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Lecture Outlines
Chapter 4
Species Interactions and
Community Ecology
Withgott/Laposata
Fifth Edition
© 2014 Pearson Education, Inc.
This lecture will help you understand:
 Species interactions
 Feeding relationships, energy flow, trophic levels,
and food webs
 Keystone species
 The process of succession
 Potential impacts of invasive species
 Restoration ecology
 Terrestrial biomes
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Central Case Study: Black and White and Spread
All Over: Zebra Mussels Invade the Great Lakes
 In 1988, zebra mussels were accidentally introduced
to Lake St. Clair in discharged ballast water
 By 2010, they had invaded 30 states
 No natural predators, competitors, or parasites
 They cause millions of dollars of damage to property
each year
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Species Interactions
 Species interactions shape and define communities
 Natural species interactions:
 Competition = relationship in which both species are
harmed
 Exploitative = one species benefits and the other is
harmed
 Predation, parasitism, and herbivory
 Mutualism = relationship in which both species
benefit
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Competition can occur when resources are
limited
 In competition, multiple organisms seek the same
limited resources and all competitors suffer
(/ interaction)
 Food, space, water, shelter, mates, sunlight
 Intraspecific competition = occurs between
members of the same species
 High population density leads to increased
competition
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Competition can occur when resources are
limited
 Interspecific competition = occurs between
members of two or more species
 Strongly affects community composition
 Leads to competitive exclusion or species
coexistence
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Competition can occur when resources are
limited
 Competitive exclusion = one species completely
excludes another species from using a resource
 Zebra mussels displaced native mussels in the Great
Lakes
 Species coexistence = neither species fully
excludes the other from resources, so both live side
by side
 This produces a stable point of equilibrium, with
stable population sizes
 Species minimize competition by using only a part of
the available resource (niche)
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Competition can occur when resources are
limited
 Fundamental niche = the full niche of a species
 Realized niche = the portion of the fundamental
niche that is actually filled
 Due to competition or other species’ interactions
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© 2014 Pearson Education, Inc.
Competition can occur when resources are
limited
 Resource partitioning = process by which species
use different resources or use shared resources in
different ways
 For example, birds all eating insects, but catching
them on different parts of the tree
 For example, one species eats small seeds and
another eats large seeds
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© 2014 Pearson Education, Inc.
Competition can occur when resources are
limited
 Character displacement = competing species
diverge in their physical characteristics due to the
evolution of traits best suited to the resources they
use
 Results from resource partitioning
 Birds that eat larger seeds evolve larger bills
 Birds that eat smaller seeds evolve smaller bills
Competition is reduced when two species
become more different
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Several types of interactions are exploitative
 Exploitation = process by which one member
exploits another for its own gain (/ interactions)
 Predation, parasitism, herbivory
 Predation = process by which individuals of one
species (predators) capture, kill, and consume
individuals of another species (prey)
 Structures food webs
 The number of predators and prey influences
community composition
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Predators kill and consume prey
 Zebra mussels eat phytoplankton and zooplankton
 Both populations decrease in lakes with zebra
mussels
 Zebra mussels don’t eat cyanobacteria
 Cyanobacteria increase in lakes with zebra mussels
 Predators may also be prey
 Zebra mussels are being eaten by diving ducks,
muskrats, crayfish, flounder, sturgeon, eels, carp, and
freshwater drum
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Predators kill and consume prey
 Predator–prey interactions can sometimes drive
cyclical population dynamics
 Increased prey population increases predators
 Predators survive and reproduce
 Increased predator population decreases prey
 Predators starve
 Decreased predator population increases prey
populations
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© 2014 Pearson Education, Inc.
Predators kill and consume prey
 Natural selection leads to evolution of adaptations
that make predators better hunters
 Individuals who are better at catching prey:
 Live longer, healthier lives
 Take better care of offspring
 Prey face strong selection pressures: they are at risk
of immediate death
 Prey have evolved elaborate defenses against being
eaten
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© 2014 Pearson Education, Inc.
