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Chapter 5
Biodiversity, Species Interactions, and Population Control
Summary
1.
Two major factors affect the number of species in a community: the latitude in terrestrial communities and
salinity/nutrients in aquatic systems.
2.
Species play different roles in a community. Native species sustain the ecosystem in which they are a part.
Some nonnative species will crowd out native species. Indicator species alert us to harmful changes in the
community. Keystone species play ecological roles in the specific community: they may assist in pollination
help regulate populations. Foundation species affect the community’s habitat to benefit other species.
3.
Species interact with each other in these different ways: interspecific competition, predation, parasitism,
mutualism, and commensalism.
4.
As environmental conditions change, one species may be replaced by other groups of species. This gradual
change in the composition of species in a given area is called ecological succession.
5.
A community has three aspects of sustaining itself: its persistence, the ability to resist being altered, its
constant population, and its resilience in repairing damage. High biodiversity may give a community some
edge in surviving, but we do not know this for certain.
Key Questions and Concepts
5-1 How do species interact?
CORE CASE STUDY. Sea otters are a keystone species found on the west coast of the United States that are
endangered. For many years they have been in recovery. Why should we be concerned about their status? Sea
otters are charismatic, they generate tourist revenue, and they are very valuable in terms of controlling
biological populations.
A. Five basic species interactions are competition, predation, parasitism, mutualism, and commensalism.
B. Competition between species for food, sunlight, water, soil, space, nest sites, etc. is interspecific
competition.
1. With intense competition for limited resources, one species must migrate, shift its feeding
habits/behavior, or face extinction.
2. As humans take more and more space, other species are compromised.
C. In competitive situations, some species evolve adaptations that reduce/avoid competition for resources.
1. Over a long time, species evolve more specialized traits that allow them to use shared resources at
different times, in different ways, or in different places; this is termed resource partitioning.
2. Predator-prey relationships define one species (the predator) feeding/preying on another.
3. Predators have a variety of ways to capture prey. Herbivores feed on immobile plant species; carnivores
use pursuit of prey or ambush to capture prey. Some predators use camouflage, and others use chemical
warfare (venom) to capture prey or deter predators.
4. Prey species escape predators in a number of different ways such as swift movement, protective shells,
camouflage, or use of chemicals to repel or poison.
SCIENCE FOCUS: Giant kelp forests are very productive and biologically diverse ecosystems. Sea urchins
are a major threat to kelp, but sea otters keep their populations in check. Polluted water and the warming of
the world’s oceans also threaten kelp forests. If kelp forests decline significantly, many other species could be
affected.
5. Coevolution is when predator and prey can exert intense natural selection pressures on one another.
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D. Parasitism, mutualism, and commensalism.
1. Parasites live on or in another species. The host of this arrangement is obviously harmed by it, but the
parasite can contribute to biodiversity by controlling the size of specific species populations.
2. Mutualism is a relationship that benefits both species; these benefits can be in dispersing pollen and
seeds for reproduction, in receiving food, or in receiving protection.
a. Mutualism is not cooperation; each species exploits the other.
E. Some species interactions help one species but does nothing for the other; this is commensalism. Examples
of this are the bromeliads and orchids (epiphytes).
5-2 What limits the growth of populations?
A. Populations change in size, density, and age distribution; most members of populations live together in
clumps or groups.
1. Three general patterns of population distribution occur in a habitat: clumping, uniform distribution, and
random dispersion. Most species live in clumps or groups.
a. Availability of resources varies from place to place.
b. Living in groups offers better protection from predators.
c. Some predator species live in packs to have a better chance of getting a meal.
d. Temporary groups may form for mating and caring for young.
2. Uniform pattern distribution may occur where a resource such as water is scarce.
B. Four variables influence/govern population size: birth, death, immigration, and emigration.
1. Increase in population occurs by birth and immigration.
2. Decrease in population occurs by death and emigration.
3. Age structure of a population is usually described as a pre-reproductive stage, the reproductive stage,
and the post-reproductive stage. A population with a large reproductive stage is likely to increase while
a population with a large post-reproductive stage is likely to decrease.
4. Each population has a range of tolerance to variations in the environment.
5. The limiting factor principle states that too much or too little of a physical or chemical factor can limit
population growth even if all other factors are optimal.
