Download 1495/Chapter 14 - Toronto District Christian High School

14.3
Factors Limiting Natural Population Growth
E X P E C TAT I O N S
Differentiate between density-independent and density-dependent population
regulating factors, and describe their effect on the growth of populations.
Describe various types of interactions among different species of animals
and plants, and explain how they affect population growth.
Compare and explain the fluctuations of various populations, emphasizing
factors such as carrying capacity, fecundity, and predation.
Density-independent Factors
Temperature or population density
Many populations that grow exponentially
eventually stop growing quickly, or crash as a result
of a very high death rate (recall Figure 14.13 on
page 478). Such crashes are frequently the result
of abiotic factors such as bad weather. For example,
certain species of bark beetles (which cause serious
damage to forests in various parts of Canada) grow
exponentially until very cold weather in early or
late winter kills many of the individuals that would
produce the next generation. When winters are
mild, or short, enough insects survive to reproduce
during the next summer, creating an exponential
growth curve.
These abiotic limiting factors are referred to as
density-independent factors because they affect
populations regardless of their density. These
factors are effective against small populations as
well as large ones. Figure 14.17 illustrates the effect
of a density-independent factor. Other density-
independent factors include events like floods or
droughts. For populations located within a small
geographic area, forest fires, hurricanes, or tornadoes
can also limit population growth. Figure 14.18 shows
a memorable example of a density-independent
regulating event.
Density-dependent Factors
As you have seen, the growth of many populations
follows a logistic rather than an exponential growth
curve. A population like this is limited by densitydependent factors. The strength with which these
factors slow a population’s growth depends on the
density of the population. When a population is
carrying capacity
temperature
population
crash
Time
Figure 14.17 Notice that for this hypothetical population,
a decline in population size is linked to temperature, a
density-independent limiting factor. The decline begins
before the population is halfway to its carrying capacity
and well before its own density would inhibit its growth.
Figure 14.18 A severe ice storm hit eastern Ontario and
parts of Québec in January 1998. The devasting ice storm
made roads impassable and knocked down power lines,
leaving hundreds of thousands of people without electricity,
some for several days. In addition, the storm acted as a
density-independent limiting factor, causing declines in the
populations of maple, birch, and cedar trees, among other
species.
Chapter 14 Population Ecology • MHR
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small enough to be far below its carrying capacity,
density-dependent factors have no effect and
population growth is rapid. Eventually, however,
the population reaches a density at which these
factors start to have an effect; after this point the
population growth slows, and it eventually stops
when the carrying capacity is reached.
Later in this chapter, Investigation 14-A will give
you an opportunity to examine the effect of both
density-dependent and density-independent
regulating factors on a species of micro-organisms.
The remainder of this section describes some specific
types of density-dependent factors in more detail.
Competition
In contrast to density-independent factors, densitydependent factors are typically biotic — they
involve living things. As has been described, these
living things are often members of the population
itself. For example, the carrying capacity of a
population’s environment may depend chiefly
on the availability of food. When the population
reaches the inflection point in its logistic growth
curve, there is no longer an abundance of food for
each member of the population. Members must
now compete with each other for the limited food
supply, which becomes even more limited as the
population size increases. The result is that the
birth rate decreases or the death rate increases, or
both (see Figure 14.19), and the population growth
slows more and more as the density increases.
This type of competition among the members
of a population is referred to as intraspecific
competition. You have encountered its effects
already, since it leads to evolutionary change as a
result of natural selection. A higher proportion of
successful competitors survive longer and have
greater reproductive success; they are said to have
higher fitness. This allows them to pass on more
copies of their alleles to subsequent generations.
The result is a change in allele frequencies within
a population — which results in evolution.
10 000
Average clutch size
Average number of seeds
per reproducing individual
12
1000
100
11
10
9
8
0
10
0
100
Seeds planted per m2
A
B
10
20
30
40
50
60
70
80
90
Number of breeding pairs
100
Survivors (%)
80
60
40
20
0
C
482
20
60
100
Density (beetles/0.5 g flour)
MHR • Unit 5 Population Dynamics
Figure 14.19 (A) A varying number of plantain seeds (a
common Ontario weed) were planted in experimental plots.
