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CHAPTER 5
Ecosystems:
Energy, Patterns,
and Disturbance
© 2011 Pearson Education, Inc.
Introduction to ecosystems
• In 1988, lightning started fires in Yellowstone
National Park
• 165,000 acres were burned
• National Park Service policies have changed over
time
• In the early years, all fires were extinguished
• Before 1988, only fires that threatened human
habitations were extinguished
• This fire started a great controversy over this policy
• Snow in September finally put the fires out
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Yellowstone recovered from the 1988
fire
• The fires burned 36% of the park
• Burned and unburned areas were interspersed
• Within 2 weeks, grasses and other vegetation
sprouted
• Within a year, vegetation covered the burned areas
• Bison and elk fed on the new vegetation
• Within 25 years, plant and animal diversity will have
completely recovered in the burned areas
• Fire is vital to many ecosystems
• It may even impact evolution
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Recovery from fire
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Characteristics of ecosystems
• Yellowstone National Park (founded in 1872) is part
of the Greater Yellowstone Ecosystem
• Because of its unique features, it is a World Heritage
Site and International Biosphere Reserve
• Ecosystems contain communities of interacting
species and their abiotic factors
• They function on different scales
• It’s hard to delineate fixed boundaries
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Scientists study ecosystems
• Biomes: ecosystems having similar vegetation and
climactic conditions
• Greater Yellowstone Ecosystem belongs to the
northern temperate forest biome
• Scientists study ecosystem properties
• Trophic levels
• Productivity
• Consumption
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Trophic levels
• During photosynthesis, plants use the Sun’s
energy
• Producing chemicals from carbon dioxide and water
• Plants are eaten by predators (a grasshopper,
mouse, etc.)
• These animals are eaten by other predators
• Food chain: describes where energy and nutrients
go as they move from one organism to another
• Energy moves “up” the food chain
• Not all energy and nutrients are passed to other
levels
• Food web: interconnection of food chains to form
complex webs of feeding relationships
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Food webs
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Trophic categories
• Autotrophs: produce organic material from
inorganic constituents through the use of an
external energy source
• Also referred to as producers
• Green plants, some single-celled organisms and
bacteria
• Heterotrophs: must consume organic material
to obtain energy
• Consumers: eat living prey
• Decomposers: scavengers, detritus feeders,
chemical decomposers eat dead organic material
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Producers are essential to every
ecosystem
• They capture energy from the Sun or chemical
reactions
• Converting CO2 to organic matter
• Most producers are green plants
• Chlorophyll: a green pigment that captures light energy
• Range in size from microscopic bacteria to gigantic
trees
• Every major ecosystem has producers
• Chemosynthesis: some bacteria use energy in
inorganic chemicals to form organic matter from CO2
and water
• Primary production: production of organic matter
through photosynthesis and growth of producers
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Consumers
• Organisms feed on organic matter for energy
• Animals, fungi (mushrooms, mold, etc.), most bacteria
• Range in size from plankton to blue whales
• Divided into subgroups according to their food
source
• Primary consumers (herbivores): feed on
producers
• Secondary consumers: feed on primary consumers
• Third (tertiary), fourth (quaternary), or higher levels
• Carnivores: secondary or higher-order meat eaters
• Omnivores: feed on both plants and animals
• Animals can occupy various levels, depending on
the food
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A grassland food chain
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Decomposers
• Detritus: dead plant material (leaves, etc.), fecal
wastes, dead bodies
• Most energy in an ecosystem goes through this
food web
• Detritus is organic and high in potential energy for
• Decomposers
• Scavengers (vultures): break down large pieces of
matter
• Detritus feeders (earthworms): eat partly
decomposed matter
• Chemical decomposers (fungi and bacteria): break
down matter on the molecular scale
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Detritus food web
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Decomposers act like any other
consumer
• Some decomposers (e.g., termites) digest woody
material
