Community Interactions

• Community – An assemblage of all the populations

interacting in an area

Populations are affected by:

• Available living space

– habitat

• Resource Availability

– niche

• Species interactions

– Symbiosis – “living together”

• commensalism

• mutualism

• parasitism

– Competition

– Predation

• Coevolution

Habitat – an organism’s mailing address

the type of place where the individuals normally live

Available Living Space

Niche –

an organism’s

profession (role)

in the

community

Resource Availability

Species Interactions

Symbiosis

• Living together for at least some part of the life cycle

• Most interactions are neutral; they have no effect on either species

– Commensalism

– Mutualism

– Parasitism

Commensalism

One species benefits and has no effect on the other

Mutualism

• Both species benefit

• Some are obligatory; partners depend

upon each other (coevolution)

– Yucca plants and yucca moth

– Mycorrhizal fungi and plants

– Anemone fish and anemone

Mutualism

Protection

photograph © Alex Wild 2004

Mutualism

Birds get food, help plant disperse seeds

Mutualism

Pollination

Yucca and Yucca Moth

• Example of an obligatory mutualism

• Each species of yucca is pollinated only by

one species of moth

• Moth larvae can grow only in that one

species of yucca

Obligate Mutualism Yucca Plant

Yucca Moth

Mycorrhizae

• Obligatory mutualism

between fungus and

plant root

• Fungus supplies

mineral ions to root

• Root supplies sugars

to fungus

Parasitism

• Parasites

– draw nutrients from hosts

• live on or in host body

• Vectors

– Convey a parasite from host to host

• Parasitoids

– Develop inside another species

– Consume and kill host

Micro Parasites

bacteria

fungus

Protozoan - flagellate

Macro parasites

tick

flea

Macro parasite – lamprey

Parasites

Parasites as biological controls

• Some parasites and parasitoids are used as

biological controls

• Adapted to specific host and habitat

– Good at locating host

– High population growth rate

– Offspring disperse

Biological Controls

• Wasp and aphid

Social Parasite

Brown-headed cowbird

Competition

• Interspecific: among different species – Exploitative competition

– Interference competition

• Intraspecific: between members of the same species – Intraspecific competition is most intense

• Territoriality

• Allelopathy

• Some species have eliminated ways of avoiding competition: – Resource partitioning

Interspecific Competition

Intraspecific Competition

Intraspecific Competition

Territoriality

Intraspecific Competition

Allelopathy (sibling harming)

Eucalyptus oils harm young saplings of same species.

Can also affect other species

Forms of Competition

• Exploitative competition

– Species have equal access to resource; compete

to exploit resource

• Interference competition

– One species prevents another from using

resource

• usually through aggressive behavior

Interference Competition

Competitive Exclusion Principle

When two species compete for identical

resources, one will be more successful and

will eventually eliminate the other

Gause’s Experiment

Paramecium caudatum

Paramecium aurelia

Species grown together

Resource Partitioning

• Apparent competitors

may have slightly

different niches

• May use resources in a

different way or time

• Minimizes competition

and allows coexistence

Predation

• Predators

– animals that feed on other living

organisms

– free-living

– do not reside on their prey

• Carnivores and omnivores

Predator – Prey Relationships:

Canada Lynx and the

Showshoe Hare

• Species are limited by

the number of

available prey

• In some cases

predators limit a prey

species

Coevolution

• Two or more species exert selection

pressure on each other

– Prey defenses evolve

– Predator responses to prey evolve

Prey Defenses

• Camouflage

• Warning coloration

• Mimicry

Camouflage

Warning Coloration & Mimicry

Predator Responses

• Predators counter prey defenses with new

adaptations

– stealth

– camouflage

– avoidance of chemical repellents

Predator Responses

Community Dynamics

• Factors affecting changes in community structure

– Intermediate Disturbance Hypothesis

– Community Instability (Alternate stable states)

– Keystone Species

– Exotic (introduced species)

– Ecological Succession

• Primary

• Secondary

Species Diversity

• Species diversity

– Variety, Abundance of Species in a Particular Place

• Species richness:

– The number of different species in a given area

• Species evenness:

– Comparative number of individuals

Variations in Species Richness and Species Evenness

Fig. 4-12, p. 93

Higher richness and evenness

lower richness and evenness

Species Diversity:

• Diversity varies with geographical location

• The most species-rich communities

– Tropical rain forests

– Coral reefs

– Ocean bottom zone

– Large tropical lakes

Species are not assembled randomly in nature.

Physical stress

(Sousa 1979 Ecology)

0

1

2

3

4

5

Low Mid High

Disturbance level

Sp

ecie

s r

ich

ne

ss

Nov-75

May-76

Oct-76

May-77

(Lubchenco 1978 Am Nat)

Herbivory

0

5

10

15

0 100 200 300

Herbivore Density (snails/m2)

#

se

aw

ee

d s

pe

cie

s

Nitrogen availability

0.25

0.50

0.75

1.00

0.0 0.2 0.4 0.6 0.8 1.0

log

# s

ea

we

ed

sp

ecie

s

log ammonium loading (micromol/L/h)

(Bracken & Nielsen 2004 Ecology)

Intermediate Disturbance Hypothesis

• Ecological Community stability is an uneasy balance

• Areas with moderate levels of disturbance have greater species diversity

Community Instability

Disturbances can cause a

community to change in ways

that persist even if the

change is reversed. In some

cases these are considered

“alternate stable states”

Keystone Species

• A species that has a disproportionately large

effect on community structure

• Removal of a keystone species can cause

drastic changes in a community

– can increase or decrease diversity

sea star community

15 species coexist

- 1 sea star is removed

8 remain in community

• Keystone species

exert an important

regulating effect

on other species

in a community.

Fig. 53.14

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Populations are limited by

competition for resources

• If Keystone

Species are

removed,

community

structure is greatly

affected.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Ecological Succession

• Over

time, one

array of

species is

replaced

by

another

Types of Succession

• Primary succession: new environments

– Begins with “foundation species”

– Ends with “climax community”

• Secondary succession: communities

destroyed or displaced

– May not have “foundation species”

– Also ends with “climax community”

Climax Community

• Stable array of species that does not change

over time

• In a particular habitat, succession produces

the same climax community

Most communities are disturbed

by events like fire, weather, or human activities

Marine communities are subject to disturbance by tropical storms.

Fig. 53.17

Volcanic Island Succession

Foundation Species Climax Community

Glacial Succession

Primary succession – begins in a new or lifeless area where soil has not yet

formed.

Lake Succession

Sand Dune Succession

• Secondary succession –

occurs where an existing community has been

cleared, but the soil is left intact.

• Relatively rapid succession

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Secondary Succession: Disturbed Habitats

Abandoned Fields

Along Roadsides

Disease Damage – Sudden Oak Death

Storm Damage

Fire Damage

Cyclic Replacement

Chaparral – fire adapted plant community

Cyclic Replacement: Fire Climax

High intensity – crown fire destroys forest Giant Sequoia

No fire –

no seedlings,

growth of white fir

Low intensity – allows Giant Sequoia to re-seed

Medium intensity – kills competing fir trees