The average number of seeds produced by an adult plant
decreased as the density of the plants in the plots
increased. (B) Many songbird populations are limited by the
amount of food available in their habitat. In some species,
the average number of eggs laid by females declines as
the density of breeding pairs increases. (C) In some cases,
increased population density reduces the ability of
individuals to survive rather than (or in addition to) reducing
reproduction. This is true for laboratory populations of flour
beetles. This graph shows the percentage of beetles that
survive from egg to adult, in relation to the population
density of the beetle.
In addition to its role in evolution, intraspecific
competition is an important density-dependent
factor limiting the growth of many populations.
Members of a population may compete for a
variety of needed resources, including food, water,
sunlight, soil nutrients, shelter, or breeding sites
(see Figure 14.20). The effect is always the same —
a reduction in the population’s growth rate.
Density-dependent limiting factors can involve
interactions between two or more populations as
well as interactions within a single population.
For instance, two species with similar habitat
requirements (for example, see Figure 14.21) may
compete with each other for soil nutrients, food, or
other resources found in that habitat. This type of
interaction between two or more populations is
referred to as interspecific competition. In some
cases, one species may eventually out-compete the
other or others, and the “losing” species disappears
from the area (see Figure 14.22 on the next page). A
result like this often indicates that the interacting
species had very similar ecological niches. Ecologists
explain the effects of interspecific competition by
referring to the competitive exclusion principle.
Essentially, this theory states that species with
niches that are exactly the same cannot co-exist; if
two species have completely overlapping niches,
one will always exclude the other, as shown by the
dashed line in Figure 14.22B. However, if the niches
of competing species are sufficiently different, they
can both live in a particular area, although the
density of one or more of the populations may be
lowered by the presence of the other (as shown by
the dashed line in Figure 14.22A).
Figure 14.20 For gannets, which nest on rocky islands such
as this one off Cape St. Mary’s in Newfoundland, the limited
number of nesting sites determines the carrying capacity of
the environment. As a result, only a certain number of pairs
can nest and reproduce at any given time. When N is low, all
birds can find a suitable nest site and reproduce, so the
population grows. Above a certain N, however, many pairs
fail to breed successfully and, as a result, population growth
slows and eventually levels off.
BIO FACT
For some species, internal rather than external cues (such
as a reduced food supply) seem to limit population size.
Small rodents kept in experimental conditions with
abundant food and shelter will reproduce quickly until their
population reaches a certain density. At this point, even
though the resources they need most are still unlimited,
reproduction decreases. High density seems to produce a
stress response in which hormonal changes delay sexual
maturation, cause shrinkage of reproductive organs, reduce
the effectiveness of the immune system, and often produce
aggressive behaviour (sometimes including cannibalism).
Exactly what cues trigger these hormonal changes is unclear,
but similar effects of crowding have been noticed in some
animal populations in nature.
Figure 14.21 In Ontario, garlic mustard (Alliaria petiolata,
a plant introduced from Europe) invades moist woodland
habitats and crowds out many native species. It is a
particular threat to some endangered plant species,
such as the wood poppy (Stylophorum diphyllum).
Chapter 14 Population Ecology • MHR
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Number of paramecia
(per mL)
A
800
grown alone
400
mixed
culture
0
Time
Paramecium caudatum
B
Number of paramecia
(per mL)
driving force of evolutionary change. In each
competing species, the individuals that are most
different from their competitors will be best able to
avoid competitive interactions and will therefore
obtain the most resources. For example, if two
species of birds compete for seeds of roughly equal
sizes, those individuals of both species that can eat
larger or smaller seeds will be able to find more food.
