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Chapter 20
Communities and Ecosystems
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Does Biodiversity Matter?
• The expanding human population threatens
– Biodiversity
– The loss of natural ecosystems
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Figure 20.00
• Healthy ecosystems
– Purify air and water
– Decompose wastes
– Recycle nutrients
• Wetlands
– Buffer coastal populations against hurricanes
– Reduce the impact of flooding rivers
– Filter pollutants
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• It is estimated that the average annual value of ecosystem services
each year in the United States is more than $33 trillion.
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THE LOSS OF BIODIVERSITY
• Biological diversity, or biodiversity, includes
– Genetic diversity
– Species diversity
– Ecosystem diversity
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Genetic Diversity
• The genetic diversity within populations of a species is the raw
material that makes microevolution and adaptation to the
environment possible.
• Genetic resources for that species are lost if
– Local populations are lost
– The number of individuals in a species declines
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Figure 20.1
Species Diversity
• Ecologists believe that we are pushing species toward extinction
at an alarming rate.
• The present rate of species loss
– May be 1,000 times higher than at any time in the past 100,000 years
– May result in the loss of half of all living plant and animal species by the
end of this century
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• Two recent victims of human-caused extinctions are
– Chinese river dolphins
– Golden toads
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A Chinese river dolphin
Golden toads
Figure 20.2
Ecosystem Diversity
• The local extinction of one species can have a negative effect on
the entire ecosystem.
• The loss of ecosystems risks the loss of ecosystem services,
including
– Air and water purification
– Climate regulation
– Erosion control
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• Coral reefs are rich in species diversity, yet
– An estimated 20% of the world’s coral reefs have been destroyed by
human activities
– 24% are in imminent danger of collapse
– Another 26% of coral reefs may succumb in the next few decades if they
are not protected
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Figure 20.3
Causes of Declining Biodiversity
• Ecologists have identified four main factors responsible for the
loss of biodiversity:
– Habitat destruction and fragmentation
– Invasive species
– Overexploitation
– Pollution
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Habitat Destruction
• Biodiversity is threatened by the destruction and fragmentation of
habitats by
– Agriculture
– Urban development
– Forestry
– Mining
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Figure 20.4
Invasive Species
• Invasive species have
– Competed with native species
– Preyed upon native species
– Parasitized native species
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Overexploitation
• People have overexploited wildlife by harvesting at rates that
exceed the ability of populations to rebound.
• Excessive harvesting has greatly affected populations of
– Tigers
– Whales
– The American bison
– Galápagos tortoises
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Figure 20.5
Pollution
• Acid precipitation is a threat to
– Forest ecosystems
– Aquatic ecosystems
• Aquatic ecosystems may be polluted by toxic
– Chemicals
– Nutrients
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COMMUNITY ECOLOGY
• An organism’s biotic environment includes
– Other individuals in its own population
– Populations of other species living in the same area
• An assemblage of species living close enough together for
potential interaction is called a community.
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Figure 20.6
Interspecific Interactions
• Interspecific interactions are interactions between species.
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• Interspecific interactions can be classified according to the effect
on the populations concerned.
– –/– interactions occur when two populations in a community compete for
a common resource.
– +/+ interactions are mutually beneficial, such as between plants and their
pollinators.
– +/– interactions occur when one population benefits and the other is
harmed, such as in predation.
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Interspecific Competition (–/–)
• In interspecific (between species) competition, the population
growth of a species may be limited by
– The population densities of competing species
– By the density of its own population
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• An ecological niche is the sum of an organism’s abiotic and biotic
resources in its environment.
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(a) Virginia’s warbler
(b) Orange-crowned warbler
Figure 20.7
• The competitive exclusion principle states that if two species
have an ecological niche that is too similar, the two species
cannot coexist in the same place.
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LM
Relative population density
Separate
cultures
Paramecium aurelia
Combined
cultures
2
P. aurelia
4
6
8
10
Days
12
14
16
LM
0
P. caudatum
Paramecium caudatum
Figure 20.8
Mutualism (+/+)
• In mutualism, both species benefit from an interaction.
• One example is the mutualistic relationship of coral animals and
the unicellular algae that live inside their cells.
– The coral gains energy from the sugars produced by the algae.
– The algae gain
–
A secure shelter
–
Access to light
–
Carbon dioxide
–
Ammonia, a valuable source of nitrogen
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Figure 20.9
Predation (+/–)
• Predation refers to an interaction in which one species (the
predator) kills and eats another (the prey).
• Numerous adaptations for predator avoidance have evolved in
prey populations through natural selection.
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• Cryptic coloration is
– Camouflage
– A way for prey to hide from predators
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Figure 20.10
• A warning coloration is a
– Brightly colored pattern
– Way to warn predators that an animal has an effective chemical defense
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Figure 20.11
• Mimicry is a form of defense in which one animal looks like
another species.
