Download continued - Linn-Benton Community College

BIOLOGY Life on Earth
WITH PHYSIOLOGY Tenth Edition
Audesirk Audesirk Byers
26
Population Growth
and Regulation
Lecture Presentations by
Carol R. Anderson
Westwood College, River Oaks Campus
© 2014 Pearson Education, Inc.
Chapter 26 At a Glance
 26.1 How Does Population Size Change?
 26.2 How Is Population Growth Regulated?
 26.3 How Are Populations Distributed in Space
and Age?
 26.4 How Is the Human Population Changing?
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 A population consists of all the members of a
particular species that live within an ecosystem, a
defined geographical area
 Each population forms an integral part of a larger
community, defined as a group of interacting
populations
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 The biosphere is the enormous ecosystem that
encompasses all of Earth’s habitable surface
 Ecology is the study of the interrelationships of
organisms with each other and with the nonliving
environment
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Changes in population size result from natural
increases and net migration
– Population size changes through
– Births
– Deaths
– Net migration
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Changes in population size result from natural
increases and net migration (continued)
– The natural increase of a population is the difference
between births and deaths
– Natural “increase” can be negative (decrease) if deaths
exceed births
– The net migration of a population is the difference
between immigration (migration into the population)
and emigration (migration out)
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Changes in population size result from natural
increases and net migration (continued)
– A population thus grows when the sum of natural
increase and net migration is positive and declines
when this sum is negative
– A simple equation for the change is
– Change in population size  natural increase  net
migration
(births  deaths) (immigration 
emigration)
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size
– The size of most natural populations of organisms
fluctuates over the course of a year because
reproduction tends to be seasonal
– Growing populations add individuals in proportion to
the population’s size, much like a bank account
accumulates compound interest
– If conditions are the same, a population will grow at a
constant percentage of its size over a given interval
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– The growth rate (r) of a population is the percentage
change in the population size per unit time
– The population growth rate is the birth rate (b) minus
its death rate (d)
– r (growth rate)  b (birth rate)  d (death rate)
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If the birth rate exceeds the death rate, the population
growth rate will be positive and population size will
increase
– If the death rate exceeds the birth rate, the growth
rate will be negative and the population size will
decrease
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– Population growth (G), which is the number of
individuals added to a population in a given time
period, can be calculated by multiplying growth rate (r)
by the original population size (N)
– Population growth (G)  r (growth rate) × N (population
size)
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
– A constant growth rate (r) produces exponential growth
– During exponential growth, an ever-larger number is
added to the population during each succeeding time
period
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
– This pattern of growth will occur in any population in
which each individual, on average over the course of its
life span, produces more than one offspring that
survives to reproduce
– If the size of an exponentially growing population is
graphed against time, a characteristic shape called a Jcurve will be produced
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
(continued)
– The age at which an organism first reproduces affects
the size of the future population
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
(continued)
– For example, consider two populations of golden
eagles that are followed for 30 years
– Individuals in one population begin reproducing at the
age of 4 years
– Individuals in the other population begin reproducing at
age 6 years
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
(continued)
– Both populations will follow a J-shaped population
growth curve, but more individuals will be added to the
earlier reproducing population, resulting in a steeper
increase in population numbers
– At 30 years, the earlier reproducing population would
be 10 times the size of the other population
© 2014 Pearson Education, Inc.
Figure 26-1 Exponential growth curves are J-shaped
2,600
reproduce at 4 years (pop. 1)
reproduce at 6 years (pop. 2)
2,400
2,200
At 24 years,
this population
has 2,504 eagles
2,000
number of eagles
Number Number
of
of
Time eagles eagles
(years) (pop. 1) (pop. 2)
0
2
2
6
8
4
1,400
12
52
18
1,200
18
362
86
1,000
24
2,504
392
800
30
17,314
1,764
1,800
1,600
600
At 24 years,
this population
has 392 eagles
400
200
0
5
© 2014 Pearson Education, Inc.
