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Chapter 13
Population Ecology
Is the Human Population Too Large?
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Chapter 13 Section 1
A Growing Human Population
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13.1 A Growing Human Population
Human PopulationEarth’s human population is 6.6 billionUN estimates that population will grow to 10.3 billion by 2050
854 million people already do not get enough to eat
55% of childhood mortality is due to poor nutrition
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13.1 A Growing Human Population
DefinitionsEcology = study of the interaction among organisms and between organisms and their environment
Population = all the individuals of a species in a given area
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13.1 A Growing Human Population
Population Structure
Estimating size of population is the most basic task. Populations can be measured in a variety of ways Basic census – all individuals counted
(humans, trees, etc.) Mark-recapture – estimate of population size
some individuals trapped, marked, and released
In later trapping, ratio of marked to unmarked individuals used to estimate population size
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13.1 A Growing Human Population - Population Structure
Example of Mark-Recapture
Figure 13.1
1 2 3Researcher captures 100 beetles ina trap, marks each with a dab ofpaint.
After one week, a trap is set again,resulting in a captured group ofmarked and unmarked individuals.
Total population is estimated asequivalent to the percentage ofmarked individuals in the second trap.
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13.1 A Growing Human Population - Population Structure
Population dispersion – how a population is distributed in space.
Clumped – high densities in resource-rich areas, low densities elsewhere
Uniform – spacing between individuals tends to be equal
Random – no compelling feature pushing individuals together or apart
Figure 13.2
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13.1 A Growing Human Population –
Human Population Growth: An Overview
Archaeologists have been able to estimate the size of human population to about 10,000 years ago Human population is an example of exponential growth
Exponential growth is in proportion to population size As population grows, growth also increases
Figure 13.3
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13.1 A Growing Human Population
Human Population Growth Growth rate was ~0.1% per year 2000 years ago By 1750 growth rate was ~2% per year Currently growth rate is ~1.2% per year Current population of earth 6.6 billion, at 1.2% growth
rate this means population increases by 77 million people/year
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13.1 A Growing Human Population
Growth Rate Explained Growth rate is represented by “r” r = (birth rate – death rate)
Example If birth rate = 20 births / 1000 = 2.0% And death rate = 8 deaths / 1000 = 0.8% Then r = 2.0 – 0.8 = 1.2%
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13.1 A Growing Human Population
Consider growth rate as time it takes a population to double
Figure 13.4b
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13.1 A Growing Human Population
The Demographic Transition
Generally in human populations, a decrease in death rate is followed by a decrease in birth rate = demographic transition
Figure 13.5
Before industrial revolution, both birth and death rates were high
With industrialization death rates decreased
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13.1 A Growing Human Population
The Demographic Transition Lag time between decrease in death rate and birth rate
has major impact on population size Many developing countries still in demographic transition
Figure 13.5
These developing countries are where majority of famines already occur
Is the human population already to large?
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13.1 A Growing Human Population
Animation—Population GrowthPLAY
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END Chapter 13 Section 1
A Growing Human Population
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END Chapter 13 Section 2
Limits to Population Growth
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13.2 Limits to Population Growth
Studies of non-humans species show that no population can grow to unlimited numbers
Elk in Yellowstone Park in 1970s had high mortality after large population degraded environment
Norway lemmings population grows so large that resources become scarce and population undertakes mass migration
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13.2 Limits to Population Growth
Carrying Capacity and Logistic Growth
Populations have the potential to grow exponentially but they are limited by
environmental resources – food, water, shelter, and space.
