Answers to Mastering Concepts Questions
18.1
1. Distinguish among ecosystems, communities, and populations?
A population includes all of the members of a species living in the same place at the same
time. A community includes the populations that interact within the same area. An
ecosystem includes the community plus the nonliving environment.
2. Name some abiotic and biotic components of your environment.
[Answers will vary.]
3. What is the biosphere?
The biosphere is the portion of Earth where life is found.
18.2
1. What is population density?
Population density is the number of individuals of a species in a specific unit area or unit
volume of a habitat.
2. What are some ways to measure a population’s density?
Aerial photos, counting subsets of the population, and mark-recapture techniques help
measure population density.
18.3
1. Under what conditions will a population grow?
A population will grow when its birth rate is greater than its death rate.
2. What factors determine the birth and death rates in a population?
The birth rate is determined by the number of times an organism reproduces, how many
offspring result from each reproductive episode, and the number of organisms in the
population at reproductive age. The death rate is determined by nutrition, predation,
accidents, and disease.
3. How does a life table relate to a survivorship curve?
The life table shows the probability of surviving at any given age, while the survivorship
curve is generated from the table and plots the number surviving at each age.
4. Describe three patterns of survivorship curves.
In a Type I survivorship curve, most individuals survive to reproductive age and then die
near the end of the maximum lifespan. In a Type II survivorship curve, the death rate is
constant throughout the lifespan. In a Type III survivorship curve, most deaths occur
early in life, and few individuals survive to old age.
18.4
1. What conditions support exponential population growth?
Exponential population growth is supported in environments where resources are
essentially unlimited.
2. How does logistic growth differ from exponential growth?
In exponential growth, the population doubles at regular intervals, and no limiting factors
retard it. In logistic growth, limiting factors slow population growth and cause it to
eventually level off.
3. Distinguish between density-dependent and density-independent factors that limit
population size, and give three examples of each.
Density-dependent factors limit population size when a population reaches a certain size.
Three examples are infectious diseases, predation, and competition. Density-independent
factors limit populations, no matter how large or small they are. Examples are severe
weather conditions, floods, and oil spills.
18.5
1. Distinguish between opportunistic and equilibrium species.
In opportunistic species, individuals tend to be small and short-lived, with rapid
reproductive rates and little parental care for the young. Equilibrium species are typically
large, long-lived, and slow to reproduce, with extensive care for the young.
2. How have studies with guppies shown that natural selection shapes life histories?
Studies with guppies have shown that the presence of predators can influence the life
history traits of prey populations. Predators shifted the prey population toward earlier
maturation, more rapid reproduction, and smaller offspring.
18.6
1. Which parts of the world have the highest and lowest rates of human population
growth?
Less-developed countries have the greatest rates of human population growth, whereas
more developed countries have the lowest rates of human population growth.
2. What factors affect birth and mortality rates worldwide?
Factors that affect birth and mortality rates are family planning programs, social factors,
economic factors, education level of women, and government policies.
3. What are some of the environmental consequences of human population growth?
Some environmental consequences of human population growth include increased
demands for resources such as food, clean water, and energy. These increased demands
result in greater expansion of settlements and farms into wild lands, deforestation, soil
erosion, water pollution, introduction of pesticides, increased soil salinity, and loss of
biodiversity.
18.7
1. How did researchers test the hypothesis that a firefly’s flash duration is correlated with
spermatophore mass?
The researchers dissected 36 females after spermatophore transfer and weighed the
spermatophores. They then plotted the weight against many male characteristics,
including flash duration, and concluded that only flash duration was positively correlated
with spermatophore mass.
2. A male firefly with a small spermatophore could “lie” by using a long flash. What
might be the consequence in the short term? In the long term, what would happen to the
reliability of the flash duration signal if many males “cheated” in this way?
In the short term the “lying” males would have increased reproductive success; however,
in the long term females would stop using flash duration as a signal for spermatophore
mass and fitness, and that form of sexual selection would cease.
Answers to Write It Out Questions
1. Rats, mustangs, koalas, and deer are examples of mammals that are overpopulated in at
least some parts of the world. Use the Internet to learn about some proposed strategies for
addressing each overpopulation problem. Is each strategy aimed at increasing death rates,
reducing birth rates, or changing distribution patterns?
Some proposed strategies include: deer are hunted, which increases their death rate;
mustangs are corralled in a round-up and made available for adoption or sale, changing
their distribution patterns; koalas are implanted with female contraceptives, reducing
birth rates; rats may be trapped or poisoned, which increases the death rate.