They will, therefore, be more likely to survive and
reproduce than will members of their own species
that cannot avoid interspecific competition. As a
result, their alleles — coding for the characteristics
(such as beaks that can handle different seeds) that
distinguish them from their competitors — will
increase in frequency in subsequent generations. In
this way, natural selection can produce increased
divergence between competing species. In Figure
14.23, natural selection may take two different
routes to lead to reduced competition. The total
range of prey sizes taken by the two species may
increase, with one species extending its preferences
to smaller items than were taken previously, while
the other may include larger foods than were eaten
before. Alternatively, the total range of prey eaten
may remain the same, but the niche of each species
may shrink; one species may become a specialist
on small prey while the other takes mainly large
prey. Over time, this can increase the diversity
of species living in a community.
Paramecium aurelia
200
grown alone
100
mixed
culture
0
Time
Figure 14.22 When two species of paramecia (Paramecium
aurelia and P. caudatum) are grown in separate cultures in a
laboratory, their populations follow a logistic curve to reach
their own carrying capacities. This is shown by the solid
lines. However, when they are grown together, P. caudatum
goes extinct in the mixed culture, shown by the dashed line
in (B), since P. aurelia is a much better competitor.
What this means is that differences between
species can be increased as a result of natural
selection, with interspecific competition as the
species B
Number of
individuals
species A
Prey size
Number of
individuals
or
Prey size
Figure 14.23 Within a species, individuals vary in many of
their features. As a result, they use a range of whatever
resources are necessary for their survival and reproduction.
In this example, the members of two species, A and B, vary
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MHR • Unit 5 Population Dynamics
Prey size
with respect to the size of their prey, but some individuals
of both species eat intermediate-sized prey. This overlap
produces interspecific competition, which may lead to
evolutionary changes in both species.
Interaction of Predator
and Prey Populations
Some populations, especially those of certain
insects, birds, and mammals, fluctuate regularly in
density. These alternating periods of high and low
populations are often referred to as population
cycles. While some small herbivorous mammals,
such as voles, have 3- to 5-year cycles, larger
herbivorous mammals, such as snowshoe hares
(Lepus americanus) and muskrats (Ondatra
zibethica), have 9- to 11-year cycles. These longer
cycles are also typical of some birds, including
ruffed grouse (Bonasa umbellus). The causes of
cycles vary with the species and from population
to population within a single species. Some may be
due to a lag in response to density-dependent
factors, as discussed in section 14.2. If such a lag
is fairly constant, a more or less regular cycle of
fluctuation above and below the population’s
carrying capacity could result.
Some of the mammal species that display
fluctuations in population density are predators
that have cycles overlapping those of their prey
(such as some of the herbivores already described).
One explanation for these cycles is the densitydependent effect of each population on the other.
For example, some populations of Canada lynx
prey almost exclusively on snowshoe hare (see
Figure 14.24). An increase in the hare population
would reduce competition for food among the lynx
(Lynx canadensis), allowing them to increase their
reproduction rate and survive longer. The result
would be an increase in the population density of
lynx. However, the presence of a large number of
these predators would eventually cause the hare
population to decrease. This, in turn, would increase
competition among lynx for food, causing a decline
in the predator population and permitting the prey
population to expand once again.
BIO FACT
Some of the most remarkable population cycles in the world
are those displayed by various species of cicadas (insects in
the order Homoptera). The life cycle of these species takes
place mostly underground, and requires 13 to 17 years to
complete. At the end of this period, they emerge as adults
in extremely high densities (as high as 600 per m2 ). When
the adults lay their eggs and die, the aboveground
population shrinks to virtually nothing. The long life cycle
may be an adaptation to reduce predation. Since these
species are around for such a short time, few predators
have learned how to prey on them efficiently.
However, there is more to this story than just the
relationship between predator and prey. Hare
populations on arctic islands where there are no
lynx also undergo a cycle, indicating that it is not
simply the effect of predators that causes hare
populations to increase or decrease. An alternative
hypothesis for fluctuations in the size of snowshoe
hare populations is that the grazing activity of a
large number of these herbivores causes serious
damage to the plants (especially willows) that they
eat. When the hare population is small, only a small
portion of each plant is consumed. The plants can
maintain high survival and reproduction rates in
this situation, resulting in high plant density. This, in
turn, allows the hare population to increase, perhaps
to a point where their grazing damages the plants,
thereby lowering plant survival and reproduction.
