Chapter 7Chapter 7

Community Community InteractionsInteractions

7-1 COMMUNITY STRUCTURE AND SPECIES

DIVERSITY

Community

• Community: populations of all species living and interacting in an area at a particular time

Four Characteristics of Community Structure

• Physical Appearance: size and distribution of its population and species

• Species Diversity/Richness: number of different species

• Species Abundance: number of individuals of each species

• Niche Structure: number of niches, how they compare, and how they interact

Differing Physical Appearances

• Patch Effects: most large communities usually consist of a mosaic of vegetation “patches”

• Edge Effects: differences in physical appearance at boundaries between ecosystems

Species Diversity• Species Rich Environments

– Tropical Rain Forests– Coral Reefs– Deep Sea– Large Tropical Lakes– Tend to have high species diversity but low

species abundance

• Factors that Affect Diversity– Latitude (terrestrial) – distance from equator– Depth (aquatic)– Pollution (aquatic)

Species Abundance

• Determined by:– Rate at which new species immigrate – Rate at which species become extinct

Niche: role an organism plays in an ecosystem

•niche is like an "occupation“ – a species’ interactions with habitat and other organisms (their role in food web)

Ecological Niche vs. HabitatHabitat: actual location where an organism lives

•habitat is like an "address"

7-2 GENERAL TYPES OF SPECIES

GENERAL TYPES OF SPECIESGENERAL TYPES OF SPECIES

•Generalist Species

•Specialist Species

•Native Species

•Nonnative Species

•Indicator Species

•Keystone Species

Generalist vs. SpecialistGeneralist vs. Specialist

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•Generalist Species have broad niches, can live many places, use a variety of resources

e.g., dandelions, cockroaches, coyotes, humans

•Specialist Species have narrow niches, live only in specific places

e.g., spotted owls, giant pandas

Native vs. Nonnative SpeciesNative vs. Nonnative Species

• Native Species: species that normally live & thrive in a particular ecosystem

• Nonnative Species:

•also called exotic, invasive, or alien species

•originate in other ecosystem

•deliberate or accidental introduction by humans causes problems

•may thrive and crowd out native species

The Case of the Killer Bees

• 1957 Brazil imported wild African bees to help increase honey production

• Displaced domestic honeybees• Actually reduced honey

production• Moved north in Central America• Established populations in

Texas, Arizona, New Mexico, Puerto Rico, and California

Snakehead Fish

CANE TOADS!

The Simpsons!

• Bart vs. Australia

Indicator SpeciesIndicator Species

• Indicator Species: species that serve as early warnings that a community or ecosystem is being damaged

•Birds are good indicator species

•Found everywhere

•Respond to environmental change quickly

•northern spotted owls are indicators of healthy old–growth forest

•Fish are good indicator species in aquatic ecosystems (i.e. trout)

Keystone SpeciesKeystone Species

Keystone Species: species that play a critical role in an ecosystem

•Ex: sea otters are keystone species because they prevent sea urchins from depleting kelp beds

•Ex: flying foxes are keystone species because they pollinate tropical trees and disperse seeds, such as durian fruit trees

“The loss of a keystone species is like a drill accidentally striking a power line. It

causes lights to go out all over.”– E.O. Wilson

7-3 SPECIES INTERACTIONS:

COMPETITION AND PREDATION

Species InteractionsSpecies Interactions

• The effects of one species on another may be negative, positive, or neutral

• Five kinds of interactions:

•Interspecific competition

•Predation

•Parasitism

•Mutualism

•Commensalism

Intra- vs. Inter- specific Intra- vs. Inter- specific competitioncompetition

• Intraspecific Competition – members of the SAME species compete for resources

•Interspecific Competition – members from 2 or more DIFFERENT species compete for resources

•When two or more species use the same limited resource (food, space, etc.) they may adversely affect each other

•niche overlap

•Ex: fire ants & native ants in North America

High

Low

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0 2 4 6 8 10 12 14 16 18

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Each species grown alone

Parameciumaurelia

Parameciumcaudatum

No CompetitionNo Competition

High

Low

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0 2 4 6 8 10 12 14 16 18

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Both species grown together

Parameciumaurelia

Parameciumcaudatum

Interspecific Competition

Resource PartitioningResource Partitioning

Species with similar resource requirements can coexist because they use limited resources:

•at different times

•in different ways

•in different places

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Resource Partitioning

Where are the 2 species competing?

