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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 36
Communities and Ecosystems
Modules 36.1 – 36.4
From PowerPoint® Lectures for Biology: Concepts & Connections
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Dining In
• Wasps and Pieris caterpillars form an unusual
three-step food chain
• The 4-mm-long wasp Apanteles
glomeratus stabs through the
skin of a Pieris rapae caterpillar
and lays her eggs
– The caterpillar will be
destroyed from within as
the wasp larvae hatch and
nourish themselves on its
internal organs
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• Ichneumon wasps can detect when a Pieris
caterpillar contains Apanteles larvae
– A female ichneumon will pierce the caterpillar
and deposit her own eggs inside of the
Apanteles larvae
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• Finally, yet another wasp, a chalcid, may lay its
eggs inside the ichneumon larvae
• Usually, only the chalcids will emerge from
the dead husk of the caterpillar
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• A biological community derives its structure
from the interactions and interdependence of
the organisms living within it
• Ecosystem functioning depends on the
complex interactions between its community
of organisms and the physical environment
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36.1 A community is all the organisms inhabiting a
particular area
• All the organisms in a particular area make up
a community
• A number of factors characterize every
community
– Biodiversity
– The prevalent form of
vegetation
– Response to disturbances
– Trophic structure
(feeding relationships)
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Figure 36.1
• Biodiversity is the variety of different kinds of
organisms that make up a community
• Biodiversity has two components
– Species richness, or the total number of
different species in the community
– The relative abundance of different species
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STRUCTURAL FEATURES OF COMMUNITIES
36.2 Competition may occur when a shared
resource is limited
• Interspecific competition occurs between two
populations if they both require the same
limited resource
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Interspecific Competition
• Intraspecific competition is usually
intense since individuals of the same species
have virtually identical niches
– If resources are limited, this is a major
factor controlling population size
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Ecological Niche
• A population's niche is its role in the community
– The sum total of its use of the biotic and abiotic
resources of its habitat
• Encompasses all aspects of a species’ way of life,
including
– Physical home or habitat
– Physical and chemical environmental factors
necessary for survival
– How the species acquires its energy and materials
– All the other species with which it interacts
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• The competitive exclusion principle
– Populations of two species cannot coexist in a
community if their niches are nearly identical
– The competitive exclusion principle was
formulated by microbiologist G. F. Gause…
High
tide
Chthamalus
Balanus
Ocean
Low
tide
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Figure 36.2
• Competition between species with identical
niches has two possible outcomes
– One of the populations, using resources more
efficiently and having a reproductive
advantage, will eventually eliminate the other
– Natural selection may lead to resource
partitioning
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Gause’s Competitive exclusion principle
– Performed laboratory experiments with
protists
– Paramecium aurelia and P. caudatum have
identical niches—invariably one excludes
the other
– However, P. aurelia and P. bursaria can
coexist as they feed in different places—
have different niches
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• When species with largely overlapping
niches are allowed to compete, their niches
may focus on a different part of the resource
spectrum
– This is called resource partitioning
– This reduces interspecific competition
– Example: North American warblers
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36.3 Predation leads to diverse adaptations in both
predator and prey
• Predation is an interaction where one species
eats another
– The consumer is called the predator and the
food species is known as the prey
• Parasitism can be considered a form of
predation
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• As predators adapt to prey, sometimes natural
selection also shapes the prey's defenses
• This process of
reciprocal
adaptation is
known as
coevolution
– Example:
Heliconius and
the passionflower
vine
Eggs
Sugar
deposits
Figure 36.3A
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• Prey gain protection against predators through
a variety of defense mechanisms
– Mechanical defenses, such as the quills of
a porcupine
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• Chemical defenses are widespread and very
effective
– Animals with effective chemical defenses are
often brightly colored to warn predators
– Example: the poison-arrow frog
Figure 36.3B
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• Camouflage renders animals inconspicuous even when in
plain sight
– May include evolved colors, patterns, and shapes that
resemble one’s surroundings
– Example: the gray tree frog
Figure 36.3C
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Camouflage
• To avoid detection by predators, some
animals have evolved to resemble objects
such as bird droppings, leaves, or thorns
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Camouflage
• Some plants have evolved to resemble rocks
to avoid detection by herbivores
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Camouflage
• Camouflage also helps predators ambush
their prey
– Examples: the cheetah blending with tall