10
15
20
time (years)
25
30
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
(continued)
– As long as birth rate exceeds death rate, population
size will follow a J-shaped rate of increase
– However, the time for each population to reach a
specific number of individuals will depend on the
magnitude of the death rate
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Populations grow based on the birth rate, the death
rate, and the population size (continued)
– If births exceed deaths, exponential growth occurs
(continued)
– The death rate has a major impact on population size
– As long as births exceed deaths, the population
eventually becomes enormous
– As the death rate increases, it takes longer to reach any
given population size
© 2014 Pearson Education, Inc.
Figure 26-2 The effect of death rates on population growth
2,500
no deaths
10% death rate
25% death rate
number of bacteria
2,000
It takes about 4
hours to produce
1,500 bacteria
It takes
about 5.5
hours to
produce
1,500
bacteria
1,500
It takes about 3.5
hours to produce
1,500 bacteria
1,000
500
0
1
© 2014 Pearson Education, Inc.
2
3
4
time (hours)
5
6
26.1 How Does Population Size Change?
 Biotic potential determines the maximum rate at
which a population can grow
– The ability to produce many offspring is an inherited
attribute
– Natural selection favors organisms whose attributes
adapt them to their environments and who pass these
adaptations on to as many healthy offspring as
possible
– Biotic potential refers to the maximum rate at which
a particular population could increase
© 2014 Pearson Education, Inc.
26.1 How Does Population Size Change?
 Biotic potential determines the maximum rate at
which a population can grow (continued)
– Several factors influence biotic potential
– The age at which the organism first reproduces
– The frequency of reproduction
– The average number of offspring produced each time
– The length of the organism’s reproductive life span
– The death rate of individuals
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 In 1859, Charles Darwin wrote: “There is no
exception to the rule that every organic being
naturally increases at so high a rate, that if not
destroyed, the Earth would soon be covered by the
progeny of a pair.”
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Population size results from the interaction between
biotic potential and environmental resistance, or
all the curbs on population growth imposed by the
living and nonliving environment
– Examples include interactions among organisms such
as predation and competition for limited resources
– Environmental resistance also includes natural events
such as freezing weather, storms, fires, floods, and
droughts
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions
– Under unusual and temporary circumstances, natural
populations exhibit exponential growth, producing Jshaped growth curves
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs in populations with boomand-bust cycles
– Exponential growth can be observed in populations that
undergo boom-and-bust cycles, in which periods of
rapid population growth are followed by a sudden,
massive die-off
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs in populations with boomand-bust cycles (continued)
– Seasonal populations are linked to changes in rainfall,
temperature, or nutrient availability
– Ideal conditions encourage rapid growth; deteriorating
conditions encourage massive die-off
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs in populations with boomand-bust cycles (continued)
– For example, each year, photosynthetic bacteria in a
lake may exhibit exponential growth when conditions
are ideal, but crash when they have depleted their
nutrient supply
© 2014 Pearson Education, Inc.
population density
Figure 26-3a A boom-and-bust cycle in photosynthetic bacteria
Nutrients are depleted, and
water temperature falls
Favorable growth “boom”
conditions occur
Jan
Mar
May
Jul
month
“bust”
Sep
Nov
A boom-and-bust cycle in photosynthetic bacteria
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs in populations with boomand-bust cycles (continued)
– Complex factors produce four-year cycles for small
rodents, such as lemmings
– Lemming populations may grow until lack of food, large
migrations, and predators and starvation cause sudden
high mortality
© 2014 Pearson Education, Inc.
number per 100 trap nights
Figure 26-3b Boom-and-bust cycles in a lemming population in the Canadian Arctic
14
12
10
8
6
4
2
0
1985
1990
1995
2000
year
Boom-and-bust cycles in a lemming population in
the Canadian Arctic.
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs temporarily when
environmental resistance is reduced
– In populations that do not experience boom-and-bust
cycles, exponential growth may occur temporarily under
special circumstances such as
– An increase of food supply or habitat
– A reduction in predation
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs temporarily when
environmental resistance is reduced (continued)
– For example, the whooping crane population has grown
exponentially since they were first protected from
hunting and human disturbance in 1940
– The whooping crane remains among the world’s rarest
birds, so continued population growth will be necessary
for its survival
© 2014 Pearson Education, Inc.
number of cranes
Figure 26-4 Exponential growth of wild whooping cranes
© 2014 Pearson Education, Inc.