Parasites exploit living hosts
 Parasitism = a relationship in which one organism
(parasite) depends on another (host) for nourishment or
some other benefit
 The parasite harms, but doesn’t kill, the host
 Some parasites are free-living
 Infrequent contact with their hosts
 Ticks, sea lampreys
 Some live within the host
 Tapeworms
 Others cause disease
 These are pathogens
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© 2014 Pearson Education, Inc.
Parasites exploit living hosts
 Parasitoids = insects that parasitize other insects
 Lay eggs in larva or adults of other species
 Kill the host
 Coevolution = process in which hosts and parasites
become locked in a duel of escalating adaptations
 Has been called an evolutionary arms race
 Each evolves new responses to the other
 It may not be evolutionarily beneficial to the parasite
to kill its host
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Herbivores exploit plants
 Herbivory = when animals feed on the tissues of
plants
 May not kill the plant, but affects its growth and
survival
 Defenses against herbivory include:
 Chemicals: toxic or distasteful
 Thorns, spines, or irritating hairs
 Other animals: protect the plant
 Like in parasitism, herbivore–plant interaction can
result in coevolution.
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© 2014 Pearson Education, Inc.
Mutualists help one another
 Mutualism = relationship in which two or more
species benefit from their interactions
 Symbiosis = mutualism in which the organisms live
in close physical contact
 Each partner provides a service the other needs
(food, protection, housing, etc.)
 Microbes within digestive tracts
 Mycorrhizae: plant roots and fungi
 Coral and algae (zooxanthellae)
 Pollination = interaction in which bees, bats, birds,
and others transfer pollen from one flower to
another, fertilizing its eggs
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© 2014 Pearson Education, Inc.
Ecological Communities
 Community = an assemblage of populations of
organisms living in the same place at the same time
 Members interact with each other
 Interactions determine the structure, function, and
species composition of the community
 Community ecologists are people interested in:
 Which species coexist, and how species relate to one
another
 How communities change, and why patterns exist
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Energy passes among trophic levels
 One of the most important species interactions
involves who eats whom.
 Matter and energy move through the community
 Trophic level = rank in the feeding hierarchy
 Producers (autotrophs)
 Consumers
 Detritivores and decomposers
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© 2014 Pearson Education, Inc.
Producers
 Producers, or autotrophs (“self-feeders”) =
organisms that capture solar energy for
photosynthesis to produce sugars
 Green plants
 Cyanobacteria
 Algae
 Chemosynthetic bacteria use the geothermal energy
in hot springs or deep-sea vents to produce their
food
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Consumers
 Primary consumers = second trophic level
 Organisms that consume producers
 Herbivores consume plants
 Deer, grasshoppers
 Secondary consumers = third trophic level
 Organisms that prey on primary consumers
 Carnivores consume meat
 Wolves, rodents (that eat insects)
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Consumers
 Tertiary consumers = fourth trophic level
 Predators at the highest trophic level
 Consume secondary consumers
 Are also carnivores
 Hawks, owls
 Omnivores = consumers that eat both producers
(plants) and consumers (animals)
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Detritivores and decomposers
 Organisms that consume nonliving organic matter
enrich soils and/or recycle nutrients found in dead
organisms
 Detritivores = scavenge waste products or dead
bodies
 Millipedes, soil insects
 Decomposers = break down leaf litter and other
nonliving material into simpler chemicals that can be
used by plants
 Fungi, bacteria
 Enhance topsoil and recycle nutrients
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Energy, biomass, and numbers decrease at
higher trophic levels
 Most energy organisms use is lost as waste heat
through cellular respiration
 Less and less energy is available in each successive
trophic level
 Each level contains only about 10% of the energy of
the trophic level below it
 There are also far fewer organisms and less
biomass (mass of living matter) at each higher
trophic level
A human vegetarian’s ecological footprint is smaller
than a meat-eater’s footprint
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© 2014 Pearson Education, Inc.
Food webs show feeding relationships and
energy flow
 Food chain = a linear series of feeding relationships
 Food web = a visual map of feeding relationships
and energy flow
 Includes many different organisms at all various
levels
 Greatly simplified; leaves out most species
 Contains many food chains
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© 2014 Pearson Education, Inc.
Some organisms play outsized roles in
communities
 Community dynamics are complex
 Species interactions differ in strength
 Keystone species = a species that has a strong or
wide-reaching impact far out of proportion to its
abundance
 Removal of a keystone species has substantial
ripple effects
 Significantly alters the food web
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© 2014 Pearson Education, Inc.
