CHAPTER 55: POPULATION ECOLOGY
WHERE DOES IT ALL FIT IN?
Chapter 55 builds on the foundations of biodiversity and provides detailed information about
environmental interactions of organisms. Students should be encouraged to recall the principles of
organismic classification and comparative anatomy. The information in chapter 55 does not stand
alone. It connects the information on organismic diversity to evolution and ecology coverage.
Students should know that animals and other organisms are interrelated and originated from a
common ancestor of all living creatures on Earth.
SYNOPSIS
Ecology is the study of complex and interwoven relationships of organisms with one another and
their environment. Several environmental elements directly affect individuals and thus a
population’s distributional patterns on Earth. Organisms are grouped in progressively larger
levels of organization: communities, ecosystems, and biomes. A population of organisms
comprises individuals of a given species that occurs in one area at one time. It has characteristic
features including range, dispersion, and size. No populations can exist in all locations, nor do
population ranges remain constant. Population ranges expand and contract as the environment
changes. Individuals in populations are arranged in one of three kinds of spacings. Clumped
spacing is most common in nature, although randomly and uniform spaced populations also
exist. Clumped distributions tend to form as a result of individuals clustering in microhabitats,
which themselves are not generally uniformly distributed.
Metapopulations are networks of distinct, interacting populations. The interactions result from
the exchange of individuals between populations. Populations in better habitats within an area
are called sources, while those in poorer areas are called sinks. Sink populations can often go
extinct if not regularly resupplied with individuals from source populations. Demography is the
statistical study of populations. Numerous factors affect population growth rates including sex
ratio, generation time and an age structure. Life tables, which depict a particular cohort’s age and
survivorship at a specific age, are used to assess how natural populations change over time.
Populations exhibit the typical survivorship curves that indicate the percentage of the original
population that survives to a given age. Type I have low mortality while young and high
mortality in postreproductive years. Individuals in type II populations are equally likely to die at
any age. Type III produce many offspring that are likely to die very early, with established adult
individuals who have low mortality.
An organism’s complete life cycle is called its life history. Life histories are very diverse and
wholly dependent on each organism. They often reflect trade-offs between reproduction, growth,
and survival. Natural selection favors maximizing lifetime reproductive success. When there is a
low cost of reproduction, as when resources are abundant, many offspring should be produced.
When there is a high cost of reproduction, for example when reproduction affects survival of the
parents or their future reproduction, fewer offspring should be produced. The number of
offspring produced is overall less important than that offspring’s ability to reproduce. All healthy
populations have the capacity to grow, although over time most populations remain relatively
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constant regardless of the number of offspring produced. The intrinsic rate of increase is the
population’s biotic potential expressed by the equation dN/dt = riN. Overall the rate of change
over time is equal to the intrinsic rate of growth of that population multiplied by the number of
individuals in that population. The normal innate capacity for population growth is exponential.
The carrying capacity of a population is its characteristic limit imposed by environmental
shortages. This capacity is a dynamic value that varies over time, fluctuating around a mean
population value. Graphing population growth over time generally produces a sigmoid curve
where the rate of increase declines as the population numbers increase. Population growth is also
affected by density-dependent and density-independent processes. The former include size of the
population, predators, behavioral changes, and emigration. The latter include factors like weather
and the physical disruption of the habitat, such as fires and floods.
Populations may exhibit regular, repeated cyclic patterns. The most common example is that of
the snowshoe hare. Increased numbers of prey result in increased predator numbers. This
decreases the prey population and leads to a decrease in number of predators. The prey
population is additionally affected by the amount of its own food source. Populations are
characterized as either r- or K- life history selected adaptations. r- selected populations exhibit
exponential growth with sudden crashes and have a high intrinsic rate of growth (r). They
reproduce early, have many offspring in a large brood. The offspring are generally small, mature
rapidly, and receive little parental care. K selected adaptation populations exhibit sigmoid growth
curves limited by carrying capacity (K).
They reproduce later in life and have small broods. Their offspring are generally large, mature
more slowly, and receive more intensive parental care. Human populations continue to grow
exponentially as humans have expanded the carrying capacity of their habitats—often at the
expense of other living organisms. Although some countries, Ethiopia for example, have reached
or exceeded their K, the Earth as a whole has not, causing an uncertain future. Several countries’
ecological footprints foreshadow hard times ahead for our planet.
LEARNING OUTCOMES
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Know the four key elements of the physical environment and understand their importance to
individuals and populations.
Describe and give examples of individual responses to environmental changes.
