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© 2011 Pearson Education, Inc. Ecological Communities AP Environmental Science Mr. Grant Lesson 29
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Ecological Communities
AP Environmental Science
Mr. GrantLesson 29
© 2011 Pearson Education, Inc.
Objectives:
• Define the term trophic level.• Characterize feeding relationships and energy flow, using
them to construct trophic levels and food webs.• Distinguish characteristics of a keystone species.• Characterize succession, community change, and the
debate over the nature of communities.• Perceive and predict the potential impacts of invasive
species in communities.• Explain the goals and the methods of restoration ecology.
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Trophic Level: Rank in the feeding hierarchy of a food chain. Organisms at higher trophic levels consume those at lower trophic levels.
Define the term trophic level.
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Characterize feeding relationships and energy flow, using them to construct trophic levels and food webs.
• Energy is transferred in food chains among trophic levels.
• Lower trophic levels generally contain more energy, biomass, and individuals.
• Food webs illustrate feeding relationships and energy flow among species in a community.
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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 how:
- Species coexist and relate to one another- Communities change, and why patterns exist
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Energy passes through trophic levels• One of the most important species interactions
- Who eats whom?• Matter and energy move through the community• Trophic levels = rank in the feeding hierarchy
- Producers (autotrophs)- Consumers- Detritivores and
decomposers
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Producers: the first trophic level
• Producers, or autotrophs (“self-feeders”) = organisms 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: consume producers
• 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
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Consumers occur at higher trophic levels
• Tertiary Consumers = fourth trophic level- Predators at the highest trophic level- Consume secondary consumers- Are also carnivores- Hawks, owls
• Omnivores = consumers that eat both plants and 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 non-
living material- Fungi, bacteria- Enhance topsoil and recycle nutrients
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Energy, biomass, and numbers decrease
• 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 10% of the energy of the
trophic level below it• There are also far fewer organisms and less biomass
(mass of living matter) at the higher trophic levelsA human vegetarian’s ecological footprint is smaller
than a meat-eater’s footprint
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Pyramids of energy, biomass, and numbers
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Food webs show relationships and energy flow
• Food chain = a 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
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Distinguish characteristics of a keystone species.
• Keystone species have impacts on communities that are far out of proportion to their abundance.
• Top predators are frequently considered keystone species, but other types of organisms also exert strong effect on communities.
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Some organisms play big roles
• Community dynamics are complex- Species interactions differ in
strength and over time• Keystone species = has a
strong or wide-reaching impact - Far out of proportion to its
abundance• Removal of a keystone species
has substantial ripple effects - Alters the food chain
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Characterize succession, community change, and the debate over the nature of communities.• Succession is a stereotypical pattern of change within a
community through time.
• Primary succession begins with an area devoid of life. Secondary succession begins with an area that has been severely disturbed.
• Communities may undergo phase shifts involving irreversible change if disturbance is severe enough.
• Clements held that communities are discrete, cohesive units. His view has largely been replaced by that of Gleason, who held that species may be added to and deleted from communities through time.
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Species can change communities
• Trophic Cascade = predators at high trophic levels indirectly affect populations at low trophic levels- By keeping species at intermediate trophic levels in
check- 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 disturbances
• Communities experience many types of disturbance- Removal of keystone species, spread of invasive
species, natural disturbances- Human impacts cause major community changes
• Resistance = community of organisms resists change and remains stable despite the disturbance
• Resilience = 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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Primary succession
• Succession = the predictable series of changes in a community - Following a disturbance
• Primary succession = disturbance removes all vegetation and/or soil life- Glaciers, drying lakes, volcanic
lava• Pioneer species = the first species
to arrive in a primary succession area (i.e. lichens)
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Secondary succession
• Secondary succession = a disturbance dramatically alters, but does not destroy, all local organisms- The remaining organisms form “building blocks”
which help shape the process of succession- Fires, hurricanes, farming, logging
• Climax community = remains in place with few changes- Until another
disturbance restarts succession
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Communities may undergo shifts
• The dynamics of community change are more variable and less predictable than thought- Conditions at one stage may promote another stage- Competition may inhibit progression to another stage- Chance factors also affect changes
• Phase (regime) shift = the overall character of the community fundamentally changes- Some crucial threshold is passed, a keystone species is
lost, or an exotic species invades- i.e. overfishing and depletion of fish and turtles has
allowed algae to dominate corals
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Community cohesion
• Frederick Clements = viewed communities as cohesive entities, with integrated parts - Its members remain associated over space and time - The community shared similar limiting factors and
evolutionary histories• Henry Gleason = maintained that each species responds
independently to its own limiting factors - Species join or leave communities without greatly
altering the community’s composition- The most widely accepted view of ecologists today
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Perceive and predict the potential impacts of invasive species in communities.
• Invasive species such as the zebra mussel have altered the composition, structure, and function of communities.
• Humans are the cause of most modern species invasions, but we can also respond to invasions with prevention and control measures.
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Invasive species threaten stability
• Invasive species = non-native (exotic) organisms that spread widely and become dominant in a community- Introduced deliberately or accidentally from
elsewhere- Growth-limiting factors (predators, disease,
competitors, etc.) are removed or absent- They have major ecological effects- Chestnut blight from Asia wiped out American
chestnut trees• Some species help people (i.e., European honeybees)
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Two invasive mussels
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Controlling invasive species
• Techniques to control invasive species- Removing them manually- Applying toxic chemicals- Drying them out- Depriving them of oxygen- Stressing them with heat, sound, electricity,
carbon dioxide, or ultraviolet light• Control and eradication are hard and expensivePrevention, rather than control, is the best policy
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Explain the goals and the methods of restoration ecology.
• Restoration ecology is the science of restoring communities to a previous, more functional or more “natural” condition, variously defined as before human or before recent industrial impact.
• Restoration ecology informs the growing practice of ecological restoration.
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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 (i.e. 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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Restoration efforts• Prairie restoration = replanting native species,
controlling invasive species• The world’s largest project = 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