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Figure 20.12
• Some insects have elaborate disguises that make them resemble
– Twigs
– Leaves
– Bird droppings
– Predators
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Figure 20.13
Herbivory (+/–)
• Herbivory is the consumption of plant parts or algae by an
animal.
• Plants have evolved numerous defenses against herbivory,
including
– Spines
– Thorns
– Chemical toxins
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Peppermint
Cinnamon
Cloves
Figure 20.14
Parasites and Pathogens (+/–)
• Plants and animals can be victims of
– Parasites, an animal that lives in or on a host from which it obtains
nutrients
– Pathogens, disease-causing
–
Bacteria
–
Viruses
–
Fungi
–
Protists
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Trophic Structure
• Trophic structure is the feeding relationships among the various
species in a community.
• A community’s trophic structure determines the passage of
energy and nutrients from plants and other photosynthetic
organisms
– To herbivores
– And then to predators
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• The trophic level that supports all other trophic levels consists of
autotrophs, also called producers.
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• All organisms in trophic levels above the producers are
heterotrophs, or consumers.
• Primary consumers are called herbivores, which eat plants.
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• Above the level of primary consumers are carnivores, which eat
the consumers from the level below.
– Secondary consumers eat primary consumers.
– Tertiary consumers eat secondary consumers.
– Quaternary consumers eat tertiary consumers.
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Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Zooplankton
Herbivore
Producers
Plant
A terrestrial food chain
Phytoplankton
An aquatic food chain
Figure 20.15-5
• Detritivores, which are often called scavengers, consume
detritus, the dead material left by all trophic levels.
• Decomposers are prokaryotes and fungi, which secrete enzymes
that digest molecules in organic material and convert them into
inorganic forms.
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Figure 20.16
Biological Magnification
• Environmental toxins accumulate in consumers at higher
concentrations up a trophic system in a process called biological
magnification.
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Increasing concentrations of PCBs
Herring
gull eggs
124 ppm
Lake trout
4.83 ppm
Smelt
1.04 ppm
Zooplankton
0.123 ppm
Phytoplankton
0.025 ppm
Figure 20.17
Food Webs
• Few ecosystems are as a simple as an unbranched food chain.
• Omnivores
– Eat producers and consumers
– Form woven ecosystems called food webs
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Quaternary,
tertiary,
and secondary
consumers
Tertiary
and
secondary
consumers
Secondary
and
primary
consumers
Primary
consumers
Producers
(plants)
Figure 20.18
Species Diversity in Communities
• Species diversity of a community consists of
– Species richness, the number of different species in the community
– Relative abundance of the different species, the proportional
representation of a species in a community
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Woodland A
Woodland B
Figure 20.19
• The next figure shows that the relative abundance of one species
in woodland A is much higher than the other three species.
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80
Key
Relative abundance of
tree species (%)
Woodland A
60
Woodland B
40
20
0
Tree species
Figure 20.20
• A keystone species is a species whose impact on its community
is much larger than its total mass or abundance indicates.
• Experiments in the 1960s demonstrated that a sea star functioned
as a keystone species in intertidal zones of the Washington coast.
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Figure 20.21
Disturbances in Communities
• Disturbances are episodes that damage biological communities,
at least temporarily, by
– Destroying organisms
– Altering the availability of resources such as mineral nutrients and water.
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• Examples of disturbances are
– Storms
– Fires
– Floods
– Droughts
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• Disturbances may cause
– The emergence of a previously unknown disease
– Opportunities for other organisms to grow
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Figure 20.22
Ecological Succession
• Disturbances may cause a gradual replacement by other species in
a process called ecological succession.
• Primary succession begins
– In a virtually lifeless area with no soil
– In places such as
–
Lava flows or
–
The rubble left by a retreating glacier
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Figure 20.23
• Secondary succession occurs where a disturbance has
– Destroyed an existing community
– Left the soil intact
• Examples of secondary succession are areas recovering from
– Fires
– Floods
– Severe storms
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Figure 20.24
ECOSYSTEM ECOLOGY
• An ecosystem includes
– The community of species in a given area
– All the abiotic factors, such as
–
Energy
–
Soil characteristics
–
Water
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• A simple terrarium is a microcosm that exhibits the two major
processes that sustain all ecosystems:
– Energy flow, the passage of energy through the components of the
ecosystem
– Chemical cycling, the use and reuse of chemical elements such as carbon
and nitrogen within the ecosystem
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al
Energy flow
Light
energy
Bacteria,
protists,
and fungi
Chemical
energy
Heat
energy
Chemical
elements
Figure 20.25
• Energy flows through ecosystems.
• Chemicals are recycled within and between ecosystems.