Identify examples of evolutionary responses to environmental variations.
Understand what constitutes a population and distinguish the changes that can occur in its
characteristic features, e.g., range expansion and contraction.
Differentiate among random, uniform, and clumped spacings of populations.
Describe several methods used by terrestrial plants to disperse their seeds.
Understand what a metapopulation is and how it is associated with source and sink
populations.
Define demography and describe factors that can affect population growth rates.
Recognize the value of life tables and be able to use the information provided by one.
Draw the three types of survivorship curves, explain, and give examples of each.
Understand why and how reproductive success is maximized by organisms using different
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reproductive strategies.
Distinguish between of the exponential and logistic growth models and explain the
significance to a population’s growth when it experiences either one.
Define carrying capacity and understand how it can affect population size.
Compare density-dependent and density independent growth regulating effects and give
examples of each.
Understand the reasons for predator-prey cycles and give examples.
Differentiate between r-selected and K-selected life history adaptations.
Be able to interpret information provided by a population pyramid.
Comprehend the concept of ecological footprints and what they mean to the future of the
Earth.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 55 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students are unaware of metapopulation effects
Students believe that predator-prey population fluctuations are uniform over time
Students do not account that predators seek many types prey when looking at population
fluctuations
Students do not account that prey are taken by several types of predators when looking at
population fluctuations
Students believe that population behavior is not subject to selection
Students believe artificial selection produces new traits
Students are unaware of evolution at the population level
Student think that selection is directional and produces superior characteristics
Students are unaware that mate selection drives population genetics
Students do not believe that random genetic drift produces population differences
Students think that all animals and plants evolved at about the same time
Students believe that most animals are vertebrates
Students believe that most plants are angiosperms
Students do not equate humans with being animals
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
The cost of reproduction can be compared to spending money now versus saving for the future.
If you buy that car now, yes, you will have it to drive around and be cool, but if you drive it too
much you wear out the tires, and you won’t have any money to buy new ones (you die). If you
wait and save your money it will gather interest, then you buy the car, drive it and the interest
you gained will provide the funds to buy new tires. Source-sink metapopulations are like a
socialistic society (or Robin Hood). Take from the rich and give to the poor. Discuss the human
population explosion and how it can be examined with density-independent and dependent
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models. We too frequently think just in terms of producing enough food for the populace without
considering the demands on other resources and accumulation of toxic and natural wastes. The
egg size versus clutch size problem is similar to what is found in many erythropoietic
diseases/difficulties. If fewer red blood cells are produced by the stem cells, they are usually
larger in size or contain more iron so that sufficient oxygen transport can still occur. This may
not occur if hemoglobin precursors are limited. Clearly differentiate between the kind of growth
that occurs when there are no environmentally limiting factors (exponential growth) and that
which occurs when limitations do exist (sigmoid growth). The two types of growth curves also
relate to whether an organism is an r strategist (exponential growth) or a K strategist (sigmoid
growth). Discuss why it is more likely for a K strategist to become extinct than an r strategist.
Discuss the human population explosion. Common sense indicates that we should want to kill
infected rats to prevent the spread of bubonic plague. New population dynamic models indicate
though, that doing so may actually make an outbreak worse. If the rats are killed, the fleas will
find other mammals to feed on. Fleas also live on mice, squirrels, chipmunks, prairie dogs, and
humans. Human plague outbreaks occur when rodent populations are low, forcing the hungry
fleas to seek blood from other sources. Killing more rodents would only increase the fleas’ need
to find new hosts.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 55.
Application
Analysis
Synthesis
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Have students explain the probable spacing of cave dwelling fish in a
coral reef.
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Have students explain the value of high genetic variation is the adaptive
survival of a population.
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Ask students how inbreeding could influence the survival of a population
in an unchanging environment.
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Have students explain if human diversity could have due to genetic drift.
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Have student describe how deforestation affects the ratio of herbivores to
carnivores in an area.
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Ask students to explain the reproductive strategy of a lizard that only has
female members in its population.
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Ask students to design an experiment to test if weed are more successful
as r-strategists or K-strategists.
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Have students design an experiment to test the effects of global warming
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on fish populations in small ponds.
Evaluation
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Ask the students come up with an application of age structure studies in
various human populations.
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Ask students assess pros and cons of mandatory human zero population
growth programs in a county.
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Ask students to access the accuracy of using animal models of resource
usage to understand human populations.
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Ask students investigate the pros and cons of protecting agricultural
animals from potentially fatal infectious diseases.
