Mastering Concepts
36.1
1. What are the relationships among ecosystems, communities, and populations?
Populations include all of the members of one species. Communities include the
populations that interact within the same area. Ecosystems include the community plus
the nonliving environment.
2. 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.
3. Distinguish among the types of distribution in a habitat.
The three kinds of distribution in a habitat are random dispersion (in which a species is
dispersed without a discernible pattern), uniform dispersion (in which a species is
dispersed in a regular pattern), and clumped dispersion (in which a species is dispersed in
groups or clusters).
4. Explain how dispersal creates subpopulations that may evolve into new species.
As organisms disperse into new areas they are more likely to mate with members in their
own location, thus forming a subpopulation. If this subpopulation becomes isolated from
other subpopulations of the species, then speciation may occur as each subpopulation
encounters novel selective forces.
36.2
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. What is the relationship between a life table and 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 the 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.
36.3
1. What is the per capita rate of increase?
The per capita rate of increase is the difference between the birth rate and the death rate.
2. What conditions support exponential population growth?
Exponential population growth is supported in environments where resources are
essentially unlimited.
3. What is environmental resistance, and what is its relationship to the carrying capacity?
Environmental resistance is an external force that keeps a population from growing at a
maximal rate. It is any condition that inhibits reproduction or increases mortality.
Environmental resistance helps to determine a habitat’s carrying capacity, the maximum
number of individuals of a species that a habitat can support.
4. 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.
5. 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.
36.4
1. Distinguish between r-selected and K-selected species.
In r-selected species, individuals tend to be small and short-lived, with rapid reproductive
rates and little parental care for the young. K-selected species are typically large, longlived, 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.
36.5
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, while more
developed countries have the lowest rates of human population growth.
2. How does the demographic transition reflect changes in mortality and birth rates?
As a nation passes through the demographic transition it passes through three general
stages. At first, birth and death rates are high so the population does not grow much.
Then living conditions improve and the death rate falls, but the birth rate remains high.
The population begins to increase. Finally, the death rate stays low as the birth rate
declines, slowing the rate of population growth once again.
3. 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.
4. 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.
36.6
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.
Write It Out
1. List some of the ways you have interacted with your surroundings today. Categorize
each item on your list as a population-, community-, or ecosystem-level interaction.
[Answers will vary]
2. Describe the difference between population density and distribution. Why aren’t
organisms always distributed evenly throughout their habitat?
Population density is the number of individuals per unit/volume area. However, the
individuals need not be distributed evenly throughout the area. Organisms may not be
evenly distributed because the environment may not be uniform or because the organisms
attract one another and tend to clump together.
3. Biologists often use rectangular sampling plots called quadrats in studies of plant
population ecology. Suppose you want to determine which type of habitat is most
favorable for the growth of a particular weed species. How could you use a quadrat of
known area to measure the weed’s population density in different habitats? Since plants
are rarely distributed uniformly throughout a habitat, how might you ensure random
sampling?
You could set up multiple, equal-sized quadrats in different habitats and then count the
number of weeds in each quadrat. Quadrat placement can introduce bias if an
investigator unconsciously chooses a location with a higher or lower population of
weeds. To reduce this type of bias and ensure random sampling, investigators often toss
the quadrats into the general sampling area rather than setting them down.
4. According to the CIA World Factbook, in 2009, the birth rate in the United States was
13.82 births per 1000 population, and the death rate was 8.38 deaths per 1000.
Immigration added 4.31 migrants per 1000 population. Calculate the overall growth rate
of the U.S. population in 2009.
13.82 + 4.31 – 8.38 = 9.75 per 1000 people per year.
5. 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.
6. 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.
7. 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 prevent 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.
8. 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.
9. Define the following terms: per capita rate of increase, environmental resistance, and
carrying capacity.