The hare population will then decline because of a
decrease in their food supply. A long-term research
Continued on page 488
Number of pelts (in thousands)
150
➥
lynx
hare
100
50
0
1845
1855
1865
1875
1885
1895
1905
1915
1925
1935
Figure 14.24 This graph shows the
number of Canada lynx and snowshoe
hare pelts traded annually to the
Hudson’s Bay Company over a 100-year
period in Canada’s arctic (data which
allowed biologists to estimate the
population sizes for both species). Note
that approximately every 10 years,
populations of both species become
very large, decline, and then become
large again. These cycles seem much
too regular to be explained by randomly
occurring abiotic factors.
Chapter 14 Population Ecology • MHR
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Investigation
SKILL FOCUS
1 4 • A
Initiating and planning
Paramecium Populations
Predicting
Cultures of small organisms, such as microscopic paramecia, can be used to study
how various abiotic and biotic factors affect population growth patterns. These
unicellular protozoa are ideal for studying changes in population size because they
have a relatively short reproductive cycle and cultures can be easily maintained,
measured, and manipulated in a laboratory environment. This investigation
focusses on how density-dependent factors and density-independent factors
affect the growth rate of paramecium populations. These observations will allow
you to suggest how abnormal pH levels in natural ecosystems, resulting from
acid precipitation or other types of contamination, might have negative effects
on organisms living in aquatic environments.
Pre-lab Questions
What is the natural food supply of micro-organisms
such as paramecium?
Describe the life cycle of paramecium.
How would lower pH levels affect populations of
organisms that inhabit aquatic ecosystems?
How would changes in food availability affect
populations of organisms that inhabit aquatic
ecosystems?
Problem
How can you determine the impact of density-dependent
factors (such as food supply) and density-independent
factors (such as pH) on the growth rate of paramecium
populations?
Prediction
Predict the effect of changes in pH level and food
supply on the growth rate of paramecium populations.
CAUTION: Handle hydrochloric acid with care.
Wash your hands after each observation.
Materials
glass jars of uniform size
culture medium (skim
milk powder)
distilled water
paramecium cultures
droppers
heating sources
thermometers
dilute hydrochloric acid
pH hydrion papers (or other
indicators that provide
accurate pH readings)
microscopes
methyl cellulose
Procedure
1. For this investigation, your class should be split into
several groups. One half of the class should focus
on testing the effects of a density-dependent limiting
factor (food supply), while the other should
investigate the effects of a density-independent
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MHR • Unit 5 Population Dynamics
Identifying variables
Performing and recording
Analyzing and interpreting
Conducting research
limiting factor (pH). Each student group will run one
control and one or more experimental set-ups.
2. Identify the specific factor or variable (such as pH
or food supply) to be investigated. This will be your
independent (manipulated) variable. Select the range
of factors to be tested. (For example, one group
could maintain one control culture at neutral pH and
an experimental culture at a slightly lower pH while
the other groups maintain cultures at different pH
levels. That way, a broad range of pH levels can
be analyzed.)
3. Maintain all cultures at constant temperature and
at uniform, medium light conditions. Avoid direct
sunlight, drafts, and contamination of cultures.
Ensure that all glassware is clean and
uncontaminated by soap or other chemical residue.
Leave each culture open to the air, but ensure that
water levels remain constant by adding roomtemperature distilled water as required.
4. To start, add about 5 g of skim milk powder to
250 mL of distilled water. Inoculate each set-up with
an identical volume of paramecium culture (about
three or four full pipettes or droppers). The control
set-up for each group should be kept at neutral pH
and be provided with the same food supply and
starter paramecium culture. In fact, you may wish to
maintain each set-up for three to four days with a
standard food supply and neutral pH, to stabilize
each paramecium population before you begin to
manipulate the environments.
5. If you are investigating food supply as a variable,
add different amounts of food to your experimental
set-ups (the class should work with a range of
0–20 g of food per experimental culture).