Some organisms play outsized roles in
communities
 Trophic cascade = a phenomenon in which
predators at high trophic levels indirectly affect
populations at low trophic levels
 Predators keep species at intermediate trophic levels
in check, allowing growth of species at a lower level
 Extermination of wolves led to increased deer
populations, which overgrazed vegetation and
changed forest structure
 “Ecosystem engineers” physically modify the
environment
 Beaver dams, prairie dogs, ants, zebra mussels
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Communities respond to disturbance in various
ways
 Disturbance = event that causes rapid changes in
the environment that alters the community or
ecosystem
 Tree falling, opening space in a forest
 Removal of keystone species, spread of invasive
species
 Natural disturbances like tornadoes, hurricanes
 Human impacts cause major community changes
 Some species become adapted to disturbance
 Plants growing in fire-prone regions may have seeds
that require fire to germinate
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Communities respond to disturbance in various
ways
 Communities have different ways of maintaining
themselves when disturbed
 Resistance = when a community of organisms resists
change and remains stable despite the disturbance
 Resilience = when a community changes in response
to a disturbance, but later returns to its original state
 A disturbed community may never return to its
original state
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Succession follows severe disturbance
 Succession = the predictable series of changes in a
community following a disturbance
 Primary succession = disturbance removes all
vegetation and soil life
 Community is built from scratch
 Glaciers, drying lakes, volcanic lava
 Pioneer species = the first species to arrive in a
primary succession area (e.g., lichens, mosses)
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© 2014 Pearson Education, Inc.
Succession follows severe disturbance
 Secondary succession = a disturbance
dramatically alters, but does not destroy, all local
organisms
 The remaining organisms and soil form “building
blocks” that help shape the process of succession
 Fires, hurricanes, farming, logging
 Climax community = community that remains in
place with few changes until another disturbance
restarts succession
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© 2014 Pearson Education, Inc.
Communities may undergo shifts
 The dynamics of community change are more variable
and less predictable than thought
 Competition may inhibit progression to another stage
 Chance factors also affect movement between stages
 Climax community may not be based on climate alone
 Phase (regime) shift = occurs when the overall
character of the community fundamentally changes
 Some crucial threshold is passed, a keystone species is
lost, or an exotic species invades
 Human activity may be creating novel or no-analog
communities that are new mixtures of species that
have not been seen before in nature
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Invasive species pose new threats to
community stability
 Invasive species = non-native (exotic) organisms
that spread widely and become dominant in a
community
 Introduced species = species that were deliberately
or accidentally brought from elsewhere
 Growth-limiting factors (predators, disease,
competitors, etc.) are removed or absent
 Have major ecological effects
 Chestnut blight from Asia wiped out American
chestnut trees
 Some species help people (e.g., European
honeybees)
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© 2014 Pearson Education, Inc.
We can respond to invasive species through
control, eradication, or prevention
 Techniques to control invasive species
 Removing them manually
 Applying toxic chemicals
 Introducing native predators or diseases
 Stressing them with heat, sound, electricity, carbon
dioxide, or ultraviolet light
 Control and eradication are hard and expensive
 Understanding the biology of invasive species can
help predict where they will be a problem
Prevention, rather than control, is the best policy
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Altered communities can be restored
 Humans have dramatically changed ecological
systems
 Severely degraded systems cease to function
 Ecological restoration = efforts to restore
communities
 Restoration is informed by restoration ecology =
the science of restoring an area to an earlier
condition to restore the system’s functionality (e.g.,
filtering of water by a wetland)
 It is difficult, time-consuming, and expensive
 It is best to protect natural systems from degradation
in the first place
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Altered communities can be restored
 Prairie restoration involves replanting native species,
controlling invasive species
 Nearly all tallgrass prairie was converted to
agriculture by the 19th century
 The world’s largest project is the Florida Everglades
 Flood control and irrigation removed water
 Populations of wading birds dropped 90–95%
 It will take 30 years and billions of dollars to restore
natural water flow
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© 2014 Pearson Education, Inc.