grass and the frogfish resembling a rock
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Protection Through Mimicry
• Mimicry refers to a situation in which one
species has evolved to resemble another
organism
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Protection Through Mimicry
• Two or more distasteful species may each
benefit from a shared warning coloration
pattern (Müllerian mimicry)
– Predators need only experience one
distasteful species to learn to avoid all with
that color pattern
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Protection Through Mimicry
• Müllerian mimicry
– Example: the cuckoo bee and the yellow
jacket
– Example: monarch and viceroy butterflies
share orange and black pattern
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Protection Through Mimicry
• Some harmless organisms can gain a
selective advantage by resembling poisonous
species (Batesian mimicry)
– Example: harmless hoverfly resembles bee
– Example: harmless mountain king snake
resembles the venomous coral snake
– This hawkmoth larva puffs up its head to
mimic the head of a snake
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Figure 36.3D
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Protection Through Mimicry
• Some animals deter predators by employing
startle coloration
– Have spots that resemble eyes of a large
predator
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Protection Through Mimicry
• In aggressive mimicry, predator
resembles a harmless animal, or part of the
environment, to lure prey within striking
distance
– Example: frogfish dangles wriggling lure
that attracts a curious fish that is then eaten
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Protection Through Mimicry
• Snowberry flies avoid by jumping spider
predation by mimicking them both visually
and behaviorally
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Chemical Warfare
• Both predators and prey have evolved
toxic chemicals for attack and defense
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Chemical Warfare
• Spiders and poisonous snakes use venom to
paralyze their prey and deter predators
• Many plants have evolved chemicals to deter
herbivores
• Bombardier beetle sprays hot chemicals
from its abdomen
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Coevolutionary Adaptations
• Plants have evolved a variety of chemicals to
deter herbivores
– Example: the toxic and distasteful
chemicals in milkweed
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Coevolutionary Adaptations
• Some animals evolve ways to detoxify these
chemicals, allowing them to eat the plants
– Plants may then evolve other toxic
substances
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Coevolutionary Adaptations
• The monarch butterfly uses deterrent
chemicals of milkweed, acquired by a feeding
caterpillar, to make itself distasteful to its
predators
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36.4 Predation can maintain diversity in a
community
• A keystone species exerts strong control on
community structure because of its ecological
role
• A keystone predator may maintain community
diversity by reducing
the numbers of the
strongest competitors
in a community
– This sea star is a
keystone predator
Figure 36.4A
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Keystone Species
• In some communities a keystone species
plays a major role in determining community
structure
• Role is out of proportion to its abundance
• Removal of keystone species dramatically
alters community
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Keystone Species
• Example: The predatory starfish Pisaster
from Washington’s rocky intertidal coast
– When removed from their ecosystem their
favored prey, mussels, increase and
competitively exclude other invertebrates
and algae, simplifying the community
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Keystone Species
• Example: Destruction of encroaching shrubs
and trees by African elephants
– Helps maintain the grass savanna which
supports many species
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Keystone Species
• Keystone species need to be identified and
protected so that human activities do not
lead to the collapse of entire communities
and ecosystems
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• Predation by killer whales
on sea otters, allowing sea
urchins to overgraze on kelp
– Sea otters represent the
keystone species
Figure 36.4B
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36.5 Symbiotic relationships help structure
communities
• A symbiotic relationship is an interaction
between two or more species that live together
in direct contact
• There are three main types of symbiotic
relationships within communities
– Parasitism
– Commensalism
– Mutualism
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Commensalism
• In commensalism, one species benefits
and the “other” is unaffected
– Example: barnacles hitching a ride on the
skin of a whale
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Parasitism
• In parasitism, the parasite benefits but
the host is harmed
– The parasite lives in or on the host and
benefits by feeding on it
– Examples: tapeworms, fleas, and diseasecausing protozoa, bacteria, and viruses,
many of which have complex life cycles
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• In the 1940s, Australia was overrun by
hundreds of millions of European rabbits
– The rabbits destroyed huge expanses of Australia
– They threatened the sheep and cattle industries
• In 1950, a parasite
that infects rabbits
(myxoma virus)
was deliberately
introduced to
control the rabbit
population
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Figure 36.5A
Parasitism
• Coevolution of parasites and hosts is intense
– Example: the malaria parasite
• Provided a strong selective pressure for
humans to carry the defective hemoglobin gene
that causes sickle-cell anemia
• Sickle-cell anemia provides protection against
malaria
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Mutualism