SCIENCE FOCUS: Many human activities have had detrimental effects of sea otter populations. Their low
reproductive rate has limited their ability to increase in numbers.
C. Rapidly growing populations typically reproduce early in life, have many offspring, and reproduce many
times with short intervals between reproductive events.
D. There are always limits to population growth in nature.
E. Environmental resistance is all factors that limit the growth of a population, and largely determine an area’s
carrying capacity.
F. Exponential growth starts slow then accelerates, yielding a J-curve.
CASE STUDY. White tailed deer populations were in decline 100 years ago, due to habitat loss and hunting.
Subsequent protection and decline in their natural predators led to a drastic increase in their numbers to the
extent that they have become a nuisance and even a danger on the urban edge. Efforts to control their
populations are very complicated, and in many cases expensive.
G. Exponential growth leads to logistic growth and may lead to the population overshooting the environment’s
carrying capacity.
1. Overshooting an environment’s resources often is a result of a reproductive time lag.
2. The reproductive time lag can produce a dieback/crash of organisms unless the organisms can find new
resources or move to an area with more resources.
H. Carrying capacities can fluctuate seasonally or from year to year.
I. Species use different reproductive patterns.
1. Some have many offspring and give them little protection.
2. Some have few offspring that are cared for by their parents.
J. Population density is the number of individuals per unit area.
1. Density-independent population controls affect a population’s size regardless of its density.
2. Density-dependent population controls have a greater affect on the population as its density increases.
Infectious disease is an example of density-dependent population control.
K. Four general types of population fluctuations in nature are: stable, irruptive, cyclic, and irregular.
1. A stable population fluctuates slightly above and below carrying capacity and is characteristic of many
species living under fairly constant environmental conditions.
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Biodiversity, Species Interactions, and Population Control
2. Some species have a fairly stable population size that may occasionally irrupt to a high peak and then
crash to below carrying capacity. This is characteristic of short-lived, rapidly reproducing species.
3. Cyclic fluctuations occur over a regular time period, generally a multiple-year cycle.
A. Humans are not exempt from population crashes. Examples include the Irish potato famine, the bubonic
plague, and the current AIDS epidemic.
5-3 How do communities and ecosystems respond to changing environmental conditions?
A. With new environmental conditions, community structures can change; one group of species is replaced by
another.
1. Ecological succession is the gradual change in species composition of a given area.
2. Primary ecological succession is the gradual establishment of biotic communities on lifeless ground; in
the soil there is no terrestrial community; in an aquatic community, there is no bottom sediment. This
process generally takes a very long time.
3. Secondary ecological succession defines a series of communities with different species developing in
places with soil or bottom sediment. The soil or sediment remains after the natural community of
organisms has been disturbed, removed, or destroyed.
a. Forest fires or deforestation, for example, can convert a particular stage of succession to an earlier
stage.
B. The classic view of ecological succession is that it is an orderly sequence, each stage leading to the next,
more stable stage until a climax community is reached. Such a community would represent the balance of
nature, one dominated by a few long-lived plant species that is in balance with its environment.
C. Changes in environmental conditions that disrupt a community can set back succession.
1. Scientists can’t predict the course of a given succession in a community toward a stable climax
community in balance with its environment.
SCIENCE FOCUS: Three factors that affect ecological succession. Facilitation is when a species makes an
area more suitable for other species. Inhibition occurs when early successional species hinder the
establishment of other species. Tolerance is when later plants are unaffected by plants that came in during
earlier stages.
D. Living systems are sustained through change.
1. Stability, the capacity of an ecosystem to withstand external stress and disturbance, is maintained by
constant change in response to changing environmental conditions.
a. Inertia or persistence is the ability of a system to survive moderate disturbances.
b. Resilience is the ability to be restored through secondary succession.
Teaching Tips
Large Lecture Courses:
Compare and contrast r-selected and K-selected species on the board. Take an example of an r-selected species
most of the group will be familiar with. Review some points about its natural history. Next, engage the class in
determining how, if they were the researchers, they might determine the carrying capacity of that species. How can
K be determined in a humane way? How do we know what K is for humans? What are the inevitable consequences
of finding out?
