Chapter 7Chapter 7Community EcologyCommunity Ecology

AP Environmental ScienceAP Environmental Science

Edinburg North High SchoolEdinburg North High School

Discussion QuestionsDiscussion Questions

• What determines the number of species in What determines the number of species in a community?a community?

• How can we classify species according to How can we classify species according to their roles in a community?their roles in a community?

• How do species interact with one another?How do species interact with one another?• How do communities respond to changes How do communities respond to changes

in environmental conditions?in environmental conditions?• Does high species biodiversity increase Does high species biodiversity increase

the stability and sustainability of a the stability and sustainability of a community?community?

Core Case Study: Why Should We Core Case Study: Why Should We Care about the American Alligator?Care about the American Alligator?

• Plays a number of roles in the ecosystems Plays a number of roles in the ecosystems where they are found.where they are found.• Dig deep depressions (gator holes).Dig deep depressions (gator holes).

• Hold water during dry spells, serve as refuges for Hold water during dry spells, serve as refuges for aquatic life.aquatic life.

• Build nesting mounds.Build nesting mounds.• provide nesting and feeding provide nesting and feeding

sites for birds.sites for birds.• Keeps areas of open Keeps areas of open

water free of vegetation.water free of vegetation.

• Alligators are a keystone species:Alligators are a keystone species:• Help maintain the structure and function of the Help maintain the structure and function of the

communities where it is found.communities where it is found.

• Survived for at least 200 m.y.Survived for at least 200 m.y.• By 1960s, was near extinction in Florida By 1960s, was near extinction in Florida

and decreased significantly in other and decreased significantly in other southern states.southern states.

• In 1967, federally listed as endangered.In 1967, federally listed as endangered.• In 1977, reclassified as threatened in FL, In 1977, reclassified as threatened in FL,

LA, and TX; in 1987, was upgraded in LA, and TX; in 1987, was upgraded in seven other states.seven other states.

Community Structure And Community Structure And Species DiversitySpecies Diversity• Community Structure: Appearance MattersCommunity Structure: Appearance Matters

• Physical appearance – relative sizes, Physical appearance – relative sizes, stratification, and distribution of populations in a stratification, and distribution of populations in a community.community.• Terrestrial communities (Fig. 7-2)Terrestrial communities (Fig. 7-2)• Aquatic communitiesAquatic communities

• Oceans, rocky shores, sandy beaches, lakes, rivers, Oceans, rocky shores, sandy beaches, lakes, rivers, wetlandswetlands

• Structure is patchy because physical Structure is patchy because physical conditions, resources, and species vary from conditions, resources, and species vary from place to place.place to place.

Fig. 7-2. Generalized Terrestrial CommunitiesFig. 7-2. Generalized Terrestrial Communities

• Edges and ecotonesEdges and ecotones• Habitat fragmentation increases edge area.Habitat fragmentation increases edge area.

• Makes species more vulnerable to physical a Makes species more vulnerable to physical a biological stress.biological stress.

• Creates barriers to gene flow and dispersalCreates barriers to gene flow and dispersal

• Species Diversity and Niche Structure: Species Diversity and Niche Structure: Different Species Playing Different RolesDifferent Species Playing Different Roles• Communities shaped by species, species Communities shaped by species, species

interactions, and species interactions with interactions, and species interactions with physical environment.physical environment.

• Species diversity: the number of different Species diversity: the number of different species it contains (species it contains (species richnessspecies richness) combined ) combined with the abundance of individuals within each of with the abundance of individuals within each of those species (those species (species evennessspecies evenness).).

• Niche structureNiche structure – how many potential – how many potential ecological niches occur, how they resemble or ecological niches occur, how they resemble or differ, and how the species occupying different differ, and how the species occupying different niches interact.niches interact.

• Geographic locationGeographic location – species diversity is – species diversity is highest in the tropics and declines as we move highest in the tropics and declines as we move from the equator toward the poles.from the equator toward the poles.• Low latitudes = Constant climate and reliable food Low latitudes = Constant climate and reliable food

supply supply greater specialization w/ narrow niches greater specialization w/ narrow niches• Higher latitudes = More variable climate and food Higher latitudes = More variable climate and food

availability availability generalist species w/ wide niches generalist species w/ wide niches

• Coral reefs, rainforest, deep seas, and tropical Coral reefs, rainforest, deep seas, and tropical lakes are high in species richness but have lakes are high in species richness but have fewer members of each species (low fewer members of each species (low evenness).evenness).