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Energy Flow in Ecosystems
• All organisms require energy for
– Growth
– Maintenance
– Reproduction
– In many species, locomotion
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Primary Production and the Energy Budgets of
Ecosystems
• Each day, the Earth receives about 1019 kcal of solar energy, the
energy equivalent of about 100 million atomic bombs.
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• Most of this energy is absorbed, scattered, or reflected by the
atmosphere or by Earth’s surface.
• About 1% is converted to chemical energy by photosynthesis.
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• The amount, or mass, of living organic material in an ecosystem
is the biomass.
• The rate at which an ecosystem’s producers convert solar energy
to the chemical energy stored in biomass
– Is primary production
– Yields about 165 billion tons of biomass per year
• Different ecosystems vary considerably in their primary
production.
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Open ocean
Estuary
Algal beds and coral reefs
Desert and semidesert scrub
Tundra
Temperate grassland
Cultivated land
Northern coniferous forest (taiga)
Savanna
Temperate broadleaf forest
Tropical rain forest
0
500 1,000 1,500 2,000 2,500
Average primary productivity (g/m2/yr)
Figure 20.26
Ecological Pyramids
• When energy flows as organic matter through the trophic levels of
an ecosystem, much of it is lost at each link in the food chain.
• Consider the example of a caterpillar.
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Plant material
eaten by caterpillar
100 kilocalories (kcal)
35 kcal
50 kcal
Cellular
respiration
Feces
15 kcal
Growth
Figure 20.27
• A pyramid of production illustrates the cumulative loss of
energy with each transfer in a food chain.
• The energy level available to the next higher level
– Ranges from 5–20%
– Is illustrated here as 10%
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Tertiary
consumers
10 kcal
Secondary
consumers
100 kcal
Primary
consumers
Producers
1,000 kcal
10,000 kcal
1,000,000 kcal of sunlight
Figure 20.28
• The energy available to top-level consumers is small compared to
the energy available to lower-level consumers.
• This explains why
– Top-level consumers require more geographic area
– Most food chains are limited to three to five levels
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Ecosystem Energetics and Human Resource Use
• The dynamics of energy flow apply to the human population as
much as to other organisms.
• When humans eat
– Plants, we are primary consumers
– Beef or other meat from herbivores, we are secondary consumers
– Fish like trout or salmon, we are tertiary consumers
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• The two energy pyramids that follow compare the amount of food
available to humans if we are strictly either:
– Vegetarians or
– Carnivores
• Eating meat of any kind is expensive economically and
environmentally.
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Trophic level
Human
meat-eaters
Secondary
consumers
Primary
consumers
Producers
Human
vegetarians
Corn
Cattle
Corn
Figure 20.29
Chemical Cycling in Ecosystems
• Life depends on the recycling of chemicals.
– Nutrients are acquired and waste products are released by living
organisms.
– At death, decomposers return the complex molecules of an organism to
the environment.
– The pool of inorganic nutrients is used by plants and other producers to
build new organic matter.
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Figure 20.30
The General Scheme of Chemical Cycling
• Biogeochemical cycles involve
– Biotic components
– Abiotic components from an abiotic reservoir where a chemical
accumulates or is stockpiled outside of living organisms
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Consumers
Producers
Decomposers
Nutrients
available
to producers
Abiotic
reservoir
Geologic processes
Figure 20.31
• Biogeochemical cycles can be
– Local
– Global
• Three important biogeochemical cycles are
– Carbon
– Phosphorus
– Nitrogen
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The Carbon Cycle
• The cycling of carbon between the biotic and abiotic worlds is
accomplished mainly by the reciprocal metabolic processes of
– Photosynthesis
– Cellular respiration
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CO2 in atmosphere
Burning
Photosynthesis
Cellular respiration
Higher-level
consumers
Plants, algae,
cyanobacteria
Primary
consumers
Wood
and fossil
fuels
Decomposition
Wastes; death
Decomposers
(soil microbes)
Plant litter;
death
Detritus
Figure 20.32
The Phosphorus Cycle
• Organisms require phosphorus as an ingredient of
– Nucleic acids
– Phospholipids
– ATP
• Phosphorus is also required as a mineral component of vertebrate
bones and teeth.
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• The phosphorus cycle does not have an atmospheric component.
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Uplifting
of rock
Weathering
of rock
Runoff
Phosphates
in rock
Animals
Plants
Assimilation
Phosphates
in solution
Rock
Solid
phosphates
Phosphates
in soil
(inorganic)
Decomposition
Detritus
Decomposers
in soil
Figure 20.33
The Nitrogen Cycle
• Nitrogen is
– An ingredient of proteins and nucleic acids
– Essential to the structure and functioning of all organisms
• Nitrogen has two abiotic reservoirs:
– The atmosphere
– The soil
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• The process of nitrogen fixation converts gaseous N2 to
ammonia and nitrates, which can be used by plants.
• Most of the nitrogen available in natural ecosystems comes from
biological fixation performed by two types of nitrogen-fixing
bacteria.