Smaller Lecture Courses:
Most species compete with one another for certain resources. This is a generalization in the biological world.
Does it apply to humans? If so, in what way? You might work with the group as a whole, or break them into groups,
each focusing on some aspect of human survival, such as agriculture or epidemiology. Are we really free from
competition, or are we constantly competing?
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Key Terms
age structure
carrying capacity
coevolution
commensalism
ecological succession
environmental resistance
inertia
interspecific competition
limiting factor
limiting factor principle
mutualism
parasitism
persistence
population
population crash
population density
predation
predator-prey relationship
primary ecological succession
range of tolerance
resilience
resource partitioning
secondary ecological
succession
Term Paper Research Topics
1.
Niche: relationships among species in a particular ecosystem, pick-a-predator, resource partitioning versus
direct competition strategies.
2.
Unusual niches: write a case study of a particular alien species, indicator species, or keystone species.
3.
Competition and predation: important features of natural selection, the competitive exclusion principle.
4.
Making peace in natural ecosystems: resource partitioning, symbiotic relationships.
5.
Ecological succession: the role of humans in succession, the role of fires and chaos in determining succession.
6.
Do diversity and stability go together?
7.
The theory and application of island biogeography.
8.
Field and laboratory methods used in ecological research: measuring net primary productivity and respiration
rates; analyzing for particular chemicals in the air, water, and soil; studying relationships among species;
population studies; computer modeling of ecological interrelationships.
9.
Research management strategies for predator control.
10. Research wildlife management strategies that rely on human control of successional stages of development.
11. Field methods of ecological research: relationships among species; computer modeling of ecological
interrelationships.
12. Restoration of degraded ecosystems such as Lake Erie; coastal zone management.
Discussion Topics
1.
Do predators fulfill a valuable ecological function or should their numbers be reduced?
2.
What lessons for human societies can be drawn from a study of species interactions in ecosystems?
3.
How should we manage predator populations?
4.
What can be done to decrease the incidence and impacts of invasions of nonnative species?
5.
What is the wisest strategy to handle fires in natural ecosystems?
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Biodiversity, Species Interactions, and Population Control
6.
What are the most appropriate applications of the theory of island biogeography?
7.
To what extent should we disrupt and simplify natural ecosystems for our food, clothing, shelter, and energy
needs and wants?
Activities and Projects
1.
Organize a class trip to a natural area such as a forest, grassland, or estuary to observe a variety of species
interactions. Arrange for an ecologist or a naturalist to provide interpretive services.
2.
Organize a class trip to an abandoned field, coastal dune, rock outcrop, or other disturbed area and observe the
various aspects of ecological succession. If possible, visit and compare two areas that have experienced
different types of disturbance.
3.
Find works of literature, art, and music that depict species interactions in natural ecosystems. Draw as many
parallels between species interaction and your own experiences as you can.
4.
Arrange a field trip providing opportunities to compare and contrast ecosystems of several different types,
including some damaged or stressed by human activities. Invite an ecologist or biologist along to identify and
discuss specific examples of species adaptation to environmental conditions. Do the boundaries between
different kinds of ecosystems tend to be sharply delineated? Can you identify factors that limit the growth of
certain species?
5.
Organize a class field trip to systematically investigate the ecological niches for plant and animal life existing
in a landscape significantly modified by human activities. If possible, arrange to travel along a gradient that
will take you from farmland to suburbs to city to central business district. (A simplified version of this exercise
could be done by walking around campus.)
6.
Organize a local field trip for the class to examine recently disturbed areas for evidence of resilience.
7.
As a class, investigate an invasion of a nonnative species in your community. Research the species and
contribute to a plan to prevent its impact on natural ecosystems.
8.
As a class, investigate hunting, trapping, and predator control issues in your community. Evaluate current
regulations and management strategies. Make proposals for change that seem necessary and appropriate to you.
9.
As a class exercise, systematically study a modern freeway or interstate highway and trace its impact on the
surrounding land in terms of succession, diversity, and stability.
Attitudes and Values
1.
Has your local ecosystem been invaded by a nonnative species? If so, how did you feel about the invasion?
2.
How do you feel toward indicator species? Keystone species?
3.
