This document contains answers to 15 "take-home message" challenge questions about key concepts in biology and ecology. The questions cover topics like the relationship between individuals, populations, communities and ecosystems; how the units of focus differ between biology and ecology; population growth patterns; natural selection; survivorship curves; litter size; maximum sustainable yield; and human population growth and limits.

1. CHAPTER 14 “TAKE-HOME MESSAGE” CHALLENGE QUESTIONS
14.1 Describe the terms individual, population, community, and ecosystem as they
related to one another.
Answer: A single organism is an individual. An interbreeding group of these organisms
is a population. Populations of different species that interact with each other in a
particular area are a community. This community, together with the non-living elements
of the area, form an ecosystem.
14.2 How do the main units of focus in biology and ecology differ?
Answer: Much of biology focuses on the individual. Ecology, however, focuses on
organisms at the level of the population.
14.3 How many of the five million eggs that a cod lays during its lifetime are likely to
survive to reproduce? How many of an elephant’s babies are likely to survive to
reproduce?
Answer: In both cases, only an average of two offspring will survive to reproduce,
replacing their parents.
14.4 Population “A” is subject to heavy density-dependent factors, such as reduced
food supplies, but not a lot of density-independent factors, such as earthquakes, while
population “B” is repeatedly affected by density-independent factors but not many
density-dependent factors. Which of these two populations is more likely to be
experiencing exponential growth? Why?
Answer: Population “B.” Population “B” is in an environment in which “bad luck”
events such as earthquakes, fires, and floods occur repeatedly. As a result, this population
might never have time to grow as high as the carrying capacity and might perpetually be
in exponential growth.
14.5 Is it a problem for science, biology, and ecology that there are well documented
exceptions to the logistic growth pattern model?
Answer: No. Nature isn’t always tidy, but there are practical reasons for finding the best
possible answers in science. If you are taking a lab with this course, you have probably
already gotten some results that were far from what you predicted. Such occurrences are
an important part of experiencing science.
14.6 What is meant by maximum sustainable yield?
Answer: The maximum sustainable yield is a special case in which as many individuals
as possible are removed from the population without impairing its growth rate. This
generally involves reducing the population size to half of the environment’s carrying
capacity.
14.7 How is an organism’s litter size shaped by natural selection?
Answer: As the term “reproductive investment” implies, there are costs and risks to an
organism in producing offspring. The number of offspring an organism produces in a
single litter can only go so high without taking such a toll that the individual is unlikely

2. to live to have another litter. Thus, for many organisms, a smaller litter size—allowing
the individual to have more litters in the future—is favored by natural selection.
14.8 How is a survivorship curve for a giant tortoise different from that of a mackerel?
Answer: A giant tortoise has high survivorship throughout most of its life, which rapidly
decreases late in life. The mackerel has high mortality early in life, but those that survive
the early years live long lives. These survivorship curves are type I and type III,
respectively.
14.9 Explain why the litter sizes of mammals living closer to the equator might be
smaller than those in higher latitudes? How are these smaller litter sizes offset?
Answer: Areas closer to the equator do not have severe winters and, thus, the breeding
seasons are long enough to allow mammals multiple reproductive episodes per year. An
offsetting evolutionary factor is that these reproductive episodes tend to produce fewer
offspring.
14.10 Does natural selection act upon a mutation that causes death at age fifty? Explain.
Answer: No. Since very little reproduction occurs after the age of fifty, death at this
time would come after reproduction had already occurred, and the mutation would
already have been passed on.
14.11 A hazard factor is a measure of the risk of death for individuals in the population
from all types of external forces. A high hazard factor will favor individuals that
reproduce early, and will not be effective in weeding out alleles causing premature death.
How does a low hazard factor influence longevity?
Answer: A low hazard factor will lead to later reproduction and will weed out alleles
causing premature death except those having their effects very late in life.
14.12 Is it possible to produce, through artificial selection, fruit flies that live twice as
long as normal? Explain.
Answer: Yes. If one simply collects eggs laid by the longest-lived flies from each
generation, the average lifespan will be dramatically increased fairly quickly.
14.13 The age pyramid for the United States shows a baby boomer “bulge” resulting
from the unusually large number of babies born from the late 1940s to the early 1960s,
who are now reaching retirement age. There is concern that the current number of
working individuals is not great enough to sufficiently contribute to cover the retirement
system payouts promised to the baby boomers. Why wouldn’t another “baby boom”
solve this problem?
Answer: This problem is immediate and individuals born today will take many years to
reach the workforce. In the meantime, they also require resources for support.
14.14 What areas of the world have not yet completed their demographic transitions and
still display dangerously fast rates of population growth?
Answer: Mexico, Brazil, Southeast Asia, and most of Africa have not yet completed
their demographic transitions, and display fast rates of population growth.
14.15 For all of the considerable success of the human species, ultimately human

3. population growth will face limits. Why?
Answer: Like every other species, human environment has a carrying capacity beyond
which the population cannot be maintained.