• Case Study: Species Diversity on Islands.Case Study: Species Diversity on Islands.• Robert MacArthur and E.O. Wilson proposed Robert MacArthur and E.O. Wilson proposed

the species equilibrium model or theory of the species equilibrium model or theory of island biogeography in the 1960’s.island biogeography in the 1960’s.

• Model projects that at some point the rates of Model projects that at some point the rates of immigration and extinction should reach an immigration and extinction should reach an equilibrium based on:equilibrium based on:• Island sizeIsland size• Distance to nearest mainlandDistance to nearest mainland

Types of SpeciesTypes of Species

• Types of Species in CommunitiesTypes of Species in Communities• Native, nonnative, indicator, keystone, and foundation Native, nonnative, indicator, keystone, and foundation

species play different ecological roles in communities.species play different ecological roles in communities.• A given species may play more than one role in an A given species may play more than one role in an

ecosystem.ecosystem.• Native speciesNative species - those that normally live and thrive in a - those that normally live and thrive in a

particular community.particular community.• Non-native, or alien speciesNon-native, or alien species - those that migrate into or - those that migrate into or

are deliberately or accidentally introduced into a are deliberately or accidentally introduced into a community.community.

• Invasive speciesInvasive species

• Indicator Species: Biological Smoke AlarmsIndicator Species: Biological Smoke Alarms• Species that serve as early warning systems of Species that serve as early warning systems of

damage to a community or ecosystem are damage to a community or ecosystem are called called indicator speciesindicator species..• The presence or absence of trout in a waters that The presence or absence of trout in a waters that

are within their thermal requirements (Fig. 3-11)are within their thermal requirements (Fig. 3-11)• Birds have been affected by habitat fragmentation Birds have been affected by habitat fragmentation

and pesticides.and pesticides.• Butterflies because many have specific host plants Butterflies because many have specific host plants

for their larvae.for their larvae.• Old idea: the canary in a coal mineOld idea: the canary in a coal mine

• Case Study: Why Are Amphibians Vanishing?Case Study: Why Are Amphibians Vanishing?• Frogs serve as indicator species because different Frogs serve as indicator species because different

parts of their life cycles can be easily disturbed.parts of their life cycles can be easily disturbed.

• Reasons for Amphibian DeclinesReasons for Amphibian Declines• Habitat loss and fragmentation.Habitat loss and fragmentation.• Prolonged drought.Prolonged drought.• Pollution.Pollution.• Increases in ultraviolet radiation.Increases in ultraviolet radiation.• Parasites.Parasites.• Viral and Fungal diseases.Viral and Fungal diseases.

• Chytrid fungus that attacks the skin of frogsChytrid fungus that attacks the skin of frogs• In 2005, Correlation between climate change and harlequin In 2005, Correlation between climate change and harlequin

frogs in Central and South America.frogs in Central and South America.• In 2008, some new evidence casts doubt on this In 2008, some new evidence casts doubt on this

hypothesis.hypothesis.

• Overhunting.Overhunting.• Natural immigration or deliberate introduction of Natural immigration or deliberate introduction of

nonnative predators and competitors.nonnative predators and competitors.

• Why care if we lose amphibians?Why care if we lose amphibians?• First, declining trend suggests that the life support First, declining trend suggests that the life support

system is deterioratingsystem is deteriorating• Habitat loss and degradationHabitat loss and degradation• Air an water pollutionAir an water pollution• Increased UV exposureIncreased UV exposure• Climate changeClimate change

• Second, they play important ecological roles in Second, they play important ecological roles in biological communities.biological communities.

• Eat insects (including mosquitoes)Eat insects (including mosquitoes)• Other animals such as fish, reptiles, birds, and mammals Other animals such as fish, reptiles, birds, and mammals

eat themeat them

• Third, a genetic storehouse of pharmaceutical Third, a genetic storehouse of pharmaceutical products waiting to be discovered.products waiting to be discovered.

• Compounds isolated from amphibian skin have been used Compounds isolated from amphibian skin have been used as painkillers, antibiotics, and as treatment for burns and as painkillers, antibiotics, and as treatment for burns and heat disease.heat disease.

• Keystone Species: Major Keystone Species: Major PlayersPlayers• Keystone speciesKeystone species – help – help

determine the types and determine the types and numbers of species in a numbers of species in a community.community.