Do you have any particular feelings toward relationships demonstrated in ecosystems? Competition?
Predation? Commensalism? Parasitism? Mutualism?
4.
How do you feel when you observe grass emerging from a sidewalk or paved parking lot?
5.
How do you feel when you observe lichens growing on a bare rock?
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6.
How do you feel toward fire in natural ecosystems?
7.
How do you feel toward mature ecosystems? Immature ecosystems?
8.
How do you feel toward island ecosystems?
News Videos
Bald Eagle Soars Again: Taken Off Endangered Species List; Environmental Science in the Headlines, 2008, DVD
ISBN: 0495561908
Monarch Watch: Researchers Track Migration of Butterflies; Environmental Science in the Headlines, 2008, DVD
ISBN: 0495561908
Circle of Life; Environmental Science in the Headlines, 2008, DVD ISBN: 0495561908
How Exotic Species are Devastating Hawaii’s Native Fauna; The Brooks/Cole Environmental Science Video
Library, 2009; DVD ISBN: 0538733551
Additional Video Resources
Last Journey for the Leatherback
A 30-minute film about sea turtle population decline and efforts toward its recovery.
http://www.greentreks.org/naturalheroes/season2/lastjourney.asp
Ocean Oasis (Documentary, San Diego Natural History Museum, 2001)
Biodiversity in the Sea of Cortez, and the deserts of Baja, Mexico.
http://www.oceanoasis.org/toc.html
Web Resources
Hare and Lynx Populations
A complete lesson plan exploring the classic population control model.
http://www.mysciencebox.org/book/export/html/81
Suggested Answers to End of Chapter Questions
Review Questions
1.
Review the Key Questions and Concepts for this chapter on p. 105 Explain how southern sea otters act as a keystone species in their environment. Explain why we should care about protecting this species from premature
extinction that could result mostly from human activities.
 One reason is that people love to look at these charismatic, cute, and cuddly animals as they play in the
water. As a result, they help to generate millions of dollars a year in tourism income in coastal areas where
they are found.
Another reason is ethical: Some people believe it is wrong to cause the premature extinction of any species.
 A third reason to care about otters—and a key reason in our study of environmental science—is that
biologists classify them as a keystone species. Without southern sea otters, scientists hypothesize that sea
urchins and other kelp-eating species would probably destroy the kelp forests and much of the rich
biodiversity associated with them.
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Biodiversity, Species Interactions, and Population Control


In giant kelp forest ecosystems, sea urchins prey on kelp, a form of seaweed. However, as keystone species,
southern sea otters prey on the sea urchins and thus keep them from destroying the kelp.
Biodiversity is an important part of the earth’s natural capital and is the focus of one of the three principles
of sustainability.
2.
Define interspecific competition, predation, parasitism, mutalism, and commensalism and give an example of
each. Explain how each of these species interactions can affect the population sizes of species in ecosystems.
Describe and give an example of resource partitioning and explain how it can increase species diversity.
 Interspecific competition occurs when members of two or more species interact to gain access to the same
limited resources such as food, water, light, and space. An example is moss and lichen.
 Predation occurs when a member of one species (the predator) feeds directly on all or part of a member of
another species (the prey). An example is lion to deer.
 Parasitism occurs when one organism (the parasite) feeds on another organism (the host), usually by living
on or in the host. An example is tick to human.
 Mutualism is an interaction that benefits both species by providing each with food, shelter, or some other
resource. An example is bee to flower.
 Commensalism is an interaction that benefits one species but has little, if any, effect on the other. An
example is epiphyte to tree.
 These relationships help maintain a balanced ecosystem with shared resources.
 Resource partitioning occurs when species competing for similar scarce resources evolve specialized traits
that allow them to use shared resources at different times, in different ways, or in different places. Some
insect-eating bird species reduce competition by feeding in different portions of certain spruce trees and by
feeding on different insect species. Resource partitioning allows species to avoid niche overlap.
3.
Distinguish between a predator and a prey species and give an example of each. What is a predator– prey
relationship? Explain why we should help to preserve kelp forests. Describe three ways in which prey species
can avoid their predators and three ways in which predators can increase their chances of feeding on their prey.