• They have a mutualistic, symbiotic relationship with
decomposer microorganisms in their guts
• Most decomposers use oxygen for cell respiration
• Some decomposers (bacteria and yeasts) partially
break down glucose in the absence of oxygen
(fermentation)
• Results in ethyl alcohol, methane gas, acetic acid
• Anaerobic (oxygen-free) respiration: in sediments
of lakes, marshes, swamps, and animal guts
• Cattle and their fermenting bacteria release methane
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A termite gut
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Limits on trophic levels
• Terrestrial ecosystems usually have three or four
trophic levels
• Marine systems sometimes have five
• Biomass: the total combined (net dry) weight of
organisms
• Each higher trophic level has about 90% less biomass
• One acre of grassland has 907 kg (2,000 lbs)
• It has 90.7 kg (200 lbs) of herbivores
• It has 9.7 kg (20 lbs) of primary carnivores
• Biomass pyramid: the different levels of producer
and consumer mass
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A biomass pyramid
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The flow of energy in ecosystems
• In most ecosystems, sunlight is the initial source
of energy
• Primary production (production of organic
molecules) is only 2% of the incoming solar
energy
• Although small, it’s enough to fuel all life
• Standing-crop biomass: the actual biomass of
primary producers in an ecosystem at any given
time
• Not always a good measure of productivity
• Biomass and primary production vary greatly
• Forests have large biomass
• Grasslands have high primary production
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The fate of food
• Between 60 and 90% of food consumed is
oxidized for energy
• 10−40% is converted to body tissues for growth,
repair, and maintenance
• Undigested food passes through the digestive
system and is excreted
• Cellulose: material in plant cell walls
• Excreted from herbivores
• Forms fiber, bulk, or roughage: a necessary part of
the diet
• Carbon dioxide, nitrogen, phosphorus: excreted
in urine
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Energy flow and efficiency
• There is a huge inefficiency at each trophic level
• Only a small fraction of energy is passed on when
energy flows from one trophic level to the next
• Much of the biomass is not consumed by herbivores
• Some food is used as energy to fuel the hetrotroph’s
cells and tissues
• Some food is not digested and is excreted as waste
• Secondary production: incorporating matter and
energy from a lower trophic level into a consumer’s
body
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Inefficiency at trophic levels
• Individuals at higher levels represent a greater
amount of the Sun’s energy for the same amount
of body tissue
• More energy is needed to produce a top-order
consumer than a producer
• It takes more time, water, and resources to
produce a top-order consumer
• Some materials are hard to excrete (e.g.,
chemicals in fat)
• They biomagnify as you go up the food chain
• They bioaccumulate (build up in tissues)
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Aquatic systems
• These systems go through the same process as
terrestrial ecosystems, with two major differences
• Less energy is required in aquatic systems
• More cold-blooded animals, which require less energy
• Less energy is needed to support body weight in
water
• With less energy needed at each level
• More energy is available to the next level
• Food chains can be longer
• Aquatic systems may have a reversed biomass
pyramid
• Larger, older fish eat algae that turn over rapidly
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A reverse pyramid in aquatic systems
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From ecosystems to biomes
• Broad ecosystem patterns translate into a
predictable set of organisms that live under
particular conditions
• Different regions have distinct biotic communities
• Creating variety in ecosystems, landscapes, and
biomes
• A biome: a large geographical biotic community
• Controlled by climate
• Is named after the dominant vegetation
• Has fuzzy boundaries
• Aquatic areas are not called biomes
• But they function similarly
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The role of climate
• Climate: a description of the average temperature
and precipitation (weather) of a region
• Climates vary widely
• Equatorial areas: warm, high rainfall, no seasons
• Above and below the equator: temperatures become
seasonal (warm/hot summers, cool/cold winters)
• Toward the poles: longer and colder winters
• Colder temperatures are also found at higher
elevations
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Effects of latitude and altitude
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Effects of precipitation on biomes
