POPULATIONS SHEET ANSWER KEY
1.
What is a population? A group of organisms of the same species living together in a specific area
2.
What is demography? The study of populations
3.
List and describe the 3 key features of a population. Size (the number of individuals), density (their
location with the community), and dispersion (the way they are arranged in their own habitat)
4.
Why are very small populations likely to become extinct? Small populations are more likely to be
affected by any changes. An increased death rate by any means has a greater impact because of lower birth
rates, which probably is what kept the population small in the first place.
5.
Why do very low density populations tend not to grow? Contact between individuals is necessary if
mating is to occur, and in low density populations, the individuals are not located too near the others; they
need to actively seek out a mating partner and travel farther to do it, unless random chance brings them into
contact with each other.
6.
What are 3 types of dispersion? Which type is the most common in nature? Random, even, and
clumped. Random dispersion usually occurs when lone individuals range over a wide territory (like
wolverines or badgers). Evenly spaced organisms have a regimented space between other individuals (rare in
nature…think of birds sitting on a phone wire), and clumped, where individuals remain in proximity to others
in the populations (think of human developments). Clumped dispersion is most common in nature
7.
Describe the stage I model of population growth. Show the formula for stage I. The stage I model
accounts only for the rate of growth in a population (how fast new individuals are being added). r is the
variable used to represent growth rate: r = birth rate- death rate
8.
Describe the stage II model of population growth. What assumption must be made when using this
model? ΔN=rN, where ΔN represents the change in number of individuals in the population, r is the
rate of growth, and N is the number of individuals in the population being studied, at the start. The
assumption that must be made is that the birth rate (b) and the death rate (d) are constant and do not
change.
9.
Is the stage II model a realistic model for populations? Use the bacteria example to help answer this
question. No, it is not completely realistic. Unsustained growth cannot be maintained because of the
resources available (space, food, etc.) If you culture bacteria in a test tube, and it reproduces ever
twenty minutes, the entire tube will be colonized in a short period, and a graph of the bacterial
population will show exponential growth (producing a J-shaped graph). However, the space and food
in the test tube is finite, and soon, the bacteria will have used it up, and crowding will begin. As toxins
from metabolism begin to poison the starving bacteria, they will begin to die off, and the population
will “Crash”
10.
Describe how wastes, resources and carrying capacity relate to the stage II model. Carrying capacity
(the number of individuals an environment can support) is not accounted for in this model, and is
determined by the resources available, and the buildup of wastes (or their return to the environment
by cycling (or recycling, if you prefer).
11.
Describe the stage III model of population growth and show the formula for it. This model takes
carrying capacity (K) into account in determining the number of individuals a certain environment
should contain. The equation used is: ΔN=rN [(K-N)]/K
12.
What does the stage III model compensate for? What is another name for this model? It takes into
account the carrying capacity of the environment (K). It is also referred to as the logisitic model,
which creates an S-shaped curve as the carrying capacity is approached.
13.
What is the key assumption for stage III? Give an example in nature. The assumption is that, as the
population grows, the birth rate slows and the death rate increases as individuals reach the end of
their reproductive years
14.
What are the two strategies of population growth in nature? In terms of population growth, some
populations are r-strategists and some are k-strategists. Note: these letters refer to the variables
from the mathematical models (equations) , so r is referring to rate of growth, and k is referring to the
carrying capacity
15.
Describe and give an example of r strategists. r-strategists reproduce in large numbers, start
reproducing at an early age, and don’t tend to take care of their young for long periods of times. This
model counts on huge numbers of offspring for survival of the species, since they tend to have short
life spans. Things like dandelions, mice, frogs, or insects are examples of r-strategists.
16.
Describe and give an example of k strategists. k-strategists rely on mature reproduction (later in life),
fewer offspring, but extended parental and even familial care, in which the extended family remains
in the group and often helps care for the young. Since such care is given to ensure that the offspring
reach maturity (an age where they can care for themselves, and begin to mate), fewer offspring are
needed to ensure survival of the species. Larger organisms, and warm-blooded animals, tend to be kstrategists. Humans, elephants, deer, wolves are all k-strategists. NOTE: remember, these strategies
exist because they “worked” (were successful) from an evolutionary standpoint, NOT because these
organisms “planned” out a reproductive strategy
17.
What kind of strategist is a human? Explain. See #16
18.
Why is the human population growing so explosively? Modern medicine and technology have affected
human population growth, because it is artificially extending the life and reproductive ability of
humans. People who, pre-technology, would not have been able to have children, are now adding to
the population. People who would have died early, perhaps even before reproducing, are remaining
alive and healthy. Increased birth rates and decreased death rates have caused the population
explosion
19.
Give 3 pieces of data that supports question #18. At the dawn of agriculture, about 8000 B.C., the
population of the world was approximately 5 million. Over the 8,000-year period up to 1 A.D. it grew to 200
million (some estimate 300 million or even 600, suggesting how imprecise population estimates of early
historical periods can be), with a growth rate of under 0.05% per year. A tremendous change occurred with
the industrial revolution: whereas it had taken all of human history until around 1800 for world population to
reach one billion, the second billion was achieved in only 130 years (1930), the third billion in less than 30
years (1959), the fourth billion in 15 years (1974), and the fifth billion in only 13 years (1987).
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During the 20th century alone, the population in the world has grown from 1.65 billion to 6 billion.
In 1970, there were roughly half as many people in the world as there are now.
The human population topped 7 billion in 2011
20.
What do you think is going to happen to the human population and its growth?
This really IS a “what do you THINK” question, but please be able to back up your predictions with
serious logic.
21.
Can you think of any ways to change or compensate for the current growth rate of the human
population? See #20