 In predation, a member of one species (the predator) feeds directly on all or part of a living organism of
another plant or animal species (the prey) as part of a food web. Together, the two different species—such
as lions (the predator, or hunter) and zebras (the prey, or hunted)—form a predator-prey relationship.
 We should conserve kelp forests because biodiversity is an important part of the earth’s natural capital and
is the focus of one of the three principles of sustainability.
 Some ways in which prey species can avoid their predators include camouflage, protective shells, chemical
warfare and a highly developed sense of sight or smell that alerts them to the presence of predators.
 Some ways that predators can increase their chances of feeding on their prey include camouflage, chemical
warfare, ability to fly faster than the prey, and better vision.
4.
Define and give an example of coevolution.
 Coevolution occurs when populations of two different species interact in such a way over a long period of
time; changes in the gene pool of one species can lead to changes in the gene pool of the other. Such
changes can help both sides become more competitive, or avoid or reduce competition. An example is bees
and flowers.
5.
Describe four variables that govern changes in population size and write an equation showing how they interact.
What is a population’s age structure and what are three major age groups called?
 Four variables—births, deaths, immigration, and emigration—govern changes in population size. A
population increases by birth and immigration (arrival of individuals from outside the population) and
decreases by death and emigration (departure of individuals from the population): Population change =
(births + immigration) – (deaths + emigration).
 Age structure refers to the number or percentage of males and females in young, middle, and older age
groups. A diagram of the age structure of the human population might show the percentages of males and
females in the total population in age categories: pre-reproductive (ages 0–14); reproductive (ages 15–44);
and post-reproductive (age 45 and older).
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6.
Distinguish between the environmental resistance and the carrying capacity of an environment, and use these
concepts to explain why there are always limits to population growth in nature. Why are southern sea otters
making a slow comeback and what factors can threaten this recovery?
 Environmental resistance is the combination of all factors that act to limit the growth of a population. It
largely determines a population’s carrying capacity: the maximum population of a given species that a
particular habitat can sustain indefinitely. The growth rate of a population decreases as its size nears the
carrying capacity of its environment because resources such as food, water, and space begin to dwindle.
 The southern sea otter cannot rapidly increase its numbers for several reasons. Female southern sea otters
reach sexual maturity between 2 and 5 years of age, can reproduce until age 15, and typically each produce
only one pup a year. The population size of southern sea otters has fluctuated in response to changes in
environmental conditions. One such change has been a rise in populations of orcas (killer whales), which
feed on them. Scientists hypothesize that orcas began feeding more on southern sea otters when populations
of their normal prey, sea lions and seals, began declining.
7.
Define and give an example of a population crash. Explain why humans are not exempt from nature’s
population controls. Describe the exploding white-tailed deer population problem in the United States and
discuss options for dealing with it. Describe two different reproductive strategies that can enhance the longterm survival of a species. Define population density and explain how it can affect the size of some but not all
populations.
 A population may suffer a dieback, or population crash, if it uses up its resource supplies and temporarily
overshoots, or exceeds, the carrying capacity of the environment. The reindeer population crashed when
they were introduced onto a small island in the Bering Sea.
 Humans are not exempt from population crashes when they have used up their resources, as seen with the
Irish potato famine.
 There are 25– 30 million white- tailed deer in the United States. Laws to protect deer have restricted
hunting and natural predators such as wolves and mountain lions have been nearly eliminated. During the
last 50 years, large numbers of Americans have moved into the wooded habitat of deer and provided them
with flowers, garden crops, and other plants they like to eat. In some forests, they are consuming native
ground cover vegetation and allowing nonnative weed species to take over. Deer also spread Lyme disease
to humans. Each year there are 1.5 million deer– vehicle collisions which injure at least 14,000 people and
kill at least 200.
 Options for dealing with the deer overpopulation include
o Changing hunting regulations to allow killing of more female deer. Since it is too dangerous to allow
widespread hunting with guns in populated communities, hire licensed archers who use bows and
arrows to help reduce deer numbers. However, animal activists argue that this is cruel and inhumane
treatment.
o Scare off deer by spraying the scent of deer predators or rotting deer meat or using electronic
equipment that emits high-frequency sounds, which humans cannot hear.
o Surround their gardens with high fencing. Such deterrents may protect one area, but cause the deer to
seek food in someone else’s yard or garden.
o Deer can be trapped and moved from one area to another. This is expensive and must be repeated
whenever deer move back into an area.
o Put deer on birth control by shooting females with darts loaded with a contraceptive.
