Chapter 8

Community Ecology: Structure, Species Interaction, Succession,

and Sustainability

Key Concepts

• Community Structure

• Roles of Species

• Species Interactions

• Changes in ecosystems

• Stability of ecosystems

Focus Questions• What determines the number of species in a

community?• How can we classify species according to

their roles?• How do species interact with one another?• How do communities change as conditions

change?• Does high species diversity increase the

stability of ecosystems?

What is Community Structure?

4 characteristics:

1. Physical appearance

2. Species diversity

3. Species abundance

4. Niche structure

1. Physical Appearance

• Types of plants• Relative sizes of plants• Stratification of plants and animals• Aquatic life zones• Edge effects: differences in physical

properties (sun, temp, wind) at ecotones

2. Species Diversity • Highest biodiversity: rain forests, coral reefs,

deep ocean• Areas with high biodiversity may have low

species abundance (few members of each species)

3 factors affect biodiversity:1. Latitude (decreases with distance from equator)2. Depth in aquatic systems (highest at surface and

bottom)3. Pollution

Biodiversity increases with:1. More solar radiation2. More precipitation3. Less elevation4. Seasonal variations

Biodiversity decreases with distance from equator

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3. Species Abundance

• Theory of Island Biogeography (aka Species Equilibrium Model): number of species on an island is determined by a balance between the immigration rate of new species and the extinction rate of species already on the island

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Number of species on island

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© 2004 Brooks/Cole – Thomson Learning

Theory of Island Biogeography, continued

Immigration and extinction rates are affected by:

1. Size of the island

2. Distance from mainland

Small islands=lower biodiversity

Small islands=higher extinction rate

General Types of Species

• Native• Non-native• Indicator• Keystone

*Scientists apply labels to clarify their niches

Types of Species

• Native: species that normally live in a particular ecosystem

• Non-native (exotic, alien): species that migrate into an ecosystem or are accidentally introduced into an ecosystem by humans

Non-native Species

• Example: 1957 Brazil imported wild African bees to increase honey production (aggressive and unpredictable)

• Displaced domestic honeybees and reduced honey supply

Indicator Species

• Species that serve as early warnings of damage to a community or ecosystem

• Example: birds-found everywhere and respond quickly to change

trout and amphibians-indicate good water quality—they require clean water with good oxygen supply

Keystone Species

• Species with much more important roles than their abundance suggests

• Critical roles include:1. Pollination by bees, hummingbirds

2. Dispersion of seeds by fruit-eating animals (bats)

3. Habitat modification (elephants, beaver dams)

4. Predation by top carnivores to control populations (wolf, lion, alligator)

5. Recycling of animal wastes (dung beetle)

Species Interactions

• Competition

• Predation

• Parasitism

• Mutualism

• Commensalism

Competition

• Intraspecific: competition between members of the same species for the same resources

Example: territoriality (mark and defend area against their own species)

• Interspecific: competition between different species for resources

Occurs when niches overlap

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© 2004 Brooks/Cole – Thomson Learning

Other types of Competition

Interference Competition: one species limits another’s access to resources

Example: hummingbird defends patches of wildflowers (nectar) so that other hummingbirds may not get to them

Exploitation Competition: species have

equal access to resources but differ in how fast they exploit it

Competitive Exclusion Principle

• One species eliminates another species in an area through competition for limited resources

Example: Paramecium

Can’t occupy the same niche with limited resources

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Parameciumaurelia

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Resource Partitioning

• Species avoid competition by dividing scarce resources amongst them

Example: using resources at different times, in different ways, or in different places (warblers)

Predator-Prey Interaction

• Predation: members of one species (predator) feed directly on another species (prey)

-At the individual level, prey is harmed

-BUT, predators kill sick, weak individuals (improves genetics)

-Remaining prey population has greater access to resources

(enough for everyone)

How do prey defend themselves against predators?

• Ability to run, swim, or fly fast• Very developed sense of sight or smell• Protective shells (turtle, armadillo)• Thick bark• Spines or thorns (porcupines, cacti)• Chemical warfare (poison)• Camouflage/Mimicry (owl butterfly)

ParasitismOne organism benefits and the

other is harmed.

Example:

Fleas

Mutualism

• Both species benefit

Example: clownfish and anemone

Commensalism

• One species benefits and the other is neither harmed nor benefits

Example: bird in

tree

How do ecosystems respond to change?

• Ecological succession: gradual change in species composition in a given area

Primary succession: establishment of biotic communities on a lifeless ground

Secondary succession: reestablishment of biotic community where some type of community is already present

What is Primary Succession?

• Begins with a lifeless area where there is no soil

Examples: bare rock exposed by retreating glacier

cooled lava

abandoned highway or parking lot

newly created shallow pond

Primary Succession, continued• First, there must be SOIL• Soil formation begins when pioneer species

attach themselves to bare rockTrap wind-blown soil particlesSecrete acids that break down rockProduce tiny bits of organic matter

• Perennial plants (live for 2 years without being reseeded) and herbs replace lichens and mosses

• Early successional plant species grow close to the ground, can tolerate harsh conditions

• Midsuccessional plant species are herbs, grasses and low shrubs

• Late successional plant species (mostly trees)

Time

Small herbsand shrubs

Heath mat

Jack pine,black spruce,

and aspen

Balsam fir,paper birch, and

white spruceclimax community

Exposedrocks

Lichensand mosses

Primary Succession

MidsuccessionalSpecies

ElkMooseDeerRuffled grouseSnowshoe hareBluebird

Late SuccessionalSpecies

TurkeyMartinHammond’s flycatcherGray squirrel

WildernessSpecies

Grizzly bearWolfCaribouBighorn sheepCalifornia condorGreat horned owl

Early SuccessionalSpecies

RabbitQuailRingneck pheasantDoveBobolinkPocket gopher

Ecological succession

Ecological Succession© 2004 Brooks/Cole – Thomson Learning

What is Secondary Succession?

• Begins in area where community has been disturbed, destroyed, or removed

• Some soil remains

Examples: abandoned farmlands

burned or cut forests

heavily polluted streams

flooded land

TimeAnnualweeds

Perennialweeds and

grasses

ShrubsYoung pine forest

Mature oak-hickory forest

Secondary Succession

How do species replace one another in ecological

succession?• Facilitation: one set of species makes

an area suitable for other species• Inhibition: early species hinder the

growth of other species• Tolerance: late successional plants are

unaffected by earlier succession plants

(thrive in mature communities without having to eliminate earlier species)

Intermediate Disturbance Hypothesis

• Communities that experience frequent disturbances have the greatest biodiversity

• Disturbances create openings for new species

1000Percentage disturbance

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What is Stability?• Complex networks of negative and positive

feedback loops that interact to provide stability and sustainability

• Stability is maintained by constant change in response to changing environmental conditions

• 3 aspects of stability/sustainability1. Inertia: ability of a system to resist

disturbance2. Constancy: ability of a system to keep

its numbers within the limits (resources available)

3. Resilience: ability of a system to bounce back after a disturbance

Precautionary Principle• Alternative view: biodiversity does not

necessarily lead to more stability, and if nature is unpredictable, there is no point in trying to manage and preserve old-growth forests and ecosystems. We should convert grasslands to cropfields, drain and develop wetlands, and not worry about extinction

• Precautionary Principle: when evidence indicates that an activity can harm the environment, we should take precautionary measures to prevent harm, even if the cause-and-effect relationships have not been fully established