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0
1940 1950 1960 1970 1980 1990 2000 2010
year
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs temporarily when
environmental resistance is reduced (continued)
– Exponential growth can occur when individuals invade
a new habitat with little competition
– Invasive species are organisms with a high biotic
potential that are introduced into ecosystems where
they did not evolve and where they encounter little
environmental resistance
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under unusual
conditions (continued)
– Exponential growth occurs temporarily when
environmental resistance is reduced (continued)
– When they are introduced into a new ecosystem,
population numbers may explode due to a lack of
natural predators
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
– Many populations that exhibit exponential growth
eventually stabilize to match the resources available
to support them
– As resources become depleted, reproduction slows and
the growth rate eventually drops to zero, causing the
population size to remain constant
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance
– This growth pattern, where populations increase to the
maximum number sustainable by their environment and
then stabilize, is called logistic population growth
– The maximum population size that can be sustained by
an ecosystem for an extended time without damage to
the ecosystem is called its carrying capacity (K)
© 2014 Pearson Education, Inc.
Figure 26-5a An S-shaped growth curve stabilizes at carrying capacity
number of individuals
carrying capacity
Growth
rate slows
Growth stops and the
population stabilizes close
to the carrying capacity
Population
grows rapidly
0
time
An S-shaped growth curve stabilizes at carrying capacity
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– When logistic growth is plotted, it results in an Sshaped growth curve, or S-curve
– In nature, an increase in population size (N) above
carrying capacity (K) can be sustained for a short time
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Animation: Population Growth and Regulation
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– If a population far exceeds the carrying capacity of its
environment, excess demands placed on the
ecosystem are likely to destroy crucial resources
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– This can permanently and severely reduce carrying
capacity, causing the population to decline to a fraction
of its former size or disappear entirely
© 2014 Pearson Education, Inc.
Figure 26-5b Consequences of exceeding carrying capacity
The population
overshoots its
carrying
capacity; the
environment
is damaged
carrying capacity
(original)
Low damage; resources
recover, and the
population fluctuates
carrying capacity
(reduced)
Extreme
damage; the
population
dies out
High damage; the
carrying capacity is
permanently lowered
0
time
Consequences of exceeding carrying capacity
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– For example, when reindeer were introduced onto an
island with no large predators, their population
increased rapidly, seriously overgrazing the vegetation
they relied on for food
– As a result, the reindeer population plummeted
© 2014 Pearson Education, Inc.
Figure 26-6 The effects of exceeding carrying capacity
2,000
number of reindeer
1,600
1,200
exponential
growth
population
crash
800
400
0
1910
© 2014 Pearson Education, Inc.
1920
1930
year
*Data not taken for 1943–1946
1940
1950
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– Logistic population growth can occur in nature when a
species moves into a new habitat
– For example, new barnacle settlers along a rocky coast
may find ideal conditions that allow their population to
grow exponentially
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– Logistic population growth can occur in nature when a
species moves into a new habitat (continued)
– As population density increases, however, individuals
begin to compete for space, energy, and nutrients
© 2014 Pearson Education, Inc.
number of barnacles (per cm2)
Figure 26-7 A logistic curve in nature
© 2014 Pearson Education, Inc.