6. If you are investigating pH as a variable, add varying
quantities of dilute HCl (start with 0.1 mol/L HCl) to
produce a range of pH, from neutral to about pH 3.
Maintain a constant pH in each set-up throughout
the experiment.
7. Use a microscope to view samples of your cultures
each day. Estimate the paramecium population of
each culture. Try the following procedure to view
and count the paramecia in your samples:
(a) Squeeze all the air out of the dropper bulb.
Use the dropper to remove a few drops of
your paramecium culture.
(b) Place a drop of methylcellulose and one or
two drops of water on the centre of a glass
microscope slide. (You can also add a few grains
of very fine, washed sand to slow down the
paramecia and prevent the cover slip from
crushing the specimens in your sample.)
(c) Carefully squeeze the bulb to deposit your
culture sample into this mixture.
(d) Gently lower a cover slip over the sample.
(e) View your sample with the low-power objective
lens of your microscope. Scan the sample and
count the number of paramecia in five different
fields of view (view the centre and the area near
each edge of the cover slip). Add up the total
number of individuals counted in all of the
samples and divide by five to determine an
average estimate of the current population size.
Enter your data in a summary data table.
(f) Use the same procedure to observe samples
daily from each set-up and record your estimates
of population size in the appropriate data table.
8. Ensure that your sampling technique can provide an
accurate estimate of the population size without
unduly disturbing the integrity of the ecosystem (and,
if possible, without causing any changes to the
appearance of individual paramecia in each culture).
9. After the data are entered in your summary table,
graph the results. If available, you may wish to use
a computer spreadsheet program to record your
experimental data and generate graphs. Combine
data from other student groups to generate graphs
depicting the class results for each experimental
variable tested.
Post-lab Questions
1. Describe any observed changes in the paramecium
population size. Graph the control and the
experimental population sizes over the duration
of your experimental procedure.
3. How did your results relate to your original
prediction?
4. Discuss possible sources of error in your procedure
(relating to such factors as the sampling procedures
used and the ability to control any extraneous
variables) that may influence your results.
5. Were your results consistent with the observations
of other groups?
Conclude and Apply
6. What can you conclude about the accuracy of your
original prediction?
7. How could your procedure be improved?
Exploring Further
8. Devise a supervised investigation to study
populations of micro-organisms in actual aquatic
ecosystems, such as ponds or streams. Have your
teacher approve your experimental design and
safety precautions in advance. Test the pH of each
sample. Identify other significant density-dependent
and density-independent regulating factors that may
have an impact on certain species of microscopic
organisms in local aquatic ecosystems (such as
rivers or ponds). Indicate your sample locations
on a local map.
9. Conduct research to assess population changes
through the seasons in the ecosystems you have
chosen.
10. What do you think is the mechanism by which
changes in pH increase or decrease the rate of
population growth? That is, why does the pH of its
environment matter to a paramecium, or to any
other organism? How would an effect on an
individual organism translate into changes in
population growth?
11. Which general type of regulating factor — densitydependent or density-independent — is most
important in regulating the size of paramecium
populations in nature? Or do you think both might
be important, but under different circumstances?
Explain your answer in full.
2. How do the size and growth rate of the control
culture compare with the populations of your
experimental paramecium cultures? Specifically,
what do your results and the class results suggest
about the impact of a density-dependent factor and
a density-independent factor on the rate of growth
of paramecium populations?
Chapter 14 Population Ecology • MHR
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project in the Yukon, led by Charles Krebs of the
University of British Columbia, suggests that both
predation and fluctuations in food supply contribute
to the cycling of hare populations.
Symbiotic Relationships
In some cases, the interactions that occur between
members of two different species are so close that
they are said to have a symbiotic relationship.
One member of the pair in such an association
is referred to as the host and the other as the
symbiont. The host is typically larger and more
independent (that is, it can live on its own) than
the symbiont (which often depends on the host
and the symbiotic relationship for its survival).
There are three general types of symbiotic
relationships. In parasitism, the symbiont is
referred to as a parasite; it benefits from the
relationship, whereas the host is harmed. In terms
of population growth, this relationship is much
like that between a predator and its prey. An
increase in host population density makes it
possible for the parasites to increase in number.
