# Answers to Mastering Concepts Questions

```Mastering Concepts
37.1
1. Define population, community, and ecosystem.
Populations include all of the members of one species. Communities include the
populations that interact within the same area. An ecosystem includes a community plus
the nonliving environment.
2. What is population density?
Population density is the number of individuals of a species per 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 (in which individuals are
distributed without a discernible pattern), uniform (in which individuals are separated by
more or less constant amounts of space), and clumped (in which individuals form groups
or clusters).
4. Explain how subpopulations 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 encounters novel
selective forces, it may become reproductively isolated from other subpopulations of the
species. In that case, a new species has evolved.
37.2
1. Under what conditions will a population grow?
A population will grow when the sum of births plus immigration exceeds the sum of
deaths plus emigration.
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 proportion of the population that
is 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. A survivorship curve is
a graph generated from the life table.
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.
37.3
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 any condition that inhibits reproduction or increases the
number of deaths. Examples include predation, disease, and competition for food. The
carrying capacity is the maximum number of individuals of a species that an ecosystem
can support. As any population gets larger it becomes more crowded, increasing
predation, disease, and competition. Therefore, as the population approaches the carrying
capacity, environmental resistance increases.
4. Use figures 37.6 and 37.8 to explain why G increases as a population grows
exponentially, but G decreases as a population under logistic growth approaches the
carrying capacity.
When a population grows exponentially, the number of individuals added during a time
period (G) is directly related to the size of the population. As the population grows, G
increases. On the other hand, G decreases as a population approaches its carrying
capacity because of increasing environmental resistance. As N increases, (K–N)/K
approaches zero, and therefore G also approaches zero.
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’s density is high.
Three examples are infectious diseases, predation, and competition, all of which are
intensified by crowding. Density-independent factors may limit populations of any size.
Examples are severe weather conditions, floods, and oil spills.
37.4
1. Explain why an organism cannot produce large numbers of offspring that receive
extended parental care.
An organism has limited resources and energy to devote to reproduction. The more
offspring an individual produces, the less parental effort each offspring can receive.
2. Distinguish between opportunistic and equilibrium life histories.
In opportunistic species, individuals tend to be small and short-lived, with high
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.
37.5
1. Which parts of the world have the highest and lowest rates of human population
growth?
Less-developed countries have the highest rates of human population growth; more
developed countries have the lowest rates of human population growth.
2. How does the demographic transition reflect changes in birth rates and death rates?
The demographic transition includes three general stages. At first, birth and death rates
are high, so population growth is slow. Then living conditions improve and the death
rate falls, but the birth rate remains high, so the population’s growth rate increases.
Finally, the death rate stays low as the birth rate declines, slowing the rate of population
growth once again.
3. What factors affect birth rates and death rates worldwide?
Factors that affect birth rates include the population’s age structure, government policies
that punish or reward having children, the availability of family planning programs, and
the education level of women. Factors that affect death rates include access to food,
health care, and clean water.
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 energy, food, and clean water. Fossil fuel use releases
gases that contribute to global climate change. Increasing demand for food means greater
expansion of settlements and farms into wild lands, causing further loss of biodiversity.
Fresh water supplies are also under increasing pressure as the population grows.
37.6
1. Predict the life history traits of mollies that live in a toxic stream near a cave opening,
where a limited amount of light is available.
Mollies in toxic streams that receive small amounts of light would likely have life history
traits that are intermediate between mollies from dark, toxic streams and mollies from
light, toxic streams. Females would produce about 10 offspring per reproductive cycle,
and each would have a mass of about 5 grams at birth.
2. Explain why mollies in light, nontoxic streams have smaller offspring than do mollies
in dark, toxic streams.
Since mollies in light, nontoxic streams have few environmental stressors, larger
offspring do not have a survival advantage over smaller offspring. Since a female has
limited energy to allocate to offspring, producing smaller offspring allows her to produce
more offspring per reproductive cycle.
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.
2. Chapter 39 describes biomes, the major types of ecosystems. Pick a biome and discuss
a community that might be found in that biome. What populations may be included in the
community you selected?
communities, and ecosystems are related.]