• In mutualism, both the host
and the “other” species benefit
– Example: lichens, which
are entities formed by
fungi and algae living together
• The algae provide the
food by photosynthesis
and the fungi provide
protection
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• Examples of
mutualism
– Nitrogen-fixing
bacteria and legumes
– Acacia trees and the
ants of the genus
Pseudomyrmex
Figure 36.5B
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Mutualism
• In mutualism, both the host and the
“other” species benefit
– Example: clownfish and sea anemones
• The fish obtain protection and anemones
obtain protection, cleaning, and scraps of food
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36.6 Disturbance is a prominent feature of most
communities
• Disturbances include events such as storms,
fires, floods, droughts, overgrazing, and human
activities
– They damage
biological
communities
– They remove
organisms from
communities
– They alter the
availability of
resources
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Figure 36.6
Succession
• Succession is usually preceded by a
disturbance
– An event that disrupts the ecosystem either
by altering the community, its abiotic
structure, or both
– Examples: volcanic eruptions and forest
fires that decimate existing ecosystems but
leave behind nutrient-rich environments
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Succession
• During succession, most terrestrial
communities go through stages
– Succession begins with arrival of a few
hardy invaders called pioneers
• They alter the ecosystem in ways that favor
other species, which eventually displace the
pioneers
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Succession
• During succession, most terrestrial
communities go through stages
– Succession often progresses to a relatively
stable and diverse climax community
– Recurring disturbances can set back the
progress of succession
• Maintain communities in subclimax stages
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Succession
• Succession takes two major forms
– Primary succession
– Secondary succession
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Primary Succession
• Primary succession occurs “from
scratch,” where there is no trace of a
previous community
– Primary succession is the gradual
colonization of barren rocks by living
organisms
– May take thousands or even tens of
thousands of years
– Examples: succession starting on bare rock,
sand, or in a clear glacial pool
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Secondary Succession
• Secondary succession occurs after a
disturbance changes, but does not obliterate
an existing community
– Often takes just hundreds of years
– Example: Secondary succession occurs after
a disturbance has removed the vegetation
but left the soil intact; leaves behind some
seeds
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Succession in Ponds and Lakes
• Lakes and ponds form when a disturbance
blocks the flow of a river or stream
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Succession in Ponds and Lakes
• Nutrient influx, sediment deposition, and
other aquatic processes can convert a body of
water into a bog, then to a dry land
community
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Climax Community
• Unless disturbances intervene, succession
usually ends with a relatively stable climax
community
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Climax Community
• Species in climax communities have
narrower niches than pioneer species
– Allows many species to coexist without
replacing one another
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Climax Community
• Climax species tend to be larger and longerlived than pioneer species
• The exact nature of the climax community at
a site reflects local geological and climatic
conditions
– Examples: type of bedrock, temperature,
and rainfall
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Climax Community
• A biome is a class of climax community that
exists over a broad geographical range
– Examples: desert, grassland, or deciduous
forest
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Subclimax State
• Frequent disturbances maintain subclimax
communities in some ecosystems
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Subclimax State
• Subclimax community example: Tallgrass
prairies that once covered northern Missouri
and Illinois
– Periodic fires prevented forest from
encroaching
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Subclimax State
• Subclimax community example: Suburban
lawns
– Mowing and herbicides keep weeds and
woody species in check
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Subclimax State
• Subclimax community example: Agriculture
– Plowing and pesticides keep competing
weeds and shrubs from replacing early
successional cereal grains
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36.7 Talking About Science: Ecologist Frank
Gilliam discusses the role of fire in ecosystems
• Ecologist Frank Gilliam is
especially interested in the
role that fire plays in shaping
ecosystems
– According to Dr. Gilliam, fire
is a key abiotic factor in
many ecosystems
– Grasslands are so dependent
on fire that its absence is
considered a disturbance
Figure 36.7A
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• Following a fire in southeastern pine forest, the
numbers and variety of nonwoody plants
usually increase dramatically
– Fire makes more nutrients available to these
plants
Figure 36.7B
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ECOSYSTEM STRUCTURE AND DYNAMICS
36.8 Energy flow and chemical cycling are the two
fundamental processes in ecosystems
• A community interacts with abiotic factors,
forming an ecosystem
• Energy flows from the sun, through plants,
animals, and decomposers, and is lost as heat
• Chemicals are recycled between air, water,
soil, and organisms
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• A terrarium ecosystem
Chemical cycling
(C, N, etc.)
