copyright © 2010 pearson education, inc. chapter 13 population ecology is the human population too...

Copyright © 2010 Pearson Education, Inc.

Chapter 13

Population Ecology

Is the Human Population Too Large?


Chapter 13 Section 1

A Growing Human Population


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



DefinitionsEcology = study of the interaction among organisms and between organisms and their environment

Population = all the individuals of a species in a given area



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


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.


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


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



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



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%



Consider growth rate as time it takes a population to double

Figure 13.4b



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



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?



Animation—Population GrowthPLAY


END Chapter 13 Section 1

A Growing Human Population



Limits to Population Growth


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



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


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



Limits on population growth: Fruit Flies

Fruit FliesWhen grown in the laboratory, as the population grows:Food becomes limitedWaste accumulatesLeads to increased mortality



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



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



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)


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


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.



Limits to Population Growth


Chapter 13 Section 3

The Future of the Human Population


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



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


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.



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



Avoiding DisasterFor humans, other factors also affect population growth rate, including:

income and women’s literacy.



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.

Copyright © 2010 Pearson Education, Inc. Figure 13.14


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


End Chapter 13 Section 3

The Future of the Human Population


End Chapter 13

copyright © 2010 pearson education, inc. chapter 13 population ecology is the human population too...

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