80
60
40
20
0
1
4
2
3
time (weeks)
5
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– Two forms of environmental resistance usually maintain
populations at or below the carrying capacity of their
environment
– Density-independent
– Density-dependent
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Logistic growth occurs when new populations stabilize
as a result of environmental resistance (continued)
– Density-independent factors limit population size
regardless of the population density
– Density-dependent factors increase in effectiveness
as the population density increases
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent factors limit populations
regardless of their density
– The most important natural density-independent factors
are climate and weather, which are responsible for
most boom-and-bust population cycles
– Many insects and annual plant populations are limited in
size by the number of individuals that can be produced
before the first hard freeze
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent factors limit populations
regardless of their density (continued)
– The most important natural density-independent factors
are climate and weather, which are responsible for
most boom-and-bust population cycles (continued)
– Hurricanes, droughts, floods, and fire can have profound
effects on local population, regardless of density
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent factors limit populations
regardless of their density (continued)
– Human activities can also limit the growth of natural
populations
– Pesticides and pollutants can cause drastic declines in
natural populations
– Overhunting has driven some species to extinction
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent factors limit populations
regardless of their density (continued)
– Habitat destruction by humans, a density-independent
factor, is the single greatest threat to wildlife worldwide
– The ivory-billed woodpecker has been driven to
extinction in the United States
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Populations of organisms with a life span of more than
a year have evolved adaptations that allow them to
survive density-independent controls imposed by
seasonal changes, such as cold and lack of food during
the winter
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Many mammals develop thick coats and store fat for
the winter
– Some mammals hibernate
– Migration is another coping mechanism
– Many birds migrate long distances to find food and a
hospitable climate
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Most trees and bushes survive the rigors of winter by
entering a period of dormancy
– For long-lived species in undisturbed habitats, the most
important elements of environmental resistance are
density dependent
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Density-dependent factors exert a negative feedback
effect on population size, because they become
increasingly effective as the population density
increases
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations
– Predators are organisms that eat other organisms,
called their prey
– Prey are killed directly and eaten, but not always
© 2014 Pearson Education, Inc.
Figure 26-8a Predators often kill weakened prey
Predators often kill weakened
prey
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations (continued)
– Predation becomes important as prey populations grow
because predators eat a variety of prey, depending on
what is most abundant and easiest to find
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations (continued)
– Predator populations often grow as their prey becomes
more abundant, which makes them even more effective
as control agents
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations (continued)
– For example, snowy owls hatch up to 12 chicks when
lemmings (their prey) are abundant, but may not
reproduce at all in years when the lemming population
has crashed
© 2014 Pearson Education, Inc.
Figure 26-8b Predator populations often increase when prey are abundant
Predator populations often
increase when prey are abundant
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations (continued)
– Some predator-prey population cycles are out-of-phase
when predators cause a dramatic decline in prey
populations, which in turn results in a decline in the
predator population at a future date
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Predators exert density-dependent controls on
populations (continued)
– Predation may maintain healthy prey populations near a
density that can be sustained by the resources of the
ecosystem
© 2014 Pearson Education, Inc.
Figure 26-9 Experimental predator–prey cycles
adult population
1,600
1,200
bean weevils (prey)
braconid wasp (predator)
A high predator
population
reduces the prey
population
The prey population
peaks when the
predator population
is low
800
400
0
5
© 2014 Pearson Education, Inc.
10
15
generation
20
25
30
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Parasites spread more rapidly among dense
populations
– A parasite feeds on a larger organism, its host,
harming it
– Parasites include tapeworms that live in the intestines of
mammals, ticks that cling to the host’s skin, and diseasecausing microorganisms
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Parasites spread more rapidly among dense
populations (continued)
– Parasites influence population size by weakening their
hosts and making them more susceptible to death from
other causes, such as harsh weather or predators
– Organisms weakened by parasites are less likely to
reproduce
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Parasites spread more rapidly among dense
populations (continued)
– Parasites, like predators, more often contribute to the
death of less-fit individuals, producing a balance in which
the host population is regulated but not eliminated
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
– Competition is the interaction among individuals who
attempt to use the same limited resource, which limits
population size in a density-dependent manner
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– There are two major forms of competition
– Interspecific competition, between individuals of
different species
– Intraspecific competition, between individuals of
the same species
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– Because the needs of members of the same species for
resources are almost identical, intraspecific competition
is an important density-dependent mechanism of
population control
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– Most plants and many insects engage in scramble
competition—a free-for-all scramble as individuals try to
beat others to a limited pool of resources
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– For example, gypsy moth females each lay a mass of up
to 1,000 eggs on tree trunks in eastern North America
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– As the eggs hatch, armies of caterpillars crawl up the
tree
– Huge outbreaks of this invasive species can
completely strip large trees of their leaves in a few
days
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– Competition for food may be so great that most of the
caterpillars die before they can metamorphose into egglaying moths
© 2014 Pearson Education, Inc.