• Precipitation varies widely in different regions
• From almost 0 to over 250 cm (100 in.)/yr
• It can be evenly distributed throughout the year or
concentrated in certain months (wet and dry
seasons)
• A given climate supports species that can tolerate
the temperature and precipitation levels of the
area
• Highest densities occur where conditions are
optimal
• A species is excluded where any condition is
beyond its limit of tolerance
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Biome examples
• Individual ranges of tolerance to temperature and
precipitation determine where a species can live
• Species’ distributions describe a biome’s placement
• Six major types of biomes exist
• Rainfall effects are primary in determining biomes
• Temperate deciduous forest: rainfall of 72–200 cm
(30–80 in.)/yr
• Grassland (prairie) biome: rainfall is less or
seasonal
• Desert biome: rainfall is less than 25 cm (10 in.)/yr
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The effects of temperature on biomes
• Temperature effects are superimposed on rainfall
effects
• It determines the kind of forests in an area with
75 cm (30 in.) or more of rainfall per year
• Tropical rain forests have broad-leaved evergreens
that cannot tolerate freezing
• Deciduous trees tolerate freezing by dropping their
leaves and becoming dormant
• Coniferous forests tolerate the harsh winters and
short summers of northern regions
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Biomes with little precipitation
• Permafrost: permanently frozen subsoil
• Prohibits tree growth because their roots cannot
penetrate the soil
• Tundra biome: has grasses, clover, and other small
plants that grow above the permafrost
• Desert: any region with less than 25 cm (10 in.) of
rain/yr
• Hot deserts have different species than cold deserts
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World distribution of the major
terrestrial biomes
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Climate and major biomes
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Aquatic systems
• Aquatic systems have major categories
• But are not called biomes
• Aquatic and wetland ecosystems are determined
by depth, salinity, and permanence of water
• Lakes, marshes, streams, rivers, estuaries, bays
• Ocean systems
• Aquatic systems can be viewed as ecosystems
• Or part of landscapes
• Or as major biome-like features (seas, oceans)
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Microclimates and other abiotic factors
• Microclimate: the conditions in a specific, localized
area
• Temperature and moisture may be different from the
overall climate of the region
• Conditions result in variations of ecosystems in a
biome
• For example, the Greater Yellowstone region is in
the northern temperate forest biome
• But it also has grassland and permafrost
• Soil and topography affect the availability of
moisture
• Oaks and hickory trees grow in rocky, sandy soils
• Beech and maple trees grow on richer soils
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Microclimates
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Biome productivity
• Biomes have different levels of primary productivity
• Highly productive biomes support organisms from
other biomes (e.g., seabirds migrating through
marshes)
• They also remove and trap CO2 from the
atmosphere
• Productive biomes aren’t better than other biomes
• Biomes can have other roles, e.g., habitat for rare
species
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Different ecosystems have different
productivity
• Tropical rain forests are highly productive
• Warm temperatures and rainfall are ideal for
photosynthesis
• Open oceans cover a large part of Earth
• But they have low productivity
• Primary production is limited by scarcity of nutrients
• They are veritable deserts
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Productivity of different ecosystems
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Ecosystem responses to disturbance
• Natural ecosystems operate in dynamic, changing
ways
• The landscape comprises a shifting mosaic of
patches
• Disturbance: a significant change that kills or
displaces many community members
• Ecological succession: transition from one biotic
community to another
• Pioneer species: colonize a newly opened area first
• Species can create conditions favorable to other
species and less favorable to them
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Succession does not go on indefinitely
• Facilitation: driving succession forward by
improving conditions for subsequent species
• Climax ecosystem: the assemblage of species
continues on in space and time
• Even these communities experience change if new
species are introduced or old ones are removed
• Patches of disturbance open space for new growth
• Fire-adapted ecosystems: some biomes (e.g.,