Per capita rate of increase: the difference between the birth rate and the death rate
Environmental resistance: the combination of external factors that keeps a population
from reaching its maximum growth rate
Carrying capacity: the maximum number of individuals that a habitat can support
indefinitely
10. The rat (Rattus norvegicus) has a per capita rate of increase (r) of 5.48 per year.
Assuming exponential growth and an initial population of 50 rats, what will be the
population size after 1 year? List three factors that prevent rats from achieving this
reproductive potential.
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Rats: G = # of individuals added per unit time = rN = 5.48/yr x 50 = 274. Added to
the 50 that started the year (and assuming no rats died during that interval), the
population by the end of the year will be 274 + 50 = 324 rats.
Disease, competition for nutrients, and competition for space keep the rats from
achieving their reproductive potential.
11. In the 1890s, a group of Shakespeare enthusiasts released 100 European starlings in
New York City. Descendants of the birds have spread across North America, and they
now number in the hundreds of millions. Propose an explanation for the exponential
growth of the starling population. How might you test your hypothesis?
The starlings may have encountered few if any natural predators in the new environment,
leading to a low death rate. One way to test this hypothesis might be to identify predators
from the starlings’ native habitat. A search of historical wildlife records would reveal
whether the same or similar predators existed in North America in the past century.
12. What are the differences between the conditions that result in exponential versus
logistic population growth?
Exponential growth occurs as long as resources are unlimited and a population can
double at regular intervals. Logistic growth occurs when resources become strained or
limited, and the population levels off.
13. 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.
14. Describe a method for estimating the number of elk in an area. What clues might help
you understand whether the elk were below or above their habitat’s carrying capacity?
To estimate the number of elk in an area, you could try counting the elk directly, count
their droppings, or use the mark-recapture method. The elk are below carrying capacity
if resources are abundant and the number of deaths is fairly small. The animals are above
the carrying capacity if plants are visibly depleted and if the elks appear starved or
diseased.
15. Bird feeders and bird baths attract dense crowds of songbirds, which, in turn, attract
hawks and other predatory birds. Droppings from the birds can spread disease. How do
these conditions illustrate density-dependent limits on songbird populations? What are
some examples of density-independent factors that might also limit songbird populations?
Predation and disease are biotic factors that increase as population numbers increase.
Density independent factors that might limit songbird numbers would include severe
weather, changing weather patterns, fire, or natural disasters.
16. 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.
17. 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.]
18. Research suggests that salmon farming threatens wild salmon in multiple ways. For
example, farmed salmon may spread parasitic lice; interbreeding with farmed salmon can
change the genetic structure of wild populations; and contaminants released from the
salmon farms may harm wild salmon. Identify each of these problems as densitydependent or density-independent.
Farmed salmon live in denser numbers and are more susceptible to disease or parasites,
so their transfer of lice to wild salmon would be a density dependent factor.
Interbreeding would also be density-dependent because the higher numbers would
increase the potential for matings. However, the release of contaminants could affect any
size population, so that would be a density-independent factor.
19. 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.
20. Some animal behaviors seem at odds with survival and reproduction. For example,
when food is scarce, a female scorpion may consume her offspring. In addition, section
31.5 describes a male spider that offers his own body for his mate to consume. Explain
each of these behaviors in terms of the trade-offs described in section 36.4A.
Though the female consumes her offspring and reduces her fitness for that breeding
season, she survives on the nutrients they provide her, and she is able to mate again in the
future. Males in that spider species are very high in numbers relative to females, so any
individual male’s likelihood of mating is low. When given the chance to mate, a male’s
reproductive fitness is greatest if he can maximize the duration of copulation. If he offers
his body for the female to consume, she allows him to mate longer, which increases his
likelihood of siring offspring.
21. Distinguish between K-selection and r-selection, and give an example of an organism
with each type of life history.
K-selected species are long-lived and late-maturing, and they produce a small number of
offspring that receive extended parental care. Examples are large mammals. r-selected
species are short-lived and early to reproduce, and they produce many offspring that each
receive little to no care. Examples are weeds and insects.
22. Review section 11.7, which describes how fishing regulations can affect fish
populations. How does commercial fishing act as a selective force that alters the life
histories of harvested fish?
