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Chapter 27
Population Growth
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc..
27.1 How Are Populations Distributed In
Space And Time?
 Organisms arrange themselves in space in
many different ways.
 Ecologists recognize three major types of
spatial distribution:
• _________
• _________
• _________
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27.1 How Are Populations Distributed In
Space And Time?
 Individuals in many populations clump together in groups,
and include social groups such as elephant herds, wolf
packs, lion prides, flocks of birds, and schools of fish.
 What are the advantages of
clumping?
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• Many eyes can search for
localized food.
• Part of a large group may
reduce the odds that one
individual will be killed by a
predator.
• A group may increase the
individual’s chance of finding
Fig. 27-1a
a mate.
27.1 How Are Populations Distributed In
Space And Time?
 Some individuals disperse themselves evenly.
• Populations with uniform distributions maintain relatively
even spacing between individuals.
• This type of distribution occurs among animals that defend
territories.
• Male Galapagos iguanas establish regularly spaced breeding
territories.
 Creosote bushes release
chemicals into the soil around
them that inhibit germination of
seeds from other plants.
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27.1 How Are Populations Distributed In
Space And Time?
 In a few populations, individuals are distributed at random.
• In a population with random distribution, the distance
between individuals varies unpredictably.
• Individuals in such populations do not form social groups.
• The resources they need are not in short supply and are
available throughout the year.
 Trees and other plants in rain forests
may be randomly distributed.
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27.2 How Do Populations Grow?
 Births, deaths, and migration determine population growth.
• A population’s size remains stable if, on average, as many
individuals join as leave.
• Individuals enter a population by ________ or
______________, and leave it by _________ or
_______________.
• A population grows when the number of births plus
immigrants exceeds the number of deaths plus emigrants.
• Populations shrink when the opposite occurs.
 A simple equation for the change in population size within a
given period is as follows:
• (births – deaths) + (immigrants – emigrants) = change in
population size
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27.2 How Do Populations Grow?
 Population growth can be expressed as a rate.
• The per capita growth rate (r) of a population is measure of how
fast a population grows, expressed as a change in population size
per individual per unit of time (that is, as a percentage).
• This value is determined by subtracting the per capita death rate
(d) from the per capita birth rate (b).
• b (births) – d (deaths) = r (growth rate)
• For example, to calculate the annual growth rate of a human
population of 10,000 in which there are 1,500 births and 500
deaths each year, we first calculate the annual per capita birth
and death rates.
• b = 1,500 births/10,000 people = 0.15 births per person per year
• d = 500 deaths/10,000 people = 0.05 deaths per person per year
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27.2 How Do Populations Grow?
• Finally, we calculate the per capita growth rate by subtracting the
death rate from the birth rate.
• r = 0.15 births per person per year – 0.05 deaths per person per year
= 0.10 (that is, 10% increase per year)
• If the death rate exceeds the birth rate, the growth rate will be
negative and the population will shrink.
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27.2 How Do Populations Grow?
 A constant growth rate increases population size rapidly.
• To calculate the number of individuals added to a population
in a year, multiply the annual per capita growth rate (r) by the
original population size (N).
• Population growth = rN
• In our example, population growth in the first year (rN) is 0.10 x
10,000 = 1,000 people; at the end of year 1, the population has
11,000 people.
• If the per capita growth rate r remains constant, the number of
people added to the population increases each year.
• This pattern of continuously accelerating increase in population
size is exponential growth.
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27.2 How Do Populations Grow?
 A population grows during periods when births exceed deaths.
 Growth will persist if, on average, each individual produces more than
one surviving offspring during its lifetime.
 Each individual, of course, has the potential to replace itself many times
during its lifetime.
 Every species has a built-in capacity for population growth, but the speed
of this potential growth varies among species, dependent upon the
following factors.
• The age of first reproduction
• The frequency of reproduction
• The average number of offspring produced each time
• The length of an organism’s reproductive life span
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Pearson Education
Inc.
• The
organism
death rate
27.3 How Is Population Growth Regulated?
 A population’s growth is influenced by its __________
potential: the maximum rate at which a population can increase,
assuming ideal conditions that allow the highest possible birth
rate and the lowest possible death rate.
• The ultimate size of a population is also affected by limits that
oppose this potential for growth.
• These limits, which are set by the living and nonliving
environments, are collectively known as _____________
resistance.
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27.3 How Is Population Growth Regulated?
 Environmental resistance is imposed by several things.
•
•
•
•
•
Availability of food and space
Interactions with competitors
Predators
Disease-causing organisms
Natural catastrophes (e.g., storms, fires, freezing weather, floods,
and droughts)
 Environmental resistance limits population size by the following
factors:
• Increasing death rates
• Decreasing birth rates
• Both factors together
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27.3 How Is Population Growth Regulated?
 Some populations fluctuate cyclically.
population density
• Short-term explosive growth occurs in populations that undergo
regular cycles of rapid population growth followed by a sudden,
massive die-off—a boom-and-bust cycle.
Nutrients are depleted
Favorable growth
conditions occur
“boom”
“bust”
Jul
Nov
Sep
month
0
Jan
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Mar
May
27.3 How Is Population Growth Regulated?
 Ecosystem changes may allow temporary rapid growth.
• A population may grow rapidly if population- controlling factors,
such as __________, are eliminated or if the ________ supply is
increased.
• Growth can also be explosive when individuals invade a new
habitat that has favorable conditions and few competitors.
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27.3 How Is Population Growth Regulated?
 Environmental resistance limits population growth.