 Some species have many, usually small, offspring and give them little or no parental care or protection. At
the other extreme are species that tend to reproduce later in life and have a small number of offspring with
fairly long life spans.
 Population density is the number of individuals in a population found in a particular area or volume.
Density-dependent factors tend to regulate a population, keeping it at a fairly constant size, often near the
carrying capacity of its environment.
8.
What is ecological succession? Distinguish between primary ecological succession and secondary ecological
succession and give an example of each. Explain why succession does not follow a predictable path.
 The gradual change in species composition in a given area is called ecological succession.
 Primary succession involves the gradual establishment of biotic communities in lifeless areas where there is
no soil in a terrestrial ecosystem or no bottom sediment in an aquatic ecosystem.
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Biodiversity, Species Interactions, and Population Control



9.
Examples include bare rock exposed by a retreating glacier (Figure 5-10), newly cooled lava, an abandoned
highway or parking lot, and a newly created shallow pond or reservoir.
Secondary succession occurs as a series of communities or ecosystems with different species develop in
places containing soil or bottom sediment. This type of succession begins in an area where an ecosystem
has been disturbed, removed, or destroyed, but some soil or bottom sediment remains. Candidates for
secondary succession include abandoned farmland (Figure 5-11), burned or cut forests, heavily polluted
streams, and land that has been flooded.
Succession does not follow a predictable path because there are many unique variables, such as weather
and available nutrients.
Explain how living systems achieve some degree of sustainability by undergoing constant change in response to
changing environmental conditions. In terms of stability, distinguish between inertia (persistence) and
resilience and give an example of each.
 Living systems contain complex networks of positive and negative feedback loops that interact to provide
some degree of stability, or sustainability. This stability, or capacity to withstand external stress and
disturbance, is maintained only by constant change in response to changing environmental conditions.
 There are two aspects of stability in living systems. One is inertia, or persistence: the ability of a living
system, such as a grassland or a forest, to survive moderate disturbances, such as mild drought. A second
factor is resilience: the ability of a living system to be restored through secondary succession after a
moderate disturbance, such as a wildfire.
10. What are this chapter’s three big ideas? Explain how changes in the nature and size of populations are related to
the three principles of sustainability.
 The three big ideas are:
o Certain interactions among species affect their use of resources and their population sizes.
o There are always limits to population growth in nature.
o Changes in environmental conditions cause communities and ecosystems to gradually alter their
species composition and population sizes (ecological succession).
 Population dynamics relate to the three principles of sustainability insofar as populations are a component
of the biodiversity within an ecosystem, their role in an ecosystem involves the use of energy ultimately
derived from the sun, and they are constantly involved in the cycling of nutrients.
Critical Thinking
The following are examples of the material that should be contained in possible student answers to the end of
chapter Critical Thinking questions. They represent only a summary overview and serve to highlight the core
concepts that are addressed in the text. It should be anticipated that the students will provide more in-depth and
detailed responses to the questions depending on an individual instructor’s stated expectations.
1.
What difference would it make if the southern sea otter (Core Case Study) became prematurely extinct mostly
because of human activities? What are three things we could do to help prevent the premature extinction of this
species?
The extinction of the sea otter would cause drastic changes to our ocean ecosystems, as the sea otter acts as a
keystone species, controlling sea urchin populations that would otherwise decimate the highly productive kelp
forests. Activities that might help prevent the demise of this species would be to join an organization that works
on their behalf, as pollution is an issue for organisms in the near-shore environment. Of course, there should be
opposition to any hunting or poaching of the species, and they could visit the regions where these organisms are
found in order to broaden their appreciation.
2.
Use the second law of thermodynamics (see chapter 2) to explain why predators are generally less abundant
than their prey.