prairies) undergo succession to other stages
without periodic fires
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Primary succession
• Primary succession: the process of initial invasion
and progression from one biotic community to
another
• In an area lacking plants and soil (e.g., a retreating
glacier)
• Mosses exploit bare rock
•
•
•
•
Their spores lodge in cracks
Moss grows and forms mats that trap soil particles
Seeds of larger plants lodge in the moss mats
Eventually, enough soil is trapped to support shrubs
and trees
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Primary succession on bare rock
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Secondary succession
• Secondary succession: an area cleared by some
disturbance (fire, floods, humans) is reinvaded by
plants and animals from surrounding areas
• Starts with pre-existing soil
• Crabgrass invades an abandoned agricultural field
• It is shaded out by taller grasses and weeds
• Pine trees grow in the direct sunlight and shade out
grasses and weeds and their own seedlings
• Hardwoods (oaks, hickories, maple, etc.) can grow in
shade and are the climax forest ecosystem
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Secondary succession of an
abandoned field
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Aquatic succession
• Natural succession also takes place in lakes and
ponds
• Soil particles erode from the land and enter the
water
• Aquatic vegetation provides detritus that also fills the
pond or lake
• Terrestrial species advance and aquatic species
move further into the lake
• The climax ecosystem can be a bog or forest
• Disturbances (e.g., drought, flood) can send
succession back to an earlier stage
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Aquatic succession of a lake
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Disturbance and resilience
• For succession to occur, plants and animals must
already be present in the area
• All stages of succession are present in any
landscape
• Disturbances constantly create gaps or patches
• Biodiversity is enhanced by disturbance
• Natural succession can be blocked or modified if
species have been eliminated
• Forests in Iceland were eliminated
• Regeneration was prevented due to a lack of seeds
and the presence of grazing sheep
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Iceland
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Fire and succession
• Fire is a major form of disturbance
• Decades ago, forest managers thought all fire
was bad
• But pine forests were replaced with economically
worthless broad-leaved trees
• Accumulated deadwood allowed insects to attack
trees
• Different species have different tolerances to fire
• Grasses and pines tolerate fire
• Broad-leaved trees are damaged by fire
• Fire releases nutrients
• Some plants need fire to germinate
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Fire climax ecosystems
• Fire climax ecosystems: ecosystems that depend on
fire to maintain their existence (e.g., grasslands, pine
forests)
• Fire can be a tool in ecosystem management
• With regular fire, deadwood doesn’t accumulate
• Crown fires are also natural
• They clear sick or dead trees and release nutrients
• The meadows they create support higher biodiversity
• Logging encourages large fires
• Removes larger, resistant trees and leaves dead
branches
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Ground fire
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Resilience
• Resilience: the ability of an ecosystem to return to
normal functioning after a disturbance
• Helps maintain ecosystem sustainability
• Resilience mechanism: the processes of
replenishment of nutrients, dispersion by plants and
animals, regrowth of plants
• Resilience has its limits
• A badly degraded ecosystem can’t carry out its
original functions
• A new, less productive ecosystem is created
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Resilience in ecosystems
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Why it matters if ecosystems are
changed
• Ecosystems provide valuable services to humans
and other species
• Flood control, maintaining soil, absorbing CO2
• Human actions tend to lower ecosystem services
• They produce ecosystems with low productivity or
low food chain length
• Ecosystems are less complex and have lower
biodiversity
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Human values and sustainability
• Natural ecosystems are models of sustainability
• We depend on them for goods and services
(ecosystem capital)
• We are threatening their sustainability
• Humans use energy that flows through ecosystems
• Converting forests and grasslands into agricultural
ecosystems
• We appropriate 40% of global net primary
productivity
• For agriculture, grazing, forestry, houses, roads, etc.