The increase in the number of parasites decreases
host
parasite
800
Table 14.1
Interspecific interactions
400
Population density
10
800
20
40
30
host
parasite
400
50
800
host survival or reproductive ability, and may
subsequently lead to a decrease in host density.
With fewer hosts available, parasite density then
declines. The result may be cyclical fluctuations of
host and parasite population densities, as shown in
Figure 14.25.
A second type of symbiotic relationship is
mutualism, in which both partners benefit from the
relationship and growth in one population typically
causes an increase in population density of the
associated population. This is shown in Table 14.1,
which summarizes the effects of different symbiotic
relationships on population growth. In this table,
a plus sign (+) indicates an increase in population
density and a minus sign (−) indicates a decrease
in density. In mutualism, for example, both species
benefit from the relationship; positive growth in
one population causes positive growth in the other.
Mutualism is common in nature (as shown in
Figure 14.26). For example, termites and various
mammalian herbivores (such as cattle and sheep)
depend on certain types of bacteria living in their
digestive systems to help them digest the plant
material they eat. The bacteria also benefit, since
they are able to live in a protected, nutrient-rich
environment.
60
70
80
Nature of relationship
between populations
Effect of growth in one population
on the other population
competitive
– / – (both are negatively affected)
predator/prey
(includes herbivore/plant)
+ / – (one population gains at the
expense of the other)
host/parasite
– / + (one population gains at the
expense of the other)
mutualistic
+ / + (both are positively affected)
host
parasite
400
90
100
110
Number of generations
Figure 14.25 In a laboratory, populations of an Adzuki bean
weevil (Callosobruchus chinensis) and its wasp parasitoid
(Heterospilus prosopidis) cycled for 112 generations or six
years (until the experiment ended). In this case the wasp is
called a parasitoid because it feeds on living tissue (like a
parasite), but kills the larvae of its host, in the process
making it like a predator in some ways.
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MHR • Unit 5 Population Dynamics
Figure 14.26 Examples of mutualism include the
relationships that exist between many types of plant and
animal species. In this case, the animal gets food (pollen or
nectar) while the plant gets help moving its pollen from plant
to plant for reproduction.
The third type of symbiotic relationship is
commensalism, in which one partner benefits and
the other is unaffected. There are few (if any)
examples of true commensalism in nature, since
it is unlikely that one participant in any type of
ecological interaction will be unaffected. Barnacles
that “hitchhike” by attaching themselves to whales
are sometimes considered to be the partner that
benefits in a commensal relationship, while the
whale is thought to be unaffected. However,
because barnacles may slightly decrease a whale’s
ability to find food or escape from predators, they
may actually lower the whale’s reproductive
success or chance of survival (and thus reduce the
density of whale populations). Other often-cited
examples of commensalism are the relationships
between cowbirds or cattle egrets and the cattle
they associate with (see Figure 14.27).
ELECTRONIC LEARNING PARTNER
Use the Electronic Learning Partner to enhance your
understanding of symbiotic relationships, including
parasitism, mutualism, and commensalism.
As you might predict, populations that are
regulated by density-dependent factors tend to be
more predictable than those regulated by weather,
fire, and other abiotic mechanisms. Ecologists and
wildlife managers make use of this predictability in
various ways, as demonstrated in the Thinking Lab
on page 490.
BIO FACT
Forestry and farming depend on a mutualistic relationship
between fungi and plant roots. A fungus’s body consists of
many thread-like extensions, which maximize the amount of
surface area exposed to its food source. This is important
since a fungus absorbs the nutrients it needs over its body
surface. When the threads of certain fungal species become
intertwined with a plant’s roots (these associations of roots
and fungal threads are called mycorrhizae), they help the
plant absorb far more water and nutrients than it could on
its own. The fungus also benefits, getting a small portion
of the photosynthetic products the plant makes with these
materials. One of the problems arising from clearcutting is
that removal of all the trees from an area often causes the
death of the soil fungi. This removal of soil fungi, in turn,
may reduce the ability of newly planted seedlings to survive
and grow.