Light
energy
Chemical
energy
Heat
energy
Figure 36.8
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36.9 Trophic structure is a key factor in ecosystem
dynamics
• A food chain is the stepwise flow of energy and
nutrients
– from plants (producers)
– to herbivores (primary consumers)
– to carnivores (secondary and higher-level
consumers)
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TROPHIC LEVEL
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Producers
Plant
Phytoplankton
A TERRESTRIAL FOOD CHAIN
AN AQUATIC FOOD CHAIN
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Figure 36.9A
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• Decomposition is the breakdown of organic
compounds into inorganic compounds
• Decomposition is essential for the continuation
of life on Earth
• Detritivores
decompose waste
matter and recycle
nutrients
– Examples: animal
scavengers, fungi,
and prokaryotes
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Figure 36.9B
36.10 Food chains interconnect, forming food webs
• A food web is a network of interconnecting
food chains
– It is a more realistic view of the trophic
structure of an ecosystem than a food chain
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Wastes and
dead organisms
Tertiary
and
secondary
consumers
Secondary
and
primary
consumers
Primary
consumers
Producers
(Plants, algae,
phytoplankton)
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Detritivores
(Prokaryotes, fungi,
certain animals)
Figure 36.10
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36.11 Energy supply limits the length of food chains
• Biomass is the amount of living organic
material in an ecosystem
• Primary production is the rate at which
producers convert sunlight to chemical energy
– The primary production of the entire biosphere
is about 170 billion tons of biomass per year
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• A pyramid of production reveals the flow of
energy from producers to primary consumers
and to higher trophic levels
Tertiary
consumers
10 kcal
Secondary
consumers
100 kcal
Primary
consumers
1,000
kcal
Producers
10,000 kcal
1,000,000 kcal of sunlight
Figure 36.11
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• Only about 10% of the energy in food is stored
at each trophic level and available to the next
level
– This stepwise energy loss limits most food
chains to 3 - 5 levels
– There is simply not enough energy at the very
top of an ecological pyramid to support another
trophic level
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36.12 Connection: A production pyramid explains
why meat is a luxury for humans
• The dynamics of energy flow apply to the
human population as much as to other
organisms
– When we eat grain or fruit, we are primary
consumers
– When we eat beef or other meat from herbivores,
we are secondary consumers
– When we eat fish like trout or salmon (which eat
insects and other small animals), we are tertiary
or quaternary consumers
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• Because the production pyramid tapers so
sharply, a field of corn or other plant crops can
support many more vegetarians than meateaters
TROPHIC LEVEL
Secondary
consumers
Primary
consumers
Human
meat-eaters
Human
vegetarians
Cattle
Corn
Corn
Producers
Figure 36.12
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36.13 Chemicals are recycled between organic
matter and abiotic reservoirs
• Ecosystems require daily infusions of energy
– The sun supplies the Earth with energy
– But there are no extraterrestrial sources of
water or other chemical nutrients
• Nutrients must be recycled between organisms
and abiotic reservoirs
– Abiotic reservoirs are parts of the ecosystem
where a chemical accumulates
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• There are four main abiotic reservoirs
– Water cycle
– Carbon cycle
– Nitrogen cycle
– Phosphorus cycle
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35.14 Water moves through the biosphere in a
global cycle
• Heat from the sun drives the global water cycle
– Precipitation
– Evaporation
– Transpiration
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Solar
heat
Water vapor
over the sea
Precipitation
over the sea
(283)
Net movement
of water vapor
by wind (36)
Evaporation
from the sea
(319)
Water vapor
over the land
Evaporation
and
transpiration
(59)
Precipitation
over the land
(95)
Oceans
Flow of water
from land to sea
(36)
Surface water
and groundwater
Figure 36.14
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36.15 The carbon cycle depends on photosynthesis
and respiration
• Carbon is taken from the atmosphere by
photosynthesis
– It is used to make organic molecules
– It is returned to the atmosphere by cellular
respiration
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CO2 in atmosphere
Burning
Cellular respiration
Plants,
algae,
cyanobacteria
Photosynthesis
Higher-level
consumers
Primary
consumers
Wood and
fossil fuels
Decomposition
Detritivores
(soil microbes
and others)
Detritus
Figure 36.15
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36.16 The nitrogen cycle relies heavily on bacteria
• Nitrogen is plentiful in the atmosphere as N2
– But plants cannot use N2
• Various bacteria in soil (and legume root
nodules) convert N2 to nitrogen compounds
that plants can use
