Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 41
Community Ecology
(Sections 41.1 - 41.5)
Albia Dugger • Miami Dade College
41.1 Fighting Foreign Fire Ants
• Red imported fire ants, Solenopsis invicta, have a venomous
sting, disrupt native wildlife communities, and are not
controlled by pesticides
• Biological control involves phorid flies, specialized parasites
(parasitoids) that kill their host by laying an egg in the ant’s
tissues – the larvae eats its way through the fire ant and
undergoes metamorphosis in its head
• community
• All species that live in a particular region
Phorid Fly and Fire Ant
• Phorid flies insert a
fertilized egg into an
ant’s thorax
• Parasitized ant that lost
its head after a
developing fly larva
moved into it
41.2 Community Structure
• The type of place where a species normally lives is its
habitat, and all species living in a habitat constitute a
community
• Communities often are nested one inside another
• habitat
• Type of environment in which a species typically lives
Species Diversity
• Communities differ in their species diversity
• Species diversity has two components:
• Species richness (number of species)
• Species evenness (relative abundance of each species)
• Community structure is dynamic
• Species richness and evenness change over time
Factors Affecting Community Structure
• Community structure can change:
• As the community forms and ages
• As a result of natural or human-induced disturbances
• With changes in physical factors such as climate and
resource availability
• Due to various types of species interactions
Species Interactions
• Species interactions can be mutually beneficial, mutually
harmful, or benefit one species while harming the other
• Example: Commensal ferns attached to the trunk of a tree;
the fern benefits from the light, and the tree is unaffected
• commensalism
• Species interaction that benefits one species and neither
helps nor harms the other
Commensalism
• A tree with a
commensal fern
• The fern benefits by
growing on the tree,
which is unaffected by
the presence of the fern
Types of Two-Species Interactions
•
•
•
•
Type of Interaction
Species 1
Species 2
Commensalism
Mutualism
Interspecific competition
Predation, herbivory
parasitism, parasitoidism
Helpful
Helpful
Harmful
None
Helpful
Harmful
Helpful
Harmful
Symbiosis
• Symbiosis (“living together”) refers to a relationship in which
two species have a prolonged close association
• Two species that interact closely for generations can coevolve
– an evolutionary process in which each species acts as a
selective agent on the other
• symbiosis
• One species lives in or on another in a commensal,
mutualistic, or parasitic relationship
Key Concepts
• Community Characteristics
• A community consists of all species in a habitat
• A habitat’s history, its biological and physical
characteristics, and interactions among species in the
habitat affect the number of species in the community and
their relative abundance
41.3 Mutualism
• In a mutualistic interaction, two species benefit by taking
advantage of one another
• Example: Pollinators eat nectar and pollen, and plants
receive pollen from other plants of the same species
• mutualism
• Species interaction that benefits both species
Mutualism and Coevolution
• In some mutualisms, neither species can complete its life
cycle without the other
• Example: Yucca plants and the moths that pollinate them
• The moth matures when yucca flowers bloom
• Mouthparts of the female moth are specialized to collect
yucca pollen
• Female flies to another flower, pierces its ovary, and lays
eggs inside – fertilizing the yucca as she leaves
• Moth eggs develop into larvae in the ovary of the yucca
Yucca Plant and Yucca Moth
Mutualism and Defense
• For some mutualists, the main benefit is defense
• Example: Sea anemone and anemone fish
• An anemone fish has a mucus layer that shields it from
stinging cells (nematocysts) of a sea anemone
• Tentacles of the anemone protect the fish from predators
• The anemone fish chases away the few fishes that are
able to eat sea anemone tentacles
Sea Anemone and Anemone Fish
Mutualistic Microorganisms
• Mutualistic microorganisms help plants obtain nutrients:
• Nitrogen-fixing bacteria on roots of legumes (peas)
provide the plant with extra nitrogen
• Mycorrhizal fungi living in or on plant roots enhance a
plant’s mineral uptake
• Other fungi interact with photosynthetic algae or bacteria
in lichens
41.4 Competitive Interactions
• Resources are limited and individuals of different species
often compete for access to them (interspecific
competition)
• Competition adversely affects both species
• interspecific competition
• Competition between two species
Ecological Niche
• Each species has an ecological niche defined by physical
and biological factors; the more similar the niches of two
species are, the more intensely they will compete
• An animal’s niches include the temperature range it can
tolerate, species it eats, and places it can breed
• A flowering plant’s niche would include its soil, water, light,
and pollinator requirements
• ecological niche
• All of a species’ requirements and roles in an ecosystem
Two Types of
Interspecific Competition
• Exploitative competition:
• Two species reduce the amount of resources available to
the other by using that resource
• Example: Deer and blue jays compete for acorns
• Interference competition:
• One species actively prevents another from accessing
some resource
• Example: One species of scavenger will often chase
another away from a carcass
Interference