Population Regulation:
Many Factors at Work
One way to think about the many factors involved
in the growth and regulation of a population’s size
is shown in Figure 14.28. The line labelled (a)
shows the biotic potential of a population — how it
would grow if it could realize its full potential for
growth. The jagged line labelled (b) shows the
regulating effects that the environment has on the
population’s growth. In this case, abiotic factors
produce the sudden dramatic declines in the
curve, while biotic factors produce less dramatic
declines and hold the population roughly to the
environment’s carrying capacity. Since both the
(b) environmental
resistance
Population size
(a) biotic
potential
Figure 14.27 Is this relationship commensal? The birds
benefit because they feed on insects flushed out of the
grass by the cows while they graze. Although the cattle are
mostly unaffected by the presence of the birds, they may
obtain some benefit since the birds sometimes eat ticks
and other parasites they find on the cows’ skin.
Time
Figure 14.28 The relationship between a population’s biotic
potential and the environmental resistance to its growth
Chapter 14 Population Ecology • MHR
489
abiotic and biotic factors are part of the population’s
environment, the combination of their effects is
sometimes referred to as environmental resistance
to population growth.
The examples discussed under “Symbiotic
Relationships” might suggest that populations of
some kinds of organisms (such as insects) are
regulated by abiotic, density-independent factors,
whereas populations of other kinds of organisms
(including birds and mammals) are regulated by
biotic, density-dependent mechanisms. This
apparent contrast led early ecologists to argue
about which form of population regulation was
THINKING
LAB
Sustainable Harvesting
You Try It
Background
Theoretically, populations regulated by density-dependent
factors can be harvested. In other words, individuals can be
removed for commercial and/or recreational purposes in
such a way that the population is not depleted. This will be
the case if the number of individuals being harvested
(referred to as the yield of the population) is just equal to
the number being added as a result of reproduction. One of
the tasks of those who manage populations of organisms
that humans use as resources (for example, forests; fish
stocks; and deer, moose, and bear populations) is to try to
determine the Maximum Sustainable Yield (MSY). MSY is
the largest number of individuals that can be removed from
the population each year (or other harvest period) without
reducing the population’s growth over the long term. In this
lab, you will see how the MSY can be assessed — in theory.
500
N0 = 5, K = 500, r = 0.2
Population size (N)
400
200
100
10
20
30
40
50
Time
Growth curve for a theoretical, density-dependent population
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1. The graph shows the growth curve for a theoretical,
density-dependent population living in an environment
in which the carrying capacity is 500 (K = 500). The
population started out with five individuals (N0 = 5) and
grew at a rate of 0.2 offspring per individual per time
period (r = 0.2). Using the curve, estimate the number
of individuals added to the population in the first five
time units. Estimate the size of the population in the
middle of this time interval (that is, at time = 2.5 units).
Follow the same procedure for intervals 5–10, 10–15,
15–20, and so on up to 45–50. Record your data in a
table, or use a spreadsheet program if available.
2. Plot the data you obtained in Step 1 for each of the
10 time intervals on a graph of “Number of individuals
added to the population” (y-axis) versus “Population
size” (x-axis). How big is the population during the
interval when the maximum number of individuals is
being added? Is N at this time near K, far below it, or
halfway to K? Explain the shape of the curve you have
plotted. Why does the curve increase to a maximum
and then decrease?
3. Suppose you are the manager of this population
and you are trying to determine its MSY. At what size
would you want to maintain the population, and why?
How would an increase in the population’s size above
the MSY point affect yield? How would a decrease
affect yield?
300
0
more important, or at least more common. These
discussions, along with more research, eventually
led to the understanding that for most populations,
both types of regulating factors play a role. For
example, most of the time a population of birds
may be regulated by density-dependent factors.