In commercial fishing, the largest fish are harvested. This policy leaves the small, slowgrowing fish as survivors. Over time, large-harvest regulations shift the selective
pressures to favor fish that are slow-growing, small, and quick to reproduce.
23. According to the CIA World Factbook, in Pakistan, 37% of the population is younger
than age 15, and 4% is older than 65. In the United Kingdom, 17% of the population is
younger than 15, and 16% is older than 65. Which population will increase faster in the
future? Explain.
Pakistan will increase faster since the future birth rate values will be far higher than the
death rate. In the United Kingdom the birth rate will be almost insignificantly higher
than the death rate, and so the population numbers will be steady.
24. Consult the websites for the U.S. Census and CIA World Factbook. What sorts of
demographic information do these two sites compile? Why might it be important to
understand human population trends within the United States and in the rest of the world?
The U.S. Census contains statistical information on many aspects of the U.S. population
such as income, poverty, housing and insurance. The CIA World Factbook contains
statistics on people, history, government, economy, etc. for countries throughout the
world. Understanding population trends in our own country can help us predict many
future needs such as schools, health care, nursing homes, housing, and sanitation.
Knowledge of the rest of the world can give insight into future resource demands,
overcrowding and possible civil wars, or potential disease outbreak areas.
25. Because of historical, cultural preferences for boys, an unintended result of China’s
“one child” policy has been a skewing of the sex ratio toward males. In China, about 110
boys are born for every 100 girls. What effects might this have on future population
growth in China? Can you foresee any social consequences of having so many females
“missing” from the population?
The low proportion of females may cause the growth rate of China’s population to
decline. The competition for scarce females may result in many social problems,
including kidnapping, and women from neighboring countries may be forced into
prostitution. In addition, fewer female leaders, business owners, and workers will be
present, increasing the power of males in the society.
26. How does the human population illustrate the concept of subpopulations? How have
improvements in global transportation influenced these subpopulations?
Concentrations of different ethnic groups occur in different areas, and people often marry
within their group. Global transportation improvements, however, have made it much
easier for migration to occur between subpopulations, so gene pools are more readily
mixed.
27. 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.
28. The rock pocket mouse (Chaetodipus intermedius) is a small mammal that lives in the
southwestern United States. Its fur comes in two colors. Light variants typically live on
sandy-colored rocks, and the dark mice live on black basalt. Propose a hypothesis that
explains this observation, then design one or more experiments to test your hypothesis.
One hypothesis might be that the mice living in each habitat are camouflaged and
protected from predation. An experiment could divide a group of light mice and place
them on both sandy-colored and black rocks; a group of dark mice would be similarly
divided. Researchers could monitor predation rates among all four groups of mice.
29. Each winter, the Audubon Society encourages birdwatchers to participate in an
annual Christmas bird count. Use the Internet to find information about this project. What
methods are used to count birds? Why is it useful to repeat counts annually?
The count area across the nation is divided into “count circles,” each overseen by a
compiler. The count is conducted over a range of dates, but each local area is assigned a
date for their count. Counts can be done out in the field with binoculars and a bird book
or in your own backyard at the feeder. The 110 years of the survey provide valuable
information about changing migratory patterns that may signal climate change or
population numbers that drop significantly and may indicate overhunting or pollution.
Pull it Together
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.
The deaths in a population may occur because of density-dependent or densityindependent factors. Logistic growth levels out at the carrying capacity. Age structure
affects a population’s birth rate.
3. How do exponential and logistic growth differ?
Exponential growth means a population’s size doubles at regular intervals. In logistic
growth, predation, competition, and other factors place limits on the population’s growth.
4. How do r- and K-selection reflect life history trade-offs?
These two life history strategies reflect a trade-off between quantity and quality. rselected species are short-lived and reproduce rapidly, investing little care in each of their
many offspring. Survival rates are therefore very low among the young. K-selected
species have longer lives and reproduce more slowly than r-selected species, investing
huge amounts of energy into each of their few offspring. Most of the young therefore
survive to adulthood.
5. 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.