Figure 26-10 Scramble competition
Gypsy moths laying eggs
© 2014 Pearson Education, Inc.
Gypsy moths caterpillars
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– Many animals have evolved contest competitions,
where social or chemical interactions determine access
to important resources
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– Territorial species—such as wolves, fish, rabbits, and
songbirds—defend areas that contain important
resources
– Only the best adapted individuals are able to defend their
territories that supply adequate food and shelter
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-dependent factors become more effective as
population density increases (continued)
– Competition for resources helps control populations
(continued)
– As population densities increase and competition
becomes more intense, some animals react by
emigrating
© 2014 Pearson Education, Inc.
Figure 26-11 Emigration
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent and density-dependent factors
interact to regulate population size
– The size of a population at any given time is the result
of complex interactions between density-independent
and density-dependent forms of environmental
resistance
© 2014 Pearson Education, Inc.
26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– Density-independent and density-dependent factors
interact to regulate population size (continued)
– For example, a caribou weakened by hunger (densitydependent) and attacked by parasites (densitydependent) is more likely to be killed by an
exceptionally cold winter (density-independent)
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations of different types of organisms show
characteristic spacing of their members, determined
by their behavioral characteristics and their
environments
 Each population exhibits patterns of reproduction
and survival that are characteristic of its species
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
– Spatial distribution describes how individuals within a
population are distributed within a given area
– Spatial distribution may vary with time, changing with
the breeding seasons
– Ecologists recognize three major types of spatial
distribution:
– Clumped
– Uniform
– Random
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– Populations whose members live in groups exhibit
clumped distribution
– Examples include elephant herds, wolf packs, prides of
lions, flocks of birds, and schools of fish
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– Advantages of clumped distributions include
– Many eyes that can search for localized food sources
– Movement of the group (e.g., schools of fish or flocks of
birds) can confuse predators by their sheer numbers
– Predators, in turn, may hunt in groups, cooperating to
bring down larger prey
© 2014 Pearson Education, Inc.
Figure 26-12a Clumped distribution
clumped
Clumped distribution
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– Organisms with a uniform distribution maintain a
relatively constant distance between individuals
– This is common among territorial animals defending
scarce resources or breeding territories
– An example among plants is desert creosote bushes,
which are spaced evenly resulting from competition
among their root systems for water and nutrients
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– Territorial behavior is more common among animals
during their breeding seasons
– Seabirds may space their nests evenly along the shore,
just out of reach of one another
– Mature desert creosote bushes are often spaced very
evenly
– This spacing comes from competition among their root
systems, which occupy a circular area around each plant
© 2014 Pearson Education, Inc.
Figure 26-12b Uniform distribution
uniform
Uniform distribution
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– Organisms with a random distribution are relatively
rare
– Such individuals do not form social groups
– The resources needed are more or less equally
available throughout the area they inhabit
– Resources are not scarce enough to require territorial
spacing
– Examples include trees and other plants in rain forests
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different spatial distributions
(continued)
– There are probably no vertebrate species that
maintain a random distribution throughout the year
– Most interact socially, at least during the breeding
season
© 2014 Pearson Education, Inc.
Figure 26-12c Random distribution
random
Random distribution
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
– Animals of different species differ considerably in their
chances of dying at any given phase of their life cycle
– Some species produce many offspring that are
provided with very few resources; most die before they
can reproduce
– Others produce few offspring, which are each given far
more resources and often survive to reproduce
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– Three types of survivorship curves are described
according to the part of the life cycle during which
most deaths occur
– Late-loss populations
– Constant-loss populations
– Early-loss populations
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– To determine the pattern of survivorship, researchers
construct survivorship tables, which track groups of
organisms (born at the same time) throughout their
lives, recording how many survive in each succeeding
year
– If these numbers are graphed, they reveal the
survivorship curves characteristic of the species in
the environment where the data were collected
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– Late-loss populations produce convex survivorship
curves
– These populations have relatively low juvenile death
rates; many or most individuals survive to old age
– Late-loss curves are characteristic of humans and other
large and long-lived animals such as elephants and
mountain sheep
– Relatively few offspring are produced by these species
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– Constant-loss populations produce straight-line
survivorship curves
– In these populations, individuals have an equal chance
of dying at any time during their life span
– This pattern is seen in some birds such as gulls and the
American robin, in some species of turtles, and in
laboratory populations of organisms that reproduce
asexually, such as hydra and bacteria
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– Early-loss populations produce concave
survivorship curves
– These curves are characteristic of organisms that
produce large numbers of offspring that receive little or
no parental care
– Many of these species engage in scramble competition
early in life
© 2014 Pearson Education, Inc.