However, occasional catastrophes may reduce the
population to a point where density has little
influence on the growth rate. Similarly, severe
weather may regularly cause the size of an insect
population to decline. If the environment contains
a limited number of protective sites where members
of this population can hide to avoid the weather,
MHR • Unit 5 Population Dynamics
4. Although MSY values have often been used to set
harvest quotas (such as the number of fish that can be
caught or deer that can be hunted), the system has not
worked well in some situations. Many populations have
been seriously depleted as a result of overfishing or
excessive hunting, in part because quotas were set
too high. Why do you think this might have happened?
What problems can you see occurring as a result of
using this method to set quotas?
density does play a role. The amount by which N
exceeds the number of hiding places will affect the
proportion of the population killed. In fact, most
populations are probably regulated by a
combination of density-dependent and densityindependent factors.
SECTION
The abiotic and biotic factors that regulate
population size also influence other characteristics
of populations. These characteristics will be
described in the last section of this chapter.
REVIEW
affected? In what way might your answer to this last
question change your interpretation of the type of
relationship exhibited here?
1.
MC What factors might eventually limit the
exponential growth rates of a particular insect species
found in such regions as the Canadian prairies?
2.
K/U What might allow some animal populations to
grow beyond the theoretical carrying capacity of their
environment and then crash to abnormally low levels?
10.
K/U What is the difference between the way in
which abiotic and biotic factors typically produce
environmental resistance to population growth?
3.
K/U What factors might limit the population size of
animals (such as vultures) that live as scavengers?
11.
4.
K/U In a density-dependent population, why does
population growth decline as population density
increases?
5.
MC Provide a real-life example of a situation in which
interspecific competition limits the population size
and growth of a particular species.
6.
K/U What combination of factors might produce
regular population cycles typical of small herbivore
species such as mice and squirrels?
MC Roosevelt elk (Cervus elaphus roosevelti) were
re-introduced to the Sunshine Coast region of British
Columbia because overhunting eliminated the original
elk population many years ago. Their population has
now grown to about 250, and some local residents
are complaining that the elk are devouring plants in
gardens, parks, and golf courses. Limited hunting of
this population is now permitted in an attempt to
control the problem. Do you think hunting of this elk
population should be permitted? How do you think
the resumption of hunting might affect the long-term
growth patterns of this population and other species
that share the same environment as the elk?
7.
I Rat populations in many urban areas in Canada
have increased dramatically in recent years. Use the
Internet to research public health records and other
web sites for data about the extent of this problem.
Describe the factors that seem to be contributing to
the growth in local rat populations and the factors
that may ultimately limit their population size.
12.
MC In ecological terms, what measures would be
most effective in controlling populations of pest
species such as rats, mice, or certain types of insects?
13.
I Having been recently appointed regional wildlife
manager, you must set a quota on the number of
moose that can be hunted during the next hunting
season. What information do you need? Design a
study that will allow you to obtain the necessary data.
14.
C You have just been hired to teach Grade 6 at
a local school. Design a lesson plan, including an
activity, that you might use to teach your students
about the factors that affect population growth in
different environments.
15.
C Although all populations eventually face
environmental resistance to continued growth, the
contribution of abiotic and biotic factors to this
resistance may vary from species to species.
Compare a micro-organism, such as E. coli, a plant
(such as a type of tree), and a mammal, such as the
snowshoe hare or the black bear, with respect to the
type of factors that typically limit the growth of
populations of each species.
8.
9.
MC Biologists have observed increasing instances
of coral bleaching in recent years. This phenomenon
involves the death of small green algae that live in the
tissues of coral polyps (the tiny animals whose bodies
secrete much of the material that forms coral reefs).
Following the death of the algae, the coral organisms
that hosted them also die. Describe the probable
type of relationship that exists between the algae
and coral organisms.
K/U Some species of sharks are often seen with
small fish called remoras attached to their bellies. The
remoras seem to feed off the scraps of food dropped
by the sharks when they are feeding on prey. The
sharks seem to derive no benefit from the presence
of the remoras. What type of symbiotic relationship
might the association of the shark and remora
illustrate? Do you think it likely that the shark is totally
unaffected by the remora? In what way might it be
Chapter 14 Population Ecology • MHR
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