Earth’s Biomes
 Widely separated regions share similarities
 Biome = major regional complex of similar
communities recognized by plant type and
vegetation structure
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© 2014 Pearson Education, Inc.
Climate influences the location of biomes
 The type of biome depends on abiotic factors
 Temperature, precipitation, soil type, atmospheric
circulation
 Climatograph = a climate diagram showing an
area’s mean monthly temperature and precipitation
 Similar latitudes have similar climates
 Similar biomes occupy similar latitudes
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© 2014 Pearson Education, Inc.
Aquatic and coastal systems resemble biomes
 Various aquatic systems comprise distinct
communities
 Coastlines, continental shelves
 Open ocean, deep sea
 Coral reefs, kelp forests
 Freshwater lakes and rivers
 Some coastal systems (estuaries, marshes, etc.)
have both aquatic and terrestrial components
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Aquatic and coastal systems resemble biomes
 Aquatic systems are shaped by
 Water temperature, salinity, and dissolved nutrients
 Wave action, currents, depth, light levels
 Substrate type and animal and plant life
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Temperate deciduous forest
 Deciduous trees lose their leaves each fall
 They remain dormant during winter
 Mid-latitude forests in Europe, eastern China,
eastern North America
 Even, year-round precipitation
 Fertile soils
 Dominant forests trees are often oak, beech, maple
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© 2014 Pearson Education, Inc.
Temperate grasslands
 More extreme temperature difference between winter
and summer than temperate deciduous forest
 Less precipitation
 Also called steppe or prairie
 Once widespread, but has been converted to
agriculture
 Bison, prairie dogs, ground-nesting birds, pronghorn
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© 2014 Pearson Education, Inc.
Temperate rainforest
 Coastal Pacific Northwest
 Great deal of precipitation
 Coniferous trees: cedar, spruce, hemlock, fir
 Moisture-loving animals
 Banana slug
 Erosion and landslides affect the fertile soil
 Logged for lumber and paper
 Most old growth is gone
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© 2014 Pearson Education, Inc.
Tropical rainforest
 Southeast Asia, west Africa, Central and South
America
 Year-round rain and warm temperatures
 Forest floor is dark and damp
 Lush vegetation
 Diverse species
 But species in low densities
 Very poor, acidic soils
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© 2014 Pearson Education, Inc.
Tropical dry forest
 Also called tropical deciduous forest
 Plants drop leaves during the dry season
 India, Africa, South America, north Australia
 Wet and dry seasons
 Warm, but seasonal rainfall and less overall than
rainforest
 Converted to agriculture
 Severe soil erosion
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© 2014 Pearson Education, Inc.
Savanna
 Grassland interspersed with trees
 Africa, South America, Australia, India
 Precipitation is only during the rainy season
 Animals gather near water holes
 Zebras, gazelles, giraffes, lions, hyenas
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© 2014 Pearson Education, Inc.
Desert
 Minimal precipitation
 Some are bare, with sand dunes (Sahara)
 Some are vegetated (Sonoran)
 They are not always hot
 Temperatures vary widely during the day
 Saline soils
 Animals are often nocturnal, nomadic
 Plants may have thick skins, spines to protect their
water
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© 2014 Pearson Education, Inc.
Tundra
 Russia, Canada, Scandinavia
 Minimal precipitation
 Extremely cold winters
 Extreme variation in day length
 Permafrost = permanently frozen soil
 Melting due to climate change
 Few animals: polar bears, musk oxen, caribou,
migratory birds
 Lichens, low vegetation, few trees
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© 2014 Pearson Education, Inc.
Boreal forest





Also called taiga
Canada, Alaska, Russia, Scandinavia
Extends for large continuous areas
A few evergreen tree species
Cool and dry climate
 Long, cold winters
 Short, cool summers
 Nutrient-poor, acidic soil
 Moose, wolves, bears, lynx, migratory birds
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© 2014 Pearson Education, Inc.
Chaparral
 Occurs in small patches around the globe
 Mediterranean Sea, coastal Chile, California,
southern Australia
 Highly seasonal biome
 Mild, wet winters
 Warm, dry summers
 Frequent fires
 Densely thicketed, evergreen shrubs
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© 2014 Pearson Education, Inc.
