Transcript Chapter 47

Community Interactions
Chapter 47
Forest of New Guinea

Community includes nine species of
pigeons that partition the food supply
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Pigeons disperse seeds of the trees that
provide their food (fruit)

These are just a few of the many
interactions that shape this community
Community

All the populations that live together in a
habitat

Type of habitat shapes a community’s
structure
Factors Shaping
Community Structure
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Climate and topography
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Available foods and resources
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Adaptations of species in community
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Species interactions
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Arrival and disappearance of species
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Physical disturbances
Niche
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
–
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Realized niche
–
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Theoretical niche occupied in the absence of any
competing species
Niche a species actually occupies
Realized niche is some fraction of the
fundamental niche
Species Interactions
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Most interactions are neutral; have no effect
on either species
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Commensalism helps one species and has
no effect on the other
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Mutualism helps both species
Species Interactions
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Interspecific competition has a negative
effect on both species
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Predation and parasitism both benefit one
species at a cost to another
Symbiosis
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Living together for at least some part of the
life cycle
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Commensalism, mutualism, and parasitism
are forms of symbiosis
Mutualism
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Both species benefit
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Some are obligatory; partners depend
upon each other
–
Yucca plants and yucca moth
–
Mycorrhizal fungi and plants
Yucca and Yucca Moth
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Example of an obligatory mutualism
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Each species of yucca is pollinated only by
one species of moth
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Moth larvae can grow only in that one
species of yucca
Mycorrhizae
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Obligatory mutualism between fungus and
plant root
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Fungus supplies mineral ions to root
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Root supplies sugars to fungus
Competition
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Interspecific - between species
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Intraspecific - between members of the same
species
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Intraspecific competition is most intense
Forms of Competition

Competitors may have equal access to a
resource; compete to exploit resource more
effectively
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One competitor may be able to control
access to a resource, to exclude others
Interference Competition
Least chipmunk is
excluded from piñon
pine habitat by the
Least
Chipmunk
competitive behavior of
yellow pine chipmunks
Yellow Pine
Chipmunk
Figure 47.3
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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
Species grown together
Paramecium aurelia
Figure 47.4
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Hairston’s Experiment
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Two salamanders species overlap in parts of
their ranges
Removed one species or the other in test
plots
Control plots unaltered
5 years later, salamander populations were
growing in test plot
Resource Partitioning
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Apparent competitors may
have slightly different niches
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May use resources in a
different way or time
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Minimizes competition and
allows coexistence
Figure 47.6
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Predation
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Predators are animals that feed on other living
organisms
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Predators are free-living; they do not take up
residence on their prey
Coevolution

Joint evolution of two or more species that
exert selection pressure on each other as an
outcome of close ecological interaction
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As snail shells have thickened, claws of
snail-eating crabs have become more
massive
Predator-Prey Models
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Type I model: Each individual predator will consume
a constant number of prey individuals over time
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Type II model: Consumption of prey by each
predator increases, but not as fast as increases in
prey density
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Type III model: Predator response is lowest when
prey density is lowest
Variation in Cycles
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An association in predator and prey
abundance does not always indicate a cause
and effect relationship
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Variations in food supply and additional
predators may also influence changes in
prey abundance
Canadian Lynx
and Snowshoe Hare
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Show cyclic oscillations
Krebs studied populations for ten years
Fencing plots delayed cyclic declines but
didn’t eliminate them
Aerial predators, plant abundance also
involved
Three-level model
Prey Defenses
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Camouflage
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Warning coloration
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Mimicry
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Moment-of-truth defenses
Predator Responses

Any adaptation that protects prey may select
for predators that can overcome that
adaptation
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Prey adaptations include stealth,
camouflage, and ways to avoid chemical
repellents
Parasitism
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Parasites drain nutrients from their hosts
and live on or in their bodies
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Natural selection favors parasites that do
not kill their host too quickly
Kinds of Parasites
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Microparasites
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Macroparasites
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Social parasites
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Parasitoids
Fungus and Frogs
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Amphibians are disappearing even in
undisturbed tropical forests
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Infection by a parasitic chytrid is one of the
causes of the recent mass deaths
Parasitic Plants
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Holoparasites
–
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Nonphotosynthetic; withdraw nutrients and water
from young roots
Hemiparasites
–
Capable of photosynthesis, but withdraw nutrients
and water from host
Parasitioids
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Insect larvae live inside and consume all of
the soft tissues of the host
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Used as agents of biological control
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Can act as selective pressure on host
Price’s Sawfly Study
Number emerging when wasp
attacks were experimentally
prevented
Number emerging after wasp attacks
Figure 47.15
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Ecological Succession
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Change in the composition of species
over time
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Classical model describes a predictable
sequence with a stable climax community
Types of Succession
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Primary succession - new
environments
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Secondary succession communities were destroyed or
displaced
Pioneer Species
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Species that colonize barren habitats
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Lichens, small plants with brief life cycles
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Improve conditions for other species who
then replace them
Climax Community
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Stable array of species that persists relatively
unchanged over time
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Succession does not always move
predictably toward a specific climax
community; other stable communities may
persist
Cyclic Changes
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Cyclic, nondirectional changes also shape
community structure
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Tree falls cause local patchiness in tropical
forests
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Fires periodically destroy underbrush in
sequoia forests
Restoration Ecology
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Natural restoration of a damaged community
can take a very long time
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Active restoration is an attempt to reestablish
biodiversity in an area
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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 reversed
Keystone Species

A species that can dictate community
structure
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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
Tidepools
Rocks exposed at high tide
Figure 47.17
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Species Introductions
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Introduction of a nonindigenous species can
decimate a community
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No natural enemies or controls
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Can outcompete native species
Exotic Species
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Species that has left its home range and
become established elsewhere
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Becomes part of its new community
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Can have beneficial, neutral, or harmful
effects on a community
Endangered Species
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A species that is extremely vulnerable to
extinction
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Close to 70 percent of endangered species
have been negatively affected by exotic
competitors
Nile Perch in East Africa
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Nile perch were introduced into Lake Victoria
as a food source
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This predator ate native cichlids; drove many
species to extinction
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Now Nile perch species is close to crashing
Rabbits in Australia
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Rabbits were introduced for food and hunting
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Without predators, their numbers soared
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Attempts at control using fences or viruses
have thus far been unsuccessful
Kudzu in Georgia
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Imported for erosion control
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No natural herbivores, pathogens, or
competitors
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Grows over landscapes and cannot be dug
up or burned out
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May turn out to have some commercial use
Diversity by Latitude
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Diversity of most groups is greatest in
tropics; declines toward poles
Ant
diversity
Figure 47.20
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Why Are Tropical
Species Rich?
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Resources are plentiful and reliable
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Species diversity is self-reinforcing
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Rates of speciation are highest in the tropics
Distance Effect
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The farther an island is from a mainland, the
fewer species
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Closer islands receive more immigrants
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Species that reach islands far from mainland
are adapted for long-distance dispersal and
can move on
Distance Effect
Figure 47.22
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Area Effect
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Larger islands tend to support more species
than smaller islands
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More habitats
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Bigger targets
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Larger populations decrease extinction risks