• PollinationPollination• Insects, birds, and batsInsects, birds, and bats

• Top predatorsTop predators• Wolf, bobcat, alligator, shark, Wolf, bobcat, alligator, shark,

Piaster orchaceusPiaster orchaceus

• Have you thanked a dung beetle?Have you thanked a dung beetle?

• Loss of keystone species can Loss of keystone species can lead to population crashes and lead to population crashes and extinctions of other species in a extinctions of other species in a community that depends on community that depends on them for certain services.them for certain services.

• Foundation Species: Other Major PlayersFoundation Species: Other Major Players• Foundation speciesFoundation species – and expansion of the – and expansion of the

keystone species concept; a species which keystone species concept; a species which plays a major role in communities by creating plays a major role in communities by creating and enhancing habitats in a way that benefits and enhancing habitats in a way that benefits others species.others species.• Elephants push over, break, or uproot trees, creating Elephants push over, break, or uproot trees, creating

forest openings promoting grass growth for other forest openings promoting grass growth for other species to utilize.species to utilize.

• Seed dispersers – frugivoresSeed dispersers – frugivores• BeaversBeavers• Mussels on the rocky shores of the Pacific Mussels on the rocky shores of the Pacific

Northwest.Northwest.

Species Interactions: Competition Species Interactions: Competition and Predationand Predation• How do species interact?How do species interact?

• When species in a community have common activities When species in a community have common activities or resources in common they interact.or resources in common they interact.

• Five basic types of interactions:Five basic types of interactions:• Interspecific competitionInterspecific competition• PredationPredation• ParasitismParasitism• MutualismMutualism• CommensalismCommensalism

• Interactions between populations of different species Interactions between populations of different species can influence abilities of individuals within those can influence abilities of individuals within those populations to survive and reproduce; interacting populations to survive and reproduce; interacting populations thus serve as agents of natural selection.populations thus serve as agents of natural selection.

• The most common interaction is The most common interaction is interspecific interspecific competitioncompetition..• No two species can occupy the same niche No two species can occupy the same niche

otherwise there will be intense competition.otherwise there will be intense competition.• One of the competing species must then migrate, shift One of the competing species must then migrate, shift

resource requirements or behaviors by natural selection, or resource requirements or behaviors by natural selection, or suffer sharp decline or extinction.suffer sharp decline or extinction.

• Humans have become serious competitors for Humans have become serious competitors for resources.resources.• As our ecological footprints grow, what can be As our ecological footprints grow, what can be

conclude about the species that share this planet conclude about the species that share this planet with us?with us?

• Reducing or Avoiding Competition: Sharing Reducing or Avoiding Competition: Sharing ResourcesResources• Over a large timescale natural selection can act Over a large timescale natural selection can act

to reduce competitionto reduce competition• Resource partitioningResource partitioning – when species – when species

competing for similar resources evolve more competing for similar resources evolve more specialized traits that allow them to use shared specialized traits that allow them to use shared resources at different times, in different ways, resources at different times, in different ways, or in different places (Fig. 7-6).or in different places (Fig. 7-6).• Leopards and lionsLeopards and lions• Hawks and owlsHawks and owls• Insectivorous birds in the Northeast (Fig. 7-7)Insectivorous birds in the Northeast (Fig. 7-7)

Fig. 7-6. Resource partitioning and niche Fig. 7-6. Resource partitioning and niche specialization as a result of competition specialization as a result of competition between two species.between two species.

Fig. 7-7. Resource partitioning of Five Warblers Fig. 7-7. Resource partitioning of Five Warblers In Maine, USA.In Maine, USA.

• Predators and Prey: Eating and Being Predators and Prey: Eating and Being EatenEaten• PredationPredation – when member of one species (the – when member of one species (the

predatorpredator) feed directly on all or part of a living ) feed directly on all or part of a living organism of another species (the organism of another species (the preyprey).).• Predator-prey relationshipPredator-prey relationship

• Some may be surprising: grizzly-army cutworm mothSome may be surprising: grizzly-army cutworm moth

• Clearly individual prey lose, but at the Clearly individual prey lose, but at the population level, predation plays a key role in population level, predation plays a key role in evolution by natural selection.evolution by natural selection.

• Perception of predation by some people?Perception of predation by some people?