26.3 How Are Populations Distributed in Space
and Age?
 Populations exhibit different age distributions
(continued)
– Early-loss populations produce concave
survivorship curves (continued)
– The death rate is high among the young, but those that
reach adulthood have a reasonable chance to survive
to old age
– Most invertebrates, many fish and amphibians, and most
plants exhibit early loss survivorship curves
© 2014 Pearson Education, Inc.
Figure 26-13 Survivorship tables and survivorship curves
Number
of
survivors
0 (birth)
100,000
10
99,124
20
98,713
30
97,754
40
96,489
50
93,698
60
87,967
70
76,241
80
54,117
90
22,312
100
2,523
A survivorship
table
© 2014 Pearson Education, Inc.
number of survivors
1,000
Age
100
late loss
(human)
10
constant loss
(American robin)
early loss
(dandelion)
0
percent of maximum life span
Survivorship curves
26.4 How Is the Human Population Changing?
 No force on Earth rivals that exerted by humans
– Humans possess enormous brainpower
– We possess dexterous hands that can shape the
environment by our demands
 Natural selection favored those with the ability and
the drive to bear and nurture offspring, which helped
make sure that few would survive
– This characteristic now threatens us and the
biosphere on which we depend
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The human population continues to grow rapidly
– In the last few centuries, the human population has
grown at nearly an exponential rate following a Jshaped growth curve
– Over the last decade, however, the human population
has been growing at a relatively constant rate,
suggesting that it may no longer be growing
exponentially
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The human population continues to grow rapidly
(continued)
– Are humans starting to enter the final bend of the Sshaped logistic growth curve that will eventually lead
to a stable population?
– Despite the fact that our annual growth rate has
declined from 1.8% in 1960 to 1.2% in 2011, Earth’s
human population is adding people faster than ever
– Having reached 7 billion in 2011, our numbers now
grow by about 83 million each year
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Animation: Human Population Growth
Figure 26-14 Human population growth
1927
1960
1975
1987
1999
2011
2025
1
All of human
history
2
3
4
5
6
7
8*
123
33
14
13
12
12
14
6
1999
5
1987
4
3
*projected
1975
year
1804
2011
billions of people
Date Billions Time to add
each billion
(years)
7
1960
2
1927
1
1804
bubonic
plague
12,000 11,000 10,000 9,000
B.C.
B.C.
B.C.
B.C.
Technical advances
© 2014 Pearson Education, Inc.
8000
7000
6000
5000
4000
3000
2000
1000
B.C.
B.C.
B.C.
B.C.
B.C.
B.C.
B.C.
B.C.
Agricultural advances
B.C./A.D.
1000
0
2000
A.D.
A.D.
Industrial
and medical
advances
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people
– Human population growth has been spurred by a
series of advances, each of which circumvented some
type of environmental resistance, increasing Earth’s
carrying capacity for people
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people (continued)
– Early humans
– Discovered fire
– Invented tools and weapons
– Built shelters
– Designed protective clothing
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people (continued)
– A series of technical advances increased carrying
capacity
– Tools and weapons allowed humans to hunt more
effectively and obtain additional high-quality food, while
shelter and clothing expanded the habitable areas of
the globe
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people (continued)