Why is there no competition here?

Resource PartitioningResource Partitioning

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Resource Partitioning ExampleResource Partitioning ExampleFive species of insect–eating warblers coexist in spruce forests of Maine:

•feed in different portions of trees

•consume somewhat different insects

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PredationPredation•Members of one species (predator) feed on another species (prey);

•+ / –

•Ex: lion feeding on zebra

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Predator-Prey Relationship

• What do predators do to increase their chances of getting a meal?

• What do prey do to avoid being eaten?

Predator-Prey Relationship

• Predators get better at catching prey

• Prey get better at avoiding capture

• Similar to an “arms race”– Ex: During the Cold War,

the US and the USSR tried to intimidate the other with bigger and better weapons

Predators

• Pursuit– Faster (cheetahs)– Better eyesight (eagles)– Hunting in packs (wolves)

• Ambush – Camouflage (praying mantis)– Mimicry (alligator snapping turtles)

Prey

• Camouflage • Ex: walking stick

• Chemical warfare • Ex: skunk

• Warning colors • Ex: poison dart frog

• Mimicry • Ex: king snake

• Behavior• Ex: blowfish

7-4 SPECIES INTERACTIONS:

PARASITISM, MUTUALISM, AND COMMENSALISM

ParasitismParasitism

One organism (parasite) lives on part of another organism (host)

•+ / –

•Ex: flea living on a dog

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MutualismMutualism

Two species interact in a way that benefits both

•+ / +

•Ex: lichens (algae & fungi)

•Ex: clownfish & anemones

•Ex: ants & acacias

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CommensalismCommensalism

One organism benefits from another, but neither helps nor harms the other organism

•+ / 0

•Ex: epiphyte growing on a tree

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Species InteractionsSpecies Interactions

• The effects of one species on another may be negative, positive, or neutral

• five kinds of interactions:POPULATION A POPULATION B

COMPETITION - -PREDATION + -PARASITISM + -

COMMENSALISM + 0MUTUALISM + +

7-5 ECOLOGICAL SUCCESSION: COMMUNITIES

IN TRANSITION

Ecological SuccessionEcological Succession

Succession: gradual & fairly predictable change in species composition over time

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Primary SuccessionPrimary Succession

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Primary Succession• Type of succession that

occurs where there was no ecosystem before

• Occurs on rocks, cliffs, and sand dunes

• Pioneer species: the first organism to colonize any newly available area and begin the process of ecological succession

Primary SuccessionPrimary Succession

Primary Succession: gradual establishment of biotic communities in an area where no life existed before

•Ex: succession on newly formed islands & after the retreat of a glacier

•Early Communities: lichens & mosses colonize bare rock

•Mid Communities: small herbs & shrubs colonize

•Late Communities: tree species colonize

Lichens

Mosses

Parking Lot

Secondary SuccessionSecondary Succession

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Secondary SuccessionSecondary SuccessionSecondary Succession: gradual reestablishment of biotic communities in an area where a biotic community was previously present

•Ex: "old field succession"

Secondary Succession: Mount St. Helens

• Erupted in 1980

• 44,460 acres were burned and flattened

• After the eruption, plants began to colonize the debris

DisturbanceDisturbance

Disturbance: a distinct event that disrupts an ecosystem or community

•disturbance initiates secondary succession

•Natural disturbances: fires, hurricanes, tornadoes, droughts, & floods

•Human-caused disturbances: deforestation, overgrazing, plowing

Intermediate Disturbance Hypothesis: moderate disturbances in communities promote greater species diversity than

small or major disturbances

Climax Community

• Climax community: the final and stable community in an ecosystem

• Will continue to change in small ways, but will remain primarily the same over time (unless disturbed)

The Circle of Life in Secondary Succession

7-6 ECOLOGICAL STABILITY AND SUSTAINABILITY

Stability - complex networks of positive and negative feedbacks loops

Ecosystem Stability

• Inertia/Persistence: the ability of an ecosystem to resist being disturbed

• Constancy: the ability of a living sysmte such as a population to keep its numbers within the limits imposed by available resources

• Resilience: the ability of an ecosystem to “bounce back” after it has been disturbed

Precautionary Principle• We should try to prevent potential

harm to an ecosystem even though we don’t understand all of the cause-and-effects

• “better safe than sorry”