause
the purple line to stay
above the blue line
indefinitely?
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Question: Carrying capacity
1.1 Suggest THREE reasons why the growth form between the period 1920 and 1935 is as it is.
1.2 What is the growth phase called between 1910 and 1920?
(1)
1.3 During which year did the jackal enter the fenced area? Give a reason for your answer from
the information supplied.
(3)
1.4 Between 1940 and 1950 the springbok population increased again. Mention a possible reason
for this increase.
(2)
1.5 Mention FOUR other factors, besides the jackals, which could have caused the decline in the
springbok population between 1935 and 1940?
(4)
1.6 What method was most probably used to determine the size of the springbok population? (1)
1.7 Do you think the line representing the carrying capacity is accurate?
Give a reason for your answer.
(3)
1.8 The population between 1965 and 1975 appears to have stabilised. Suggest how the farmer
might be controlling the population.
(2)
1.9 What do you notice about the growth from 1915 – 1925 and 1940 – 1950?
(1)
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(20)
Density dependent and factors
Density dependent
Density independent
a density-dependent factor affects a
greater percentage of individuals in a
population as the number of individuals
increases; it will also affect each
individual more strongly. Population
growth declines because death rate
increases, birth rate decreases or both.
Resource limitation is one such density
dependent factor. A reduction in
available food often limits reproductive
output as each individual produces
fewer eggs or seeds. Health and
survivorship also decrease as crowding
results in smaller, less robust individuals.
Many predators concentrate on a
particular prey when its population
density is high, taking a greater
percentage than usual.
Density-independent factors affect the
same percentage of individuals
regardless of population size. Weather,
climate and natural disasters such as
freezes, seasonal changes, hurricanes
and fires are examples; the severity and
time of occurrence being the
determining factor on what number of
organisms is affected. In some natural
populations, these effects routinely
control population size before densitydependent factors become important
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Geometric growth form (J- curve)
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Logistic growth form (S-shaped curve)
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Question : growth patterns
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4. A certain fast growing unicellular micro-organism is cultivated in a sugar solution in a
closed test tube at 250 C. At regular intervals, samples were taken in order to calculate the
population size. The graph below was drawn from the data obtained. Study the graph and
answer the questions that follow.
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4.1 Which specific organism was most probably cultivated in the test tube?
(1)
4.2 Name the growth phases indicated by A, B and C respectively.
(3)
4.3 Give an explanation for the specific growth pattern of each of the phases A,
B and C.
(6)
4.4 What is phase Y called and what is the possible cause, thereof, for this specific
population in particular?
(3)
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Comparison between geometric (J-curve) and Logistic (S-curve)
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