– Ammonium (NH4+) and nitrate (NO3–)
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• Some bacteria break down organic matter and
recycle nitrogen as ammonium or nitrate to
plants
• Other bacteria return N2 to the atmosphere
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Nitrogen (N2) in atmosphere
Assimilation
by plants
Amino acids
and proteins in
plants and animals
Denitrifying
bacteria
Nitrogen
fixation
Detritus
Nitrogen-fixing
bacteria in root
nodules of legumes
Nitrates
(NO3–)
Detritivores
Decomposition
Nitrifying
bacteria
Nitrogen-fixing
bacteria in soil
Nitrogen
fixation
Ammonium (NH4+)
Figure 36.16
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36.17 The phosphorus cycle depends on the
weathering of rock
• Phosphates (compounds containing PO43-) and
other minerals are added to the soil by the
gradual weathering of rock
• Consumers obtain phosphorus in organic form
from plants
• Phosphates are returned to the soil through
excretion by animals and the actions of
decomposers
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Uplifting
of rock
Phosphates
in organic
compounds
Weathering
of rock
Phosphates
in rock
Animals
Plants
Runoff
Detritus
Phosphates
in solution
Phosphates
in soil
(inorganic)
Decomposition
Rock
Precipitated
(solid) phosphates
Detritivores
in soil
Figure 36.17
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ECOSYSTEM ALTERATION
36.18 Connection: Ecosystem alteration can upset
chemical cycling
• Experimental studies have been performed to
determine chemical cycling in ecosystems
• A study to monitor nutrient dynamics has
been ongoing in the Hubbard Brook
Experimental Forest since 1963
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• Dams were built
across streams at
the bottom of each
watershed to
monitor water and
nutrient losses
Figure 36.18A
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• In 1966, one of the valleys was completely
logged
– It was then
sprayed with
herbicides for
3 years to
prevent plant
regrowth
– All the original
plant material
was left in
place to
decompose
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Figure 36.18B
• Researchers found that the total removal of
vegetation can increase the runoff of water and
loss of soil nutrients
Figure 36.18C
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• Environmental changes caused by humans can
unbalance nutrient cycling over the long term
– Example: acid rain
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36.19 Talking About Science: David Schindler
talks about the effects of nutrients on
freshwater ecosystems
• Eutrophication is a process in which nutrient
runoff from agricultural lands or livestock
operations causes photosynthetic organisms in
ponds and lakes to multiply rapidly
– The result is algal bloom
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• Algal bloom can cause a pond or lake to lose
much of its species diversity
– Human-caused eutrophication wiped out
fisheries in Lake Erie in the 1950s and 1960s
Figure 36.19B
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• Dr. David Schindler is an ecologist who worked
at the Experimental Lakes Project in northern
Ontario
– He performed
several classic
experiments on
eutrophication
that led to the ban
on phosphates in
detergents
Figure 36.19A
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• According to Dr. Schindler, there are three
serious threats to freshwater ecosystems
– Acid precipitation
– Climate warming
– Changes in land use
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36.20 Connection: Zoned reserves are an attempt
to reverse ecosystem disruption
• The human alteration of ecosystems threatens
the existence of thousands of species
• To slow the disruption of ecosystems, some
nations are establishing zoned reserves
– These are undisturbed wildlands surrounded
by buffer zones of compatible economic
development
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• Costa Rica has established eight zone reserves
• Costa Rica looks to its zoned reserve system to
maintain at least 80% of its native species
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– On this map, the reserves are shown in green
and the buffer zones in yellow
NICARAGUA
COSTA
RICA
Guanacaste
Caribbean Sea
Llanuras de
Tortuguero
La
Amistad
Arenal
Bajo
Tempisque
Cordillera
Volcanica Central
Pacifico Central
Peninsula de Osa
Pacific Ocean
Figure 36.20
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Nutrient Cycles
• Nutrients are elements and small molecules
that form all the chemical building blocks of
life
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Nutrient Cycles
• Macronutrients are required by organisms in
large quantities
– Examples: water, carbon, hydrogen, oxygen
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Nutrient Cycles
• Micronutrients are required only in trace
quantities
– Examples: zinc, molybdenum, iron,
selenium
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Nutrient Cycles
• Nutrient cycles (or biogeochemical
cycles) describe the pathways nutrients
follow between communities and the
nonliving portions of ecosystems
– Reservoirs are sources and storage sites of
nutrients
– Major reservoirs are usually in the abiotic
environment
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The Carbon Cycle