Altitudes create patterns analogous to
latitudinal
 Vegetative communities change along mountain
slopes
 The climate varies with altitude in the same way it
varies with latitude
 A mountain climber in the Andes begins in the
tropics and ends on a glacier
 Rainshadow effect = phenomenon that occurs
when air going over a mountain releases moisture
on one side, creating an arid region on the other
side
Hiking up a mountain in the southwest U.S. is like
walking from Mexico to Canada
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© 2014 Pearson Education, Inc.
Conclusion
 Biomes and communities help us understand how
the world functions
 Species interactions affect communities
 Predation, parasitism, competition, mutualism
 Cause weak and strong, direct and indirect effects
 Feeding relationships are represented by trophic
levels and food webs
 Humans have altered many communities
 Ecological restoration attempts to undo the negative
changes that we have caused
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QUESTION: Review
Mutualism is a form of species interaction where
a) both species are harmed.
b) one species benefits, but the other is harmed.
c) both species benefit.
d) one species excludes another from a particular
area.
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QUESTION: Review
Which of the following is NOT an exploitative
interaction?
a) Predation
b) Herbivory
c) Parasitism
d) All of these are exploitative interactions.
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QUESTION: Review
An example of resource partitioning is
a) one species eating larger berries and another
species eating smaller berries.
b) one species moving out of an area to find new
resources.
c) a host species becoming more vulnerable to
parasitism.
d) a pine tree evolving thicker pinecones to reduce
consumption by squirrels.
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QUESTION: Review
Which statement about trophic levels is NOT true?
a) Plants are autotrophs and occupy the first trophic
level.
b) Detritivores consume waste products or dead
bodies.
c) Biomass and energy decrease going up the food
chain.
d) There are more predator individuals than prey
individuals in a typical area.
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QUESTION: Review
Primary succession would take place on all of the
following EXCEPT
a) the slopes of a newly formed volcanic island.
b) a forest in northwest Oregon after it had been
logged.
c) a receding glacier.
d) a drying lake.
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QUESTION: Review
Because communities can undergo phase (regime)
shifts, we must remember that
a) secondary succession results in a predictable series
of stages.
b) we can count on being able to reverse damage
caused by human disturbance.
c) we cannot count on being able to reverse damage
caused by human disturbance.
d) changes humans set in motion will not be
permanent.
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QUESTION: Review
All of the following are ways to control invasive species
EXCEPT
a) removing individuals from the area.
b) stressing them by noise.
c) introducing a predator.
d) encouraging them to hybridize with another species.
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QUESTION: Review
Which biome has year-round rain and warm
temperatures, is dark and damp, and has lush
vegetation?
a) Tropical rainforest
b) Temperate grasslands
c) Chaparral
d) Taiga
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QUESTION: Review
Which biome is dominated by a few evergreen
species, has long, cold winters, and has moose,
wolves, bear, and lynx?
a) Tropical rainforest
b) Temperate grasslands
c) Temperate rainforest
d) Taiga
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QUESTION: Weighing the Issues
Would you be willing to decrease the amount of meat
you consume (i.e., eat lower on the food chain) to
decrease your ecological footprint?
a) Yes, if the extra food was sent to countries with
starving people.
b) Yes, because it would decrease environmental
degradation.
c) I don’t eat meat now.
d) No, I don’t see the need to eat lower on the food
chain.
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QUESTION: Weighing the Issues
Although mustangs are not native to the United States,
they exist in several western states on federally owned
land. As an introduced species, what should be done with
them?
a) As an exotic species, they should immediately be
removed and adopted or killed.
b) Although they are an exotic species, they are part of our
heritage and should be allowed to stay.
c) They have been here so long, we should just leave them
alone.
d) Many countries eat horse flesh, so we should round
them up and export them to horse-eating countries.
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QUESTION: Interpreting Graphs and Data
What does this figure illustrate?
a) A predator–prey cycle
b) Competitive exclusion
c) Resource partitioning
d) Succession
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QUESTION: Interpreting Graphs and Data
According to this climatograph for Vaigach, Russia, in
the tundra biome, winters are
a) long and warm.
b) short and cool.
c) long and cold.
d) short and warm.
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