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Nitrogen (N2) in atmosphere
Assimilation
by plants
Plant
Animal
Organic
compounds
Organic
compounds
Death; wastes
Nitrogen
fixation
Denitrifying
bacteria
Nitrates
In soil
(NO3–)
Nitrogen-fixing
bacteria in
root nodules
Detritus
Decomposers
Nitrifying
bacteria
Free-living
nitrogen-fixing
bacteria
Decomposition
Nitrogen fixation
Ammonium (NH4+)
in soil
Figure 20.34
Nutrient Pollution
• The growth of algae and cyanobacteria in aquatic ecosystems is
limited by low nutrient levels, especially of phosphorus and
nitrogen.
• Nutrient pollution occurs when human activities add excess
amounts of these chemicals to aquatic ecosystems.
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Figure 20.35
• Nitrogen runoff from Midwestern farm fields has been linked to
an annual summer dead zone in the Gulf of Mexico.
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Mississippi River
Light blue lines represent rivers draining into the
Mississippi River (shown in dark blue)
Summer
Winter
Figure 20.36
CONSERVATION AND RESTORATION
BIOLOGY
• Ecologists have discovered many environmental problems caused
by human enterprises.
• Ecological research is the foundation for
– Finding solutions to these problems
– Reversing the negative consequences of ecosystem alteration
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• Conservation biology is a goal-oriented science that seeks to
understand and counter the loss of biodiversity.
• Restoration ecology uses ecological principles to develop
methods of returning degraded areas to their natural state.
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Biodiversity “Hot Spots”
• Conservation efforts are often focused on biodiversity hot spots,
relatively small areas that have
– A large number of endangered and threatened species
– An exceptional concentration of endemic species, those that are found
nowhere else
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Equator
Figure 20.37
Conservation at the Ecosystem Level
• Conservation biology increasingly aims at sustaining the
biodiversity of entire
– Communities
– Ecosystems
– Landscapes, a regional assemblage of interacting ecosystems, such as an
area with forest, adjacent fields, wetlands, streams, and streamside
habitats
• Landscape ecology is the application of ecological principles to
the study of land-use patterns.
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• Edges between ecosystems
– Are prominent features of landscapes, whether natural or altered by
humans
– Have their own sets of physical conditions, such as
–
Soil type
–
Surface features
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Natural edges
Edges created by human activity
Figure 20.38
• A movement corridor
– Is a narrow strip or series of small clumps of suitable habitat
– Connects otherwise isolated patches
• Corridors
– Can promote dispersal and help sustain populations
– Are especially important to species that migrate between different
habitats seasonally
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Figure 20.39
The Process of Science: How Does Tropical Forest
Fragmentation Affect Biodiversity?
• Observation: Forests were becoming fragmented when cleared
for agriculture.
• Question: How does fragmentation of tropical forests affect
species diversity within the fragments?
• Hypothesis: Species diversity declines with the size of the forest
fragment.
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Figure 20.40
• Prediction: Predators will only be found in the largest areas.
• Results: Fragmentation of forests into smaller pieces does lead to
declines in species diversity.
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Restoring Ecosystems
• Bioremediation uses living organisms to detoxify polluted
ecosystems.
• Researchers are investigating the use of plants to remove toxic
substances from contaminated soil.
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Figure 20.41
• The Kissimmee River was straightened into a canal between
1962–1971, draining the floodplain.
• The Kissimmee River restoration project is reversing the
engineering of the river by
– Removing water control structures such as dams and reservoirs
– Filling in about 35 km of the canal
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Lake
Kissimmee
Widened
canal
Water control
structure remaining
Former canal
Water control
structure removed
Phase 1
completed
River channel
restored
Canal backfilled
Detail
ATLANTIC
OCEAN
Water control
structure to
be removed in
phase 2
Kissimmee
River
Floodplain
FLORIDA
GULF OF
MEXICO
0
Miles
10
Lake
Okeechobee
Figure 20.42
The Goal of Sustainable Development
• As the world population grows and becomes more affluent, the
demand increases for the provisioning services of ecosystems,
such as
– Food
– Wood
– Water
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• The goal of sustainable development is to acquire the ecological
information necessary for the responsible development,
management, and conservation of Earth’s resources.
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• Sustainable development depends on
– Continued research
– Application of ecological knowledge
– The connection of the life sciences with
–
Social sciences
–
Economics
–
Humanities
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• Sustainable development aims to
– Conserve biodiversity
– Improve the human condition
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Evolution Connection:
Biophilia and an Environmental Ethic
• Edward O. Wilson makes the case that biophilia, the human
desire to affiliate with other life in its many forms, is innate.
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Figure 20.43
• Most biologists have
– Embraced the concept of biophilia
– Turned their passion for nature into careers
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Figure 20.44
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