• Chains of carbon atoms form the framework
of all organic molecules, the building blocks
of life
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The Carbon Cycle
• Carbon enters communities through capture
of CO2 during photosynthesis
– Producers on land get CO2 from the
atmosphere
– Aquatic producers get CO2 dissolved in the
water
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The Carbon Cycle
• Primary consumers eat producers and
acquire carbon stored in their tissues
– These herbivores release some of the carbon
through respiration as CO2
– They store the rest, which may be
consumed by higher trophic levels
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The Carbon Cycle
• If not eaten, when organisms die their bodies
are broken down by detritus feeders and
decomposers
• Cellular respiration by organisms releases
CO2 into the atmosphere and oceans
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The Carbon Cycle
• Fossil fuels are formed when the remains
of prehistoric organisms are buried and
subjected to high temperatures and
pressures for millions of years
– Burning fossil fuels releases stored energy
in hydrocarbons and releases carbon into
the atmosphere as CO2
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Nitrogen Cycle
• Nitrogen is a crucial component of proteins,
many vitamins, DNA, and RNA
• While nitrogen gas (N2) makes up 79% of the
atmosphere, this form of nitrogen cannot be
utilized by plants
• Plants utilize nitrate (NO3–) or ammonia
(NH3) as their nitrogen source
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The Nitrogen Cycle
• N2 is converted to ammonia by specific
bacteria
– Some of these bacteria live in water and soil
– Others live in symbiotic associations with
plants called legumes
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The Nitrogen Cycle
• Primary consumers, detritus feeders, and
decomposers obtain nitrogen from their food
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The Nitrogen Cycle
• Some nitrogen is released in wastes and dead
bodies
• Decomposer bacteria convert this back to
nitrate and ammonia in the soil or water,
which is then available to plants
• Denitrifying bacteria break down nitrate,
releasing N2 back to the atmosphere
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Nitrogen Cycle
• Human-dominated ecosystems have
dramatically altered nitrogen cycles
– Application of chemical fertilizers may
change plant community composition
– Burning of forests and fossil fuels releases
nitrogen that causes habitat acidification
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The Phosphorous Cycle
• Phosphorus is a crucial component of ATP
and NADP, nucleic acids, and phospholipids
of cell membranes
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The Phosphorous Cycle
• The major reservoir of the phosphorus cycle
is in rock bound to oxygen as phosphate
– Phosphate in exposed rock can be dissolved
by rainwater
– It is absorbed by autotrophs, where it is
incorporated into biological molecules that
pass through food webs
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The Phosphorous Cycle
• The major reservoir of the phosphorus cycle
is in rock bound to oxygen as phosphate
– At each level, excess phosphorus is excreted
and decomposers release phosphate
– Phosphate may be reabsorbed by
autotrophs or reincorporated into rock
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Phosphorous Cycle
• Phosphate-rich fertilizers are obtained by
mining rock
• Soil erosion from fertilized fields carries
large quantities of phosphate into lakes,
streams, and oceans
– Stimulates growth of algae and bacteria,
disrupting natural community interactions
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The Hydrologic Cycle
• Water molecules remain chemically
unchanged during the hydrologic cycle
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The Hydrologic Cycle
• The major reservoir of water is the ocean
– Contains more than 97% of Earth’s water
• Solar energy evaporates water, and it comes
back to Earth as precipitation
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The Hydrologic Cycle
• Water that has fallen on land takes various
paths
– Some evaporates from the soil, lakes, and
streams
– Some runs off the land back to the ocean
– A small amount enters underground
reservoirs
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The Hydrologic Cycle
• Most water evaporates from the surface of
the ocean
• Plants absorb water through roots, but most
is evaporated back to the atmosphere from
leaves
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Hydrologic Cycle
• Consumers get water from their food or by
drinking
– Their bodies are roughly 70% water
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The Hydrologic Cycle
• With human population growth, fresh water
has become scarce
– Water scarcity limits crop growth
– Pumping water from underground aquifers
is rapidly depleting many of them
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The Hydrologic Cycle
• With human population growth, fresh water
has become scarce
– Contaminated drinking water is consumed
by over 1 billion people in developing
countries each year, killing millions of
children
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