– Domesticated crops and animals had supplanted
hunting and gathering in many parts of the world by
8000 B.C.
– These agricultural advances provided a larger and
more stable food supply for people, further increasing
Earth’s carrying capacity for humans
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people (continued)
– Human population growth continued slowly for
thousands of years until major industrial and medical
advances permitted a population explosion
– These advances began in England in the mideighteenth century
– Medical progress dramatically decreased the death rate
by reducing environmental resistance caused by
disease
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 A series of advances has increased Earth’s carrying
capacity to support people (continued)
– The discovery of bacteria and their role in infection
resulted in better control of bacterial diseases through
improved sanitation and antibiotics
– Vaccines for diseases such as smallpox reduced
deaths from viral infections
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size
– In developed countries, people benefit from a
relatively high standard of living, with access to
modern technology and medical care, including
readily available contraception
– Developed countries include Australia, New Zealand,
Japan, and countries in North America and Europe
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– Average income in developed countries is relatively
high
– Education and employment opportunities are
available to both sexes
– Death rates from infectious diseases are low
– Less than 20% of the world’s population lives in
developed countries
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– In the developing countries of Central and South
America, Africa, and much of Asia—home to more
than 80% of humanity—the average person lacks
these advantages
– The historical rate of population growth in developed
countries has changed over time in reasonably
predictable stages, producing a pattern called
demographic transition
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– Pre-industrial stage: The population was relatively
small and stable, with high birth rates and high death
rates
– Transitional stage: Food production increased and
health care improved, which caused death rates to
fall; because birth rates remained high, there was an
explosive population increase
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– Industrial stage: Birth rates fell as contraceptives were
more available, and as people moved from farms to
cities, where children were less important as a source
of labor
– Post-industrial stage: Populations are relatively
stable, with low birth and death rates
© 2014 Pearson Education, Inc.
Figure 26-15 The demographic transition
increase in rates or size
Pre-industrial
Stage
birth rate
death rate
population
size
Birth and death
rates are high
Population
grows rapidly
Transitional Stage
Industrial Stage
Population
growth slows
Birth rate
remains high
Population
stabilizes
natural rate
of population
increase
Birth rate
declines
Birth and death
rates are low
Population
remains low
Death rate
declines
time
© 2014 Pearson Education, Inc.
Post-industrial
Stage
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– A population’s fertility rate reflects the average
number of children that each woman bears
– If immigration and emigration rates are balanced, a
population will eventually stabilize if parents have just
the number of children to replace themselves
– This is called replacement-level fertility (RLF)
– RLF is 2.1 children per woman because not all children
survive to maturity
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
– In developing countries, medical advances have
decreased death rates and increased life span, but
birth rates remain relatively high
– Although China is a developing country, its population
has approached one billion
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
(continued)
– Most other developing countries are within the latetransitional or the industrial stage of the demographic
transition
– Adult children provide financial security for aging
parents
– Young children may also contribute significantly to the
family income by working on farms or factories
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
(continued)
– Most other developing countries are within the latetransitional or the industrial stage of the demographic
transition (continued)
– Social factors drive population growth in countries
where children confer prestige because religious beliefs
promote large families
– Many individuals who would like to limit their family size
lack access to contraceptives
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
(continued)
– A lack of education and a lack of access to
contraceptives then contributes to continued high birth
rates
– Of the 7 billion people on Earth in 2011, about 5.8
billion resided in developing countries
– The prospect for world population stabilization in the
near future is nonexistent
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth
– Age structure diagrams show age groups on the
vertical axis and the numbers (or percentages) of
individuals in each age group on the horizontal axis,
with males and females shown on opposite sides
– Age structure diagrams all rise to a peak that reflects
the maximum human life span
– The shape of the rest of the diagram reveals whether
the population is expanding, stable, or shrinking
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– If adults of reproductive age (15 to 44 years) are
having more children (the 0- to 14-year age group)
than are needed to replace themselves, the
population is above RLF and is expanding
– The age-structure diagram will be roughly triangular
© 2014 Pearson Education, Inc.
Figure 26-16a Africa: A rapidly growing population
100
90
Africa 2010
female
male
80
age
70
60
50
40
30
20
10
0
6
4
2
4
2
0
percent of population
Africa: A rapidly growing population
© 2014 Pearson Education, Inc.