3. 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 of habitat, although the
individuals may not be evenly distributed. The environment may not be uniform, so
individuals may form clumps in the best patches of habitat. In addition, individuals may
form groups related to hunting, breeding, or safety.
4. Why might an ecologist be interested in studying population dynamics?
Understanding how and why populations grow or shrink is relevant to disease prevention,
land management, the protection of endangered species, and many other fields.
5. Calculate the anticipated size of the rotifer population in figure 37.6 after 21 days.
The population will reach 7989 rotifers by the end of the 21st day. After 20 days, the
population size (N) was 6548. On the 21st day, the growth rate (G) equals the product of r
and N (0.22 times 6548 equals 1441 individuals added on day 21). Add the individuals
produced on day 21 to the day 20 total (1441 plus 6548 equals 7989 rotifers).
6. Refer to the logistic model of population growth shown in figure 37.8. What would be
the growth rate (G) of the population if the population size (N) equaled the carrying
When population size (N) equals the carrying capacity (K), then K minus N equals zero.
Therefore, G equals r times N times zero divided by K. A number multiplied by zero
equals zero, so G equals zero. The population has stopped growing.
7. Suppose that in a population with an equilibrium life history, 60% of individuals are
younger than reproductive age. Is the population likely to grow, remain stable, or
decline? Why? Why might this question not apply to a population with an opportunistic
life history?
The population is likely to grow. As the young members of the population enter
reproductive age, the birth rate of the population will increase. This question does not
apply to an opportunistic species because nearly all of their offspring die before reaching
reproductive age.
8. Define the following terms: per capita rate of increase, environmental resistance, and
carrying capacity.
Per capita rate of increase is the difference between the birth rate and the death rate.
Environmental resistance is the combination of external factors that keeps a population
from reaching its maximum growth rate. The carrying capacity is the maximum number
of individuals of a species that an ecosystem can support indefinitely.
9. Use the Internet to find a species that is growing out of control and a species that is
endangered. Compare their birth rates, death rates, and other factors that are leading to
the differences in the populations’ fates? What techniques do researchers use to study
each species?
Answers will vary. In general, however, the growing population likely has a high birth
rate and a low death rate, whereas the declining population likely has a low birth rate and
a high death rate.
10. 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.]
11. 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
35.7 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 37.4A.
Although a female scorpion might consume her offspring and reduce her fitness for one
breeding season, she survives on the nutrients they provide her, and she is able to mate
again in the future. The male spiders described in section 35.7 are very numerous
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.
12. Distinguish between opportunistic and equilibrium life histories, and give an example
of an organism with each type of life history.
Opportunistic species are short-lived and are early to reproduce. They produce many
offspring, each of which receives little to no care. Examples are weeds and insects.
Equilibrium species are long-lived and late-maturing, and they produce a small number
of offspring that receive extended parental care. Examples are large mammals.
13. 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.
14. 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, lifestyle, energy efficiency, and overall 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 (including
fossil fuels) must be greatly reduced.
15. Search the Internet for an ecological footprint calculator. What is your ecological
footprint? What can you do to reduce the size of your footprint?
Pull it Together
1. Add G (the number of individuals added during any time interval) to the concept map.
Explain how G changes throughout exponential and logistic growth.
“G” connects with “may be high when the population has a high rate of” to “Births.” “G”
connects with “may be low when the population has a high rate of” to “Deaths.” G
increases as a population grows exponentially. In logistic growth, G starts low, increases,
and then decreases again as the population approaches the carrying capacity.
2. Add the following terms to this concept map: density-dependent, density-independent,
carrying capacity, and age structure.
Both “Density-dependent” and “Density-independent” connect with the phrase “factors
cause” to “Deaths.” “Logistic growth” connects with the phrase “levels out at the” to
“Carrying capacity.” “Age structure” connects with the phrase “influences the rate of” to
“Births.”
3. Describe the trends for human birth and death rates in more- and less-developed
countries.
In more-developed countries the birth rate is typically about equal to the death rate, and
population growth is zero (in the absence of immigration). In less-developed countries,
the birth rate is often much higher than the death rate, so the populations are growing.
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