• Humans are the dominant biological force on Earth
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Global primary productivity in
terrestrial ecosystems
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Involvement in nutrient cycling
• Nutrients are replenished through breakdown of
organic compounds
• Maintaining the sustainability of ecosystems
• Humans are changing nutrient cycles
• Climate change is one of the most serious
consequences
• Burning fossil fuels has increased CO2 in the
atmosphere
• Methane is released from melting permafrost and
cattle
• Many ecosystems may not be able to function
sustainably
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Value of ecosystem capital
• Developing countries use natural ecosystems
directly
• For survival and economic gains
• For fuelwood, construction materials, food
• Access may be uncertain
• Developed countries give little thought or value to
natural ecosystems
• People are unconcerned about—and insulated
from—human impacts
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The value of the world’s ecosystems
• “The Value of the World’s Ecosystem Services and
Natural Capital” (1977) reported that
• Natural goods and services are not easily seen in
markets
• They may not be counted or valued at all
• We can’t consider human welfare without these
services
• Incremental value of services: changes in the
quantity or quality of services that influence human
welfare
• Can be calculated and given an economic value
• The global value of ecosystem services = $41 trillion/yr
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Using ecosystem value in decision
making
• Vital ecosystem services should be included in
calculating costs and benefits of a proposed change in
land use
• Ecosystem capital stock: ecosystems and populations
must be given adequate weight in public-policy decisions
involving changes to that stock
• But the services are often ignored or undervalued
• Ignoring services results in changes to natural systems
• But the social costs far outweigh the benefits
• For example, mangrove forests are converted to shrimp
farms even though the forest’s value exceeds the farm’s
value by 70%
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A new look
• Examining the benefit-cost consequences of
converting ecosystems to human uses shows that
• In every case, the net balance of value was a loss
• The services lost outweighed the services gained
• $250 billion/yr is lost through habitat conversion alone
• Why are ecosystems still being changed?
• Benefits of conversion are exaggerated by subsidies
• The market does not measure ecosystem benefits well
• Benefits of conversion are local; costs are larger-scale
• Conservation is achievable when all costs are
included
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Can ecosystems be restored?
• Humans have the capacity to restore ecosystems
• Restoration ecologists: try to restore ecosystems
• The three assumptions of ecosystem restoration
• Abiotic factors must be unchanged or able to return
to their original state
• Viable populations of species must still exist
• Foreign species must be eliminated or not interfere
with survival of reintroduced native species
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The process of restoration ecology
• Consists of developing a model of the desired
ecosystem
•
•
•
•
Designing and implementing a plan for restoration
Stating clear standards to evaluate progress
Monitoring the plan
Developing strategies for long-term protection and
maintenance of the system
• We should restore ecosystems
• For aesthetic reasons, human use, other species
• Nature has value separate from humans
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The future
• Human population growth and rising consumption
will challenge ecosystem sustainability
• There is unprecedented pressure on Earth to
provide goods and services
• For example, agricultural demands require more
land
• We can choose other alternatives
• We must understand how essential ecosystems are
• We need wisdom and political will to promote
ecosystem sustainability
• We must manage ecosystems
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Managing ecosystems
• No ecosystem can escape human impact
• Good ecosystem management is based on
understanding:
• How ecosystems function
• How they respond to disturbances
• What goods and services they provide
• U.S. agencies have adopted ecosystem
management as their management paradigm
• Forest Service, Bureau of Land Management,
National Park Service, Environmental Protection
Agency
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Principles of ecosystem management
• It looks at ecosystems at both small and large scales
• It preserves the range of possible landscapes
• Managers learn from experiments and use new
knowledge
• Managers are not just government agencies
• Valuable goods and services are recognized
• All stakeholders are included
• It incorporates the objective of ecological
sustainability
• According to the U.S. Forest Service, sustainability
should be the guiding star for stewardship of national
forests and grasslands
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Conclusions from Living Beyond Our
Means
• The Millennium Ecosystem Assessment’s report
Living Beyond Our Means: Natural Assets and
Human Well-Being concluded that ecosystems may
no longer be able to sustain future generations
• We depend on nature and ecosystem services
• Humans have profoundly changed ecosystems
• These changes have improved lives but weakened
nature’s ability to provide other services
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More conclusions from Living Beyond
Our Means
• Outstanding problems include overfishing, loss of
services, climate change, nutrient pollution
• Humans have caused a massive wave of species
extinction
• Loss of services is a barrier to solving poverty, hunger,
disease
• Pressures on ecosystems will increase unless attitudes
and actions change
• Conservation succeeds when local people are involved
• Technology and knowledge can reduce human impacts
• Protection of natural assets requires coordinated efforts
• We are living off the future—an unsustainable situation
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CHAPTER 5
Ecosystems:
Energy, Patterns,
and Disturbance
Active Lecture Questions
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Review Question-1
The process of initial invasion and progression
from one biotic community to the next is called
a. primary succession.