Carrying capacity = the maximum number of individuals that can be supported indefinitely in a given environment
Figure 13.6
NoGrowth
Carrying capacity
Time
Popula
tion s
ize
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13.2 Limits to Population Growth - Carrying Capacity and Logistic Growth
Limits on population growth:Density-dependent factors – factors that increase with population size
Limited food supply, increased risk of disease, increase in waste levels
Can cause decrease in birth rates, increase in death rates, or both
Density-independent factors – not influenced by population numbers
Droughts, temperature extremes, natural disasters
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13.2 Limits to Population Growth - Carrying Capacity and Logistic Growth
Limits on population growth: Fruit Flies
Fruit FliesWhen grown in the laboratory, as the population grows:Food becomes limitedWaste accumulatesLeads to increased mortality
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13.2 Limits to Population Growth - Carrying Capacity and Logistic Growth
Limits on population growth: Water Fleas
Water FleasWhen grown in an aquarium, as the population grows:Food becomes limitedFemales do not have enough energy for egg productionLeads to decreased birth rate
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13.2 Limits to Population Growth - Carrying Capacity and Logistic Growth
Limits on population growth: White-tailed Deer
White-tailed DeerWhen natural habitat becomes crowded:Food becomes limitedFemales do not have enough energy to carry pregnancy to termLeads to decreased birth rate
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13.2 Limits to Population Growth
Earth’s Carrying Capacity for Humans
There is uncertainty about future human population growth ratesHuman population growth rates at highest in 1960s, ~2.1%Growth rates now about 1.2%Unclear what future trends are, UN has released 3 projections
Figure 13.8
Year
Medium growth rate
High growth rate
Low growth rate
Hum
an p
opula
tion (
bill
ions)
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13.2 Limits to Population Growth - Earth’s Carrying Capacity for Humans
Signs that Earth is Not Near Carrying Capacity One reason for declining growth rates is choice Earth’s resources can be measured by net
primary productivity – the total amount of plant growth Estimates of net primary productivity indicate
that a human population of 20 billion could be supported But this assumes that all plant growth
fed humans and nothing else
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13.2 Limits to Population Growth - Earth’s Carrying Capacity for Humans
Signs that Population is Nearing Carrying CapacityPopulations require more than simply food, thus net primary productivity estimates might be too high.
Humans need clean water, but also produce waste. Many essential supplies are non-renewable
resources. Material consumption is affected by lifestyle and
affluence. USA accounts for ~5% of world’s population but
consumes 24% of global energy resources.
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END Chapter 13 Section 2
Limits to Population Growth
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Chapter 13 Section 3
The Future of the Human Population
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13.3 The Future of the Human Population
A Possible Population Crash?
The cycle of growth beyond carrying capacity leads to population crashes and, sometimes, rebounds.
Figure 13.11
Boom-and-bust cycle may persist… …or population may stabilize at carrying capacity.
Boom
Crash
Carryingcapacity
Time Time
Pop
ula
tion s
ize
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13.3 The Future of the Human Population
Example: Easter Island (Rapa Nui)
The island is very isolated in the Pacific Could only use resouces
on island
Island was once a tropical forest and supported 7000 people
They destroyed the forest and population crashed by 1775 to 700
Figure 13.11
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13.3 The Future of the Human Population - A Possible Population Crash?
Demographic momentum = time lag between when humans reduce birth rates and when population growth begins to slow Even while parents are reducing family size,
their children grow and have children. Even when parents have two children
(replacement), population will continue to grow for 60 or 70 years until stable.
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13.3 The Future of the Human Population - A Possible Population Crash?
A population pyramid with a large base indicates that the majority of the population is young and that the population is still growing
Figure 13.13
Males(population in millions)
FemalesMales(population in millions)
Females
Reproductiveyears
(a) South Africa in 2000 (b) United States in 2000
Ag
e
Ag
e
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13.3 The Future of the Human Population
Avoiding DisasterFor humans, other factors also affect population growth rate, including:
income and women’s literacy.
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13.3 The Future of the Human Population - A Possible Population Crash?
Avoiding DisasterThus, social policies can have large impacts on human population growth.
The question of how many people the earth can support is not just one of science, but of values and ethics.
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13.3 The Future of the Human Population - A Possible Population Crash?
Quality of Life?The “American Lifestyle” is only supported by using resources from other countriesBut global carrying capacity only provides enough food and water
But who wants such a world? To have a rich, rewarding life, we must be well
below the carrying capacity, ensuring a bountiful nature and diversity of other species
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End Chapter 13 Section 3
The Future of the Human Population
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End Chapter 13
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