• Populations that grow rapidly must eventually stabilize or crash.
• They tend to stabilize at or below their ecosystem’s carrying
capacity, the maximum population of a particular species that an
ecosystem can support indefinitely.
• As a growing population approaches the carrying capacity of
the environment, its growth rate gradually declines and
finally stops when the population reaches a state of
___________.
• In this equilibrium, the birth rate is balanced by the death rate, and
population size is stable.
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27.3 How Is Population Growth Regulated?
• The factors that usually maintain populations at or
below the carrying capacity of their environment can be
classified into two broad categories.
• ___________-_____________factors limit population size
regardless of the population density (number of
individuals per given area).
• ___________-_____________factors increase in
effectiveness as the population density increases.
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27.3 How Is Population Growth Regulated?
 Density-independent factors limit population size.
• The most important natural density-independent factor is
___________.
• hurricanes, droughts, floods, and fire
• Human activities, such as the use of __________ and
___________, can also limit population growth in ways
independent of population density.
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27.3 How Is Population Growth Regulated?
 Density-dependent factors have a greater effect as
population density increases.
• Density-dependent factors become increasingly effective as
population density increases, thus exerting negative feedback that
limits the size of populations.
• Conversely, density-dependent factors become less effective
as population density decreases, allowing population size to
stabilize or grow.
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27.3 How Is Population Growth Regulated?
• The most important density-dependent factors are predation and
competition.
• In predation, one organism feeds on another, harming it in the
process.
• Often, one organism (the __________) kills another (its ______)
in order to eat it, but the prey is not always killed.
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27.3 How Is Population Growth Regulated?
• Predation plays an increasingly important role in population
control as prey populations increase.
• The increased frequency of encounters with predators
increases the prey populations’ death rate, because
predators tend to eat larger numbers of whichever prey
species is most abundant and easiest to find.
• Increasing numbers of prey organisms also increase the
numbers of predators, and this tends to control prey
numbers.
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27.3 How Is Population Growth Regulated?
• Prey also often survive the special form of predation known
as ___________.
• The predator (the parasite) lives on or inside another
organism (its _______) and feeds on the its body without
killing it—at least not immediately.
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27.3 How Is Population Growth Regulated?
 Parasites spread faster when the host population density is
high.
• Like other forms of predation, parasitism is densitydependent.
• Most parasites have limited ability to move, so they spread
more readily from host to host at high host-population
densities.
• Parasites can affect the death rate of a host population
because the damage inflicted by the parasite on its host’s
body may kill the host.
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27.3 How Is Population Growth Regulated?
 Populations can soar or crash when predator-prey relationships
are disrupted.
• The population balance in ecosystems can be disrupted
when predators are introduced to regions in which they were
not previously present and where prey species have had no
opportunity to evolve defenses against them.
• Rats, snakes, and mongooses were introduced in Hawaii and
many other Pacific islands, and have drastically reduced or
exterminated many native bird populations.
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27.3 How Is Population Growth Regulated?
• Prey populations can grow out of control when introduced to areas
where they have no predators.
• The prickly pear cactus was introduced into Australia from Latin
America, and, lacking predators, it spread uncontrollably.
• In the 1920s, a cactus moth was imported from Argentina to feed
on the cacti.
• Within a few years, the cacti were almost eliminated, and today,
the moth keeps the population density of its prey very low.
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27.3 How Is Population Growth Regulated?
 Competition for resources helps curb population size.
• The resources that determine the carrying capacity are
limited.
• Therefore, organisms must compete for access to resources,
and the competition among individuals limits population size
in a density-dependent manner.
• There are two forms of competition: ____________
competition among individuals of different species, and
___________ competition among individuals of the same
species.
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27.3 How Is Population Growth Regulated?
 Competition may be indirect or direct.
• Some organisms, including most plants and
many insects, engage in scramble
competition, a kind of free-for-all in which
individuals independently seek resources
without interacting directly.
• Many animals have evolved contest
competition, in which individuals interact to
contest access to important resources.
• Only the best competitors are able to defend
adequate territories; poor competitors fail to
secure territories and are unlikely to
reproduce or survive.
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27.3 How Is Population Growth Regulated?
 Different age groups may experience different death rates.
• The effects of population-limiting factors do not necessarily fall
equally on all members of a population.
• For example, the death rate in a population differs among
individuals of different ages; survivorship varies with age.
• A survivorship curve shows the likelihood that an individual
will survive to a given age.
number of survivors
1,000
100
late loss
(human)
constant loss
(American robin)
10
early loss
(dandelion)
0
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age
(in percentage of maximum life span)
27.3 How Is Population Growth Regulated?
 Survivorship in populations follows three basic patterns.
• Convex late-loss survivorship curves reflect populations in
which the death rate is low for juveniles and in which most
individuals survive to old age.
• These curves are characteristic of humans and other large, long-lived
animals, such as elephants and mountain sheep.
• These species produce relatively few offspring that receive a
great deal of parental care.
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27.3 How Is Population Growth Regulated?
• Early-loss survivorship produces a concave curve and is
characteristic of organisms that produce large numbers of
offspring that receive little or no parental care.
• The death rate of juveniles is very high, but individuals than manage
to reach adulthood have a good chance of surviving to old age.
• Early-loss survivorship curves are common among invertebrates,
plants, and fish.
• Populations with straight-line, constant-loss survivorship
curves have a fairly constant death rate that does not vary
much with age.
• This pattern has been found in some bird and lizard species, and in
populations of some asexually-reproducing invertebrate species.
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