b. secondary succession.
c. a climax ecosystem.
d. fire.
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Review Question-1 Answer
The process of initial invasion and progression
from one biotic community to the next is called
a. primary succession.
b. secondary succession.
c. a climax ecosystem.
d. fire.
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Review Question-2
True or False: Forest fires are destructive to
ecosystems and should be avoided if at all
possible.
a. True
b. False
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Review Question-2 Answer
True or False: Forest fires are destructive to
ecosystems and should be avoided if at all
possible.
a. True
b. False
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Review Question-3
Resilience mechanisms might include
a. replenishment of nutrients.
b. rapid regrowth of plant cover.
c. succession in a forest.
d. all of the above.
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Review Question-3 Answer
Resilience mechanisms might include
a. replenishment of nutrients.
b. rapid regrowth of plant cover.
c. succession in a forest.
d. all of the above.
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Review Question-4
When an ecosystem reaches a dynamic
balance between all of the species and the
physical environment, the ecosystem is
considered
a. at climax.
b. in primary succession.
c. in secondary succession.
d. in aquatic succession.
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Review Question-4 Answer
When an ecosystem reaches a dynamic
balance between all of the species and the
physical environment, the ecosystem is
considered
a. at climax.
b. in primary succession.
c. in secondary succession.
d. in aquatic succession.
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Review Question-5
A small human action that catalyzes a major
change in the state of an ecosystem is called
the
a. turning point.
b. dew point.
c. tipping point.
d. point of no return.
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Review Question-5 Answer
A small human action that catalyzes a major
change in the state of an ecosystem is called
the
a. turning point.
b. dew point.
c. tipping point.
d. point of no return.
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Interpreting Graphs and Data-1
According to Fig. 5-11, the temperature and
precipitation of the moist tundra biome can be
described as
a. cold and wet.
b. cold and dry.
c. hot and wet.
d. hot and dry.
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Interpreting Graphs and Data-1 Answer
According to Fig. 5-11, the temperature and
precipitation of the moist tundra biome can be
described as
a. cold and wet.
b. cold and dry.
c. hot and wet.
d. hot and dry.
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Interpreting Graphs and Data-2
According to Fig. 5-20, areas shaded in black
have
a. no net primary
production.
b. low net primary
production.
c. medium net
primary production.
d. high net primary
production.
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Interpreting Graphs and Data-2 Answer
According to Fig. 5-20, areas shaded in black
have
a. no net primary
production.
b. low net primary
production.
c. medium net
primary production.
d. high net primary
production.
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Thinking Environmentally-1
All of the following are categories of consumers
except
a. herbivores.
b. photosynthesizers.
c. omnivores.
d. parasites.
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Thinking Environmentally-1 Answer
All of the following are categories of consumers
except
a. herbivores.
b. photosynthesizers.
c. omnivores.
d. parasites.
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Thinking Environmentally-2
Which of the following might be considered
primary stakeholders in an ecosystem?
a. government decision makers
b. scientists studying the ecosystem
c. people living within the ecosystem
d. conservation organizations
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Thinking Environmentally-2 Answer
Which of the following might be considered
primary stakeholders in an ecosystem?
a. government decision makers
b. scientists studying the ecosystem
c. people living within the ecosystem
d. conservation organizations
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