• Sensing the Environment to Find Food and Sensing the Environment to Find Food and MatesMates• EM radiationEM radiation

• Birds and beesBirds and bees• Pit-vipersPit-vipers• Acute night visionAcute night vision

• SoundSound• PinnaePinnae• SonarSonar

• Volatile chemicalsVolatile chemicals• OlfactionOlfaction

• How do predators increase their chance fo How do predators increase their chance fo getting a meal?getting a meal?• HerbivoresHerbivores

• Walk, swim, or fly up to plants to feed on them.Walk, swim, or fly up to plants to feed on them.

• CarnivoresCarnivores• PursuitPursuit• AmbushAmbush

• Chemical WarfareChemical Warfare• SpidersSpiders• SnakesSnakes

• How do prey defend against or avoid predators?How do prey defend against or avoid predators?• Run, swim, or fly awayRun, swim, or fly away• Highly developed senses of sight, hearing and/or smellHighly developed senses of sight, hearing and/or smell• Protective shells, thick bark, spines, and thornsProtective shells, thick bark, spines, and thorns• Camouflage (Fig. 7-8a,b))Camouflage (Fig. 7-8a,b))• Chemical warfare (Fig. 7-8c-e)Chemical warfare (Fig. 7-8c-e)

• Poisons and venomsPoisons and venoms• IrritatingIrritating• Foul smellingFoul smelling• Bad tastingBad tasting

• Warning coloration (Fig. 7-8d,e)Warning coloration (Fig. 7-8d,e)• Mimicry (Fig. 7-8f)Mimicry (Fig. 7-8f)• Deceptive looks and behavior (Fig. 7-8g,h)Deceptive looks and behavior (Fig. 7-8g,h)• Gregarious behaviorGregarious behavior

Fig. 7-8. Some predator avoidance strategies.Fig. 7-8. Some predator avoidance strategies.

Species Interactions: Parasitism, Species Interactions: Parasitism, Mutualism, and CommensalismMutualism, and Commensalism

• Parasites: Sponging Off OthersParasites: Sponging Off Others• ParasitismParasitism – when one species (the – when one species (the parasiteparasite) )

feeds on part of another organism (the feeds on part of another organism (the hosthost).).• One benefits, the other harmedOne benefits, the other harmed

• Enodparasites, some pathogenicEnodparasites, some pathogenic• EctoparasitesEctoparasites• Some parasite have little contact with there Some parasite have little contact with there

hostshosts• Parasites can promote biodiversity and control Parasites can promote biodiversity and control

populations by helping keep some species from populations by helping keep some species from becoming so plentiful that they eliminate othersbecoming so plentiful that they eliminate others

• Mutualism: Win-Win Mutualism: Win-Win RelationshipRelationship• MutualismMutualism – when two – when two

species or a network species or a network of species interact in a of species interact in a way that both benefit.way that both benefit.• Pollination and seed Pollination and seed

dispersaldispersal• Being supplied with Being supplied with

foodfood• Receiving protectionReceiving protection

• Each species benefits Each species benefits by exploiting the other.by exploiting the other.

• Commensalism: Commensalism: Using without Using without HarmingHarming• CommensalismCommensalism – –

interaction that interaction that benefits one species benefits one species but has little, if any but has little, if any effect on the other effect on the other species.species.• Silverfish and army Silverfish and army

antsants• Remoras and sharksRemoras and sharks• Epiphytes Epiphytes

Ecological Succession: Ecological Succession: Communities in TransitionCommunities in Transition• Ecological Succession: How Communities Ecological Succession: How Communities

Change Over TimeChange Over Time• Ecological successionEcological succession – gradual change in – gradual change in

species structure and composition of a species structure and composition of a community in response to changing community in response to changing environmental conditionsenvironmental conditions• Primary successionPrimary succession• Secondary successionSecondary succession

• Pioneer (colonizing or early succession Pioneer (colonizing or early succession species) species arrive first.species) species arrive first.

• Primary Succession: Starting from ScratchPrimary Succession: Starting from Scratch• Primary succession begins with an essentially Primary succession begins with an essentially

lifeless are where there is no soil in a terrestrial lifeless are where there is no soil in a terrestrial ecosystem (Fig. 7-11) or no bottom sediment in ecosystem (Fig. 7-11) or no bottom sediment in aquatic systems.aquatic systems.• Early successional, or pioneer species: lichens and Early successional, or pioneer species: lichens and

mossesmosses• Midsuccession species: herbs, grasses and low Midsuccession species: herbs, grasses and low

shrubs. Trees that need lots of sunlight replace shrubs. Trees that need lots of sunlight replace these.these.