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Predators are generally found at higher trophic levels than their prey as they eat higher up the food chain. For
example, a lion eats a zebra that feeds on grass. The lion, a carnivore and secondary consumer, is at the third
trophic level; the zebra, an herbivore and primary consumer, is at the second trophic level; and the grass, a
producer, is at the first trophic level. There is less energy and biomass when going from lower to higher trophic
levels. This follows the second law of thermodynamics as some energy is degraded and lost as heat at each
energy conversion from one trophic level to the next. Generally, there is a 90% loss of usable chemical energy
that is transferred as biomass from one trophic level to another; this is referred to as ecological efficiency. As a
result, large numbers of predators (or large amounts of biomass) cannot occur at the end of a food chain, so the
number of predators will be less than the numbers of their prey.
3.
Explain why most species with a high capacity for population growth (such as bacteria, flies, and cockroaches)
tend to have small individuals, while those with a low capacity for population growth (such as humans,
elephants, and whales) tend to have large individuals).
Species with a high biotic potential tend to be born small and have minimal time and energy to invest into
growth and development before reproduction begins. Species with fewer offspring tend to give birth to few
large slowly-maturing individuals. These then grow large in order to better compete for resources.
4.
Which reproductive strategy do most insect pest species and harmful bacteria use? Why does this make it
difficult for us to control their populations?
Insects and bacteria tend to reproduce very rapidly, with their populations following the J-curve. It is difficult
to control their populations, because when problems arise, it is generally because we have provided them with
abundant resources. In the case of insect pests, populations rise rapidly to take advantage of the food source
that our crops provide.
5.
List three factors that have limited human population growth in the past that we have overcome. Describe how
we overcame each of these factors. List two factors that may limit human population growth in the future. Do
you think that we are close to reaching those limits? Explain.
In the past, human populations have been limited by disease, limitations in food productivity, and limitations in
terms of the landscape they could occupy. Factors that may continue to limit human populations in the future
would be water limitation and diseases.
6.
If the human species suffered a population crash, name three species that might move in to occupy our part of
the niche.
Any number of inventive answers might suffice, depending on how students interpret the role that humans play
in their environment. Humans occupy a very broad niche, and when niches are vacated, often invasive species
tend to take over. Student responses should highlight generalist and invasive species that may occupy the
vacated human niche.
7.
How would you reply to someone who argues that we should not worry about our effects on natural systems
because succession will heal the wounds of human activities and restore the balance of nature?
Succession may be able to restore the balance of nature and heal some of the wounds that humans have
inflicted, however succession takes place over hundreds or thousands of years and will not be able to take place
at a fast enough rate to restore the imbalances that humans are causing in the short term. We do need to worry
about human effects on natural systems.
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Biodiversity, Species Interactions, and Population Control
8.
How would you reply to someone who contends that efforts to preserve natural systems are not worthwhile
because nature is largely unpredictable?
Preservation efforts must look beyond the snapshot view of nature and embrace the notion that nature is in a
constant state of flux. These efforts should be directed at allowing natural processes to continue with little
disturbance from humans.
9.
In your own words, restate this chapter’s closing quotation by Sir Francis Bacon. Do you agree with this notion?
Why or why not?
Sir Francis Bacon’s quote that “we cannot command nature except by obeying her,” ultimately points to the
balance we find in ecosystems. Our efforts to alter these systems generally have unforeseen and far-reaching
effects. Whether or not these effects should be of concern to us should be determined, with supporting
arguments, by the students.
10. List two questions that you would like to have answered as a result of reading this chapter.
Student answers will vary and provide a good starting point for class discussion.
Data Analysis
The graph below shows the changes in size of an Emperor Penguin population in terms of breeding pairs on Terre
Adelie in the Antarctic. Use the graph to answer the questions below.
1. What was the approximate carrying capacity of the penguin population on the island from 1960 to 1975? What
was the approximate carrying capacity of the penguin population on the island from 1980 to 2000?
2. What is the percentage decline in the penguin population from 1975 to 2000?
Answers
1. The carrying capacity from 1960 to 1975 is approximately 5,500 emperor penguins. The carrying capacity from
1980 to 2000 is approximately 3,000 emperor penguins.
2. The population of emperor penguins on Terre Adelie has declined by about 45%. 5,500 –3,000 = 2,500 drop in
the number of penguins.
2,500/5,500 x 100 = 45%
Instructor's Manual: Chapter 5
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