Community Interactions
Chapter 29
Forest of New Guinea
• Community includes nine species of
pigeons that partition the food supply
• Pigeons disperse seeds of the trees
that provide their food (fruit)
• These are just a few of the many
interactions that shape this community
• All the populations that live together in a
• Habitat is the type of place where
individuals of a species typically live
• Type of habitat shapes a community’s
Factors Shaping
Community Structure
• Climate and topography
• Available foods and resources
• Adaptations of species in community
• Species interactions
• Arrival and disappearance of species
• Physical disturbances
Sum of activities and relationships in
which a species engages to secure and
use resources necessary for survival
and reproduction
Realized &
Fundamental Niches
• Fundamental niche
– Theoretical niche occupied in the absence
of any competing species
• Realized niche
– Niche a species actually occupies
• Realized niche is some fraction of the
fundamental niche
Species Interactions
• Most interactions are neutral; have no
effect on either species
• Commensalism helps one species and
has no effect on the other
• Mutualism helps both species
Species Interactions
• Interspecific competition has a negative
effect on both species
• Predation and parasitism both benefit
one species at a cost to another
• Living together for at least some part of
the life cycle
• Commensalism, mutualism, and
parasitism are forms of symbiosis
• Both species benefit
• Many examples in nature
• Some mutualisms are obligatory;
partners depend upon each other
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
• Obligatory mutualism between fungus
and plant root
• Fungus supplies mineral ions to root
• Root supplies sugars to fungus
• Interspecific - between species
• Intraspecific - between members of the
same species
• Intraspecific competition is most intense
Forms of Competition
• Competitors may have equal access to
a resource; compete to exploit resource
more effectively
• One competitor may be able to control
access to a resource, to exclude others
Competitive Exclusion
When two species compete for identical
resources, one will be more successful
and will eventually eliminate the other
Competitive Exclusion Expt
Paramecium caudatum
Paramecium aurelia
Keystone Species
• A species that can dictate community
• Removal of a keystone species can
cause drastic changes in a community;
can increase or decrease diversity
Lubchenco Experiment
Periwinkles promote or limit diversity in different habitats
Rocks exposed at high tide
Resource Partitioning
• Apparent competitors
may actually have
slightly different niches
• Species may use
resources in a different
way or time
• Minimizes competition
and allows coexistence
• Predators are animals that feed on other
living organisms
• Predators are free-living; they do not
take up residence on their prey
• Natural selection promotes traits that
help prey escape predation
• It also promotes traits that make
predators more successful at capturing
Predator-Prey Cycles
• Predator and prey populations may
show an apparent correspondence
Variation in Cycles
• An association in predator and prey
abundance does not always indicate a
cause and effect relationship
• Variations in food supply and additional
predators may also influence changes
in prey abundance
Prey Defenses
• Camouflage
• Warning coloration
• Mimicry
• Moment-of-truth defenses
Predator Responses
• Any adaptation that protects prey may
select for predators that can overcome
that adaptation
• Prey adaptations include stealth,
camouflage, and ways to avoid
chemical repellents
• Parasites drain nutrients from their
hosts and live on or in their bodies
• Natural selection favors parasites
that do not kill their host too quickly
Types of Parasites
• Microparasites
• Macroparasites
• Social parasites
• Parasitoids
Change in the composition of species
over time
Types of Succession
• Primary succession - new
• Secondary succession communities were destroyed
or displaced
Pioneer Species
• Species that colonize barren habitats
• Lichens, small plants with brief life
• Improve conditions for other species
who then replace them
Climax Community
• Stable array of species that persists
relatively unchanged over time
• Succession does not always move
predictably toward a specific climax
community; other stable communities
may persist
Cyclic Changes
• Cyclic, nondirectional changes also
shape community structure
• Tree falls cause local patchiness in
tropical forests
• Fires periodically destroy underbrush in
sequoia forests
Restoration Ecology
• Natural restoration of a damaged
community can take a very long time
• Active restoration is an attempt to
reestablish biodiversity in an area
• Ecologists are actively working to
restore reefs, grasslands, and wetlands
Community Instability
• Disturbances can cause a
community to change in ways that
persist even if the change is
Species Introductions
• Introduction of a nonindigenous species
can decimate a community
• No natural enemies or controls
• Can outcompete native species
Exotic Species
• Species that has left its home range
and become established elsewhere
• Becomes part of its new community
• Can have beneficial, neutral, or
harmful effects on a community
Endangered Species
• A species that is extremely vulnerable to
• Close to 70 percent of endangered
species have been negatively affected
by exotic competitors
Nile Perch in East Africa
• Nile perch were introduced into Lake
Victoria as a food source
• This predator ate native cichlids; drove
many species to extinction
• Now Nile perch species is close to
Rabbits in Australia
• Rabbits were introduced for food and
• Without predators, their numbers
• Attempts at control using fences or
viruses have thus far been unsuccessful
Kudzu in Georgia
• Imported for erosion control
• No natural herbivores, pathogens, or
• Grows over landscapes and cannot be
dug up or burned out
• May turn out to have some commercial
Diversity by Latitude
• Diversity of most groups is greatest
in tropics; declines toward poles
Why Are Tropical
Species Rich?
• Resources are plentiful and reliable
• Species diversity is self-reinforcing
• Rates of speciation are highest in the
Distance Effect
• The farther an island is from a
mainland, the fewer species
• Closer islands receive more immigrants
• Species that reach islands far from
mainland are adapted for long-distance
dispersal and can move on
Distance Effect

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