6
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– If adults of reproductive age have just the number of
children needed to replace themselves, the population
is at RLF
– A population that has been at RLF for many years will
have an age structure diagram with relatively straight
sides
© 2014 Pearson Education, Inc.
Figure 26-16b North America: A slowly growing population
100
90
North America 2010
female
male
80
age
70
60
50
40
30
20
10
0
6
4
2
4
2
0
percent of population
6
North America: A slowly growing population
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– In a shrinking population, the reproducing adults have
fewer children than are required to replace
themselves
– The age-structure diagram will be narrow at the base
– The median age depends on the age structure
– The lower the median age, the more rapidly the
population will expand
© 2014 Pearson Education, Inc.
Figure 26-16c Europe: A slowly declining population
100
90
Europe 2010
female
male
80
age
70
60
50
40
30
20
10
0
6
4
2
4
2
0
percent of population
Europe: A slowly declining population
© 2014 Pearson Education, Inc.
6
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– Average-age structure diagrams have been plotted for
developed and developing countries for 2012, with
predictions for 2050
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– These diagrams reveal that even if developing
countries were to achieve RLF immediately, their
population increases would continue for decades
– A large population of children today creates a
momentum for future growth as they enter their
reproductive years
© 2014 Pearson Education, Inc.
Figure 26-17a Developed countries
2010
2050
100
90
female
male
80
postreproductive
(45100 years)
70
age
60
50
40
reproductive
(1544 years)
30
20
prereproductive
(014 years)
10
0
300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300
millions of people
Developed countries
© 2014 Pearson Education, Inc.
Figure 26-17b Developing countries
2010
2050
100
90
male
female
80
70
age
60
50
40
30
20
10
0
300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300
millions of people
Developing countries
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The age structure of a population predicts its future
growth (continued)
– The United Nations has developed high, medium, and
low projections for future growth based on
assumptions about fertility rates
– For the year 2050, the medium projection is that
Earth’s population will have increased by about 33%
to over 9.3 billion
– Eight billion people live in developing nations
© 2014 Pearson Education, Inc.
world population (billions)
Figure 26-18 United Nations world population projections
12
11
high 10.6
10
9.3
9
medium
8
low
8.1
7
6
2000
© 2014 Pearson Education, Inc.
2010
2020
2030
2040
2050
26.4 How Is the Human Population Changing?
 Fertility in some nations is below replacement level
– A comparison of growth rates for various world
regions shows Europe as the only one with an
average rate of change in population that is negative
– The average fertility rate is 1.6, which is substantially
below RLF
– Concerns about the availability of future workers and
taxpayers have prompted several countries to offer
incentives for couples to have children at an earlier age
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 Fertility in some nations is below replacement level
(continued)
– Japan’s government is concerned about the country’s
low fertility rate (1.4) and provides subsidies to
encourage larger families
– Japan is about the size of the state of Montana in the
United States and home to 128 million people
(equivalent to 41% of the entire U.S. population)
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 Fertility in some nations is below replacement level
(continued)
– The current economic structures in countries
throughout the world are based on growing
populations
– The difficult adjustments necessary as populations
decline—or even merely stabilize—motivate
governments to adopt policies that encourage more
childbearing and continued growth
© 2014 Pearson Education, Inc.
Table 26-1
© 2014 Pearson Education, Inc.
26.4 How Is the Human Population Changing?
 The U.S. population is growing rapidly
– The United States has a population of more than 313
million and a growth rate of about 0.7% (adding one
person every 15 seconds) and is the fastest-growing
developed country in the world
– Continued immigration, which accounts for about 30%
of the population increase, will ensure growth for the
indefinite future
© 2014 Pearson Education, Inc.
Figure 26-19 United States population growth
325
300
275
U.S. population (in millions)
250
225
200
175
150
125
100
75
50
25
0
© 2014 Pearson Education, Inc.
1800
1850
1900
year
1950
2000
26.4 How Is the Human Population Changing?
 The U.S. population is growing rapidly (continued)
– The rapid growth of the U.S. population has major
environmental implications for local ecosystems and
for the planet
– The average U.S. resident uses nearly four times as
much energy as the average person worldwide
© 2014 Pearson Education, Inc.