2. Population biologists often tag animals with radio collars equipped with Geographic
Positioning System units. What type of useful information might ecologists gain by
learning more about animal movements?
In general, observing animal movements can help ecologists learn more about how
animals migrate or use their territories. Understanding the movements of endangered
animals could help ecologists plan reserves and wildlife corridors that would minimize
conflict with humans. At the other extreme, ecologists could tag nuisance or pest animals
to learn more about their rate of spread to new habitats.
3. Decades of overfishing led to the collapse of the cod fishing industry off the coast of
North America in the 1980s and 1990s. Given the effect of birth rates, death rates, and
age structure on a population, propose an explanation for the decline of the cod
population. What sorts of policies might protect the cod from extinction?
During the decades of overfishing, the death rates were considerably higher than the birth
rates, and fishing most likely eliminated the cod in their reproductive stages. If the fish
were unable to reproduce fast enough to counter their losses, the population would
decline quickly. Policies that might prevent the collapse of the fishery might include
overall catch limits and setting aside areas (especially breeding areas) that are off-limits
for fishing.
4. Why does a population’s age structure help predict the population’s future?
A population’s age structure indicates the upcoming reproductive potential and, therefore,
helps indicate future birth and death rates.
5. Give three examples of how a habitat’s carrying capacity might change over time.
A season of heavy rains may result in additional food production or conversely a drought
may drastically reduce food availability. Alternately, a prolonged winter or early hard
frost could shorten the growing season, limiting available food resources.
6. What prevents predator populations from eliminating prey populations?
Competition prevents predator populations from eliminating prey populations. When the
predator population is large, prey populations become scarce. The next generation of
predator populations will have a hard time acquiring enough food to support their young.
As the predator population declines, the prey population may rebound.
7. Cite three recent environmental upheavals that may have had a density-independent
influence on wildlife populations.
[Answers will vary, but could include such events as typhoons, hurricanes, volcanic
eruptions, and oil spills.]
8. The grey seal population on Sable Island cannot continue its exponential growth
forever. What are three examples of density-dependent factors that might eventually limit
population growth for these marine mammals?
The increasing numbers will lead to increasing resource competition. Eventually, more
pups die, and survivorship in the early years will plummet. Also, increased density may
lead to disease outbreaks that sweep through the population. Finally, the high
concentration of pups may attract new predators seeking an easy meal.
9. Some animal behaviors seem at odds with survival and reproduction. For example,
when food is scarce, a female scorpion may consume her offspring. Explain each of these
behaviors in terms of evolutionary trade-offs.
Though the female consumes her offspring and reduces her fitness for that breeding
season, the nutrients they provide her, help her to survive, and she is able to mate again in
the future.
10. Describe the trends for human birth and death rates in more- and less-developed
countries.
In more-developed countries the sum of the birth rate plus the immigration rate is
typically about equal to the death rate, and population growth is zero. In less-developed
countries, the birth rate is often much higher than the death rate, so the populations are
growing.
11. What is the relationship between the human population and the ecological footprint?
Why does the ecological footprint vary around the world? Overall, humans are using
many resources at a faster rate than they can be replenished. What actions would be
necessary to reduce humanity’s ecological footprint to a sustainable level?
As the human population increases, the amount of land area necessary to sustain it
increases. However, while the land area is finite, currently the population size is
increasing exponentially. Different countries around the world vary in population
numbers, rates of population increase, and demand for natural resources. Ecological
footprints, therefore, vary. To reduce the ecological footprint, the increase in population
numbers must be significantly slowed by decreasing birth rates around the world.
Furthermore, the use of nonrenewable resources like fossil fuels must be greatly reduced.
Answers to Pull it Together Questions
1. What are the additions to and subtractions from a population?
Additions are births and migration into the population. Subtractions are deaths and
migration out of the population.
2. Add the following terms to this concept map: density-dependent, density-independent,
carrying capacity, and age structure.
"Deaths" connects with the phrase "may be" to both "Density-dependent" and "Densityindependent". "Logistic (growth)" connects with the phrase "levels out at the"
to "Carrying capacity". "Age structure" connects with the phrase "affects the rate of" to
"Births".
3. What conditions result in exponential growth? Logistic growth?
Exponential growth will occur when a population utilizes a new habitat or when
resources are abundant. Logistic growth occurs when resources become scarce and the
population begins to reach the carrying capacity.