• Late successional species: mostly trees that can Late successional species: mostly trees that can tolerate shade.tolerate shade.

• Newly created ponds also go through succession.Newly created ponds also go through succession.

Fig. 7-11. Primary Ecological Fig. 7-11. Primary Ecological SuccessionSuccession

• Secondary Succession: Starting Over with Secondary Succession: Starting Over with Some HelpSome Help• Secondary succession begins in an area where Secondary succession begins in an area where

the natural community has been disturbed, the natural community has been disturbed, removed, or destroyed (Fig. 7-12).removed, or destroyed (Fig. 7-12).

• Numbers and types of animals and Numbers and types of animals and decomposers also change.decomposers also change.

• Intermediate disturbance hypothesisIntermediate disturbance hypothesis – fairly – fairly frequent but moderate disturbances lead to the frequent but moderate disturbances lead to the greatest species diversity.greatest species diversity.

Fig. 7-12. Secondary Succession of a Fig. 7-12. Secondary Succession of a Plant Communities on a Farm Field in Plant Communities on a Farm Field in North Carolina.North Carolina.

• Can we predict the path of succession, and Can we predict the path of succession, and is nature in balance?is nature in balance?• Traditional view: succession proceeds until a Traditional view: succession proceeds until a

climax communityclimax community is established – one is established – one dominated by a few long-lived plant species dominated by a few long-lived plant species and is in balance with the environment.and is in balance with the environment.

• Most ecologists now recognize that mature Most ecologists now recognize that mature late-successional communities are not in a late-successional communities are not in a state of permanent equilibriumstate of permanent equilibrium• In a state of continual disturbance and change.In a state of continual disturbance and change.• Not preordained to progress to an ideal climax Not preordained to progress to an ideal climax

communitycommunity• Succession reflects the ongoing struggle by different Succession reflects the ongoing struggle by different

species for enough light, nutrients, food, and space.species for enough light, nutrients, food, and space.

Ecological Stability and SustainabilityEcological Stability and Sustainability

• Stability of Living Systems: Surviving by ChangingStability of Living Systems: Surviving by Changing• Communities like other living systems constantly Communities like other living systems constantly

change in response to changing environmental change in response to changing environmental conditions.conditions.

• Negative and positive feedback loops interact to provide some Negative and positive feedback loops interact to provide some degree of stability over each system’s expected life span.degree of stability over each system’s expected life span.

• Three aspects of stability and sustainability:Three aspects of stability and sustainability:• Inertia, or persistence – resists disturbance or changeInertia, or persistence – resists disturbance or change• Constancy – keep within the limits imposed by environmentConstancy – keep within the limits imposed by environment• Resilience – bounce back and repair damage that is not too Resilience – bounce back and repair damage that is not too

drasticdrastic

• Community Productivity and SustainabilityCommunity Productivity and Sustainability• Research suggests that communities with more Research suggests that communities with more

species tend to have a higher NPP and can be species tend to have a higher NPP and can be more resilient than simpler ones.more resilient than simpler ones.• Perhaps because species diversity allow species to Perhaps because species diversity allow species to

exploit a different portion of available resourcesexploit a different portion of available resources

• It is difficult to assess just how much diversity is It is difficult to assess just how much diversity is necessary to achieve greater stability or necessary to achieve greater stability or sustainability.sustainability.

• Why should we bother to protect natural systemsWhy should we bother to protect natural systems• Developers say that because biodiversity does not Developers say that because biodiversity does not

necessarily lead to increased ecological stability,necessarily lead to increased ecological stability,• Cut down old-growth forests, replace with tree plantationsCut down old-growth forests, replace with tree plantations• Convert grasslands to croplandsConvert grasslands to croplands• Drain and develop wetlands, dump toxic and radioactive Drain and develop wetlands, dump toxic and radioactive

wastes into deep oceanswastes into deep oceans• No worries about premature extinctionsNo worries about premature extinctions

• Ecologists say that just because natural systems are Ecologists say that just because natural systems are not in balance does not mean these systems are an not in balance does not mean these systems are an unimportant part of earth’s natural capital in unimportant part of earth’s natural capital in unimportant.unimportant.

• Human activities are disrupting, destroying, degrading Human activities are disrupting, destroying, degrading and simplifying the world’s ecosystems which and simplifying the world’s ecosystems which threatened the ecosystem services that support and threatened the ecosystem services that support and sustain all life and all economies.sustain all life and all economies.