VISUAL RESOURCES
Show simple interspecific competition between two microorganisms in a petri dish. Fungi
“wars” can be quite impressive. Inoculate a series of plates over a period of time to show the
progress of each organism. Many microorganisms produce antibiotics to kill off other
microorganisms and out-compete them in the same habitat.
Have students conduct a 24 hour “garbage survey” of what they throw away in one day. They
should keep a log and submit to a collector who will collate the results. If the data are presented
graphically they will lead to good in-class discussions about the “throw away” society and the
ecological footprint that the United States places on the Earth.
Have groups of students construct a simple life table using different mortality rates and presents
to the class on an overhead projector.
Show photographs of different organisms (plant, animals, and fungi) and let students respond to
which survivorship curve best fits (I, II, III).
Show photographs of different organisms (plants, animals, and fungi) and let students respond to
which is an r-strategist and which is a K-strategist.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Guess the Hotspot
Introduction
This demonstration has the instructor lead students through the rationale that biologists
use to define biodiversity loss hotspots.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Biodiversity Hotspots website at
http://www.biodiversityhotspots.org/xp/Hotspots
Procedure & Inquiry
1. Tell the class they will be viewing different locations around the world defined as
biodiversity decline hotspots.
2. Load up the Biodiversity Hotspots website and click on the Interactive Map
3. Ask the class to describe the locations of the hotspots on the map
4. Then pass the cursor over each hotspot for its environmental description
5. Ask the students why certain areas are declining in biodiversity
6. Click select hotspots for more information and discuss the human factors leading to
biodiversity loss in different regions of the world.
7. Have students briefly describe probable biodiversity changes in your area
USEFUL INTERNET RESOURCES
1. The World Wildlife Fund has an informative website on biodiversity. Information on
biodiversity advocacy and research can be shared with the class as part of a lecture on
Chapter 55. The site is available at http://www.worldwildlife.org/bsp/
2. Save America’s Forests organization is a valuable on biodiversity. It offers a radio
interview about biodiversity issues discussed by Dr. Edward O. Wilson. Also included is
a PowerPoint presentation that accompanies the interview. The website can be found at
http://www.saveamericasforests.org/wilson/intro.htm
3. Case studies are valuable for reinforcing the concepts in a lesson on ecology. The
University of Buffalo has a case study called “The Rocky Mountain Locust: Extinction
and the American Experience?” which has students investigating a model study of
extinction. The website can be found at http://www.sciencecases.org/locusts/locusts.pdf.
4. Case studies are an effective tool for stimulating interest in a lesson on population
ecology. The University of Buffalo has a case study called “Improving on Nature?”
which has students investigating the issues associated with adjusting natural populations
to favor tourism. The case study can be found at
http://www.sciencecases.org/nature/nature.pdf.
LABORATORY IDEAS
A. “Squirrel” Population Dynamics
This activity has students has students investigate a theoretical model for squirrel
population dynamics.
a. Explain that they will be using beans to represent population growth in a theoretical
squirrel population.
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b. Provide the students with the following:
a. Graph paper
b. Two colors of dried beans
c. Large polystyrene cups to hold beans - Squirrels
d. Pennies – Predators
e. Peanuts
f. Fake bait worms
c. Tell each student to take a bean of each color.
d. Tell them that one bean is a male squirrel and one bean is a female squirrel and that they
are going to record the squirrel population for ten years under conditions with no limiting
factors. Squirrels produce one generation of offspring per year and are sexually mature
after one year.
e. Tell them that each pair of squirrels gives birth to four squirrels, two males and two
females.
f. Have the students collect beans to build new population as the ten years pass. Have the
students get record the population numbers on the graph paper.
g. Have students recognize the distinct characteristics of the squirrel population without
limiting factors.
h. Next have the students add the following limiting factors to another ten years:
a. Predators – Kills two squirrels per predator per year. They should add a certain
number of predators to look at the effects of predator population on the squirrel
population.
b. Food – One peanut feeds a squirrel. Add and subtract peanuts to look at the
effects of food on the squirrel population.
c. Parasites – One “worm” halves the reproductive rate of one squirrel pair. Add
and subtract worms to look at the effects of food on the squirrel population.
i. Have students describe how changes in limiting factors effected the population of the
squirrels.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
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Have students volunteer at a nature center.
Have students tutor high school students studying ecology.
Have students do a presentation on biodiversity to elementary school students.
Have students volunteer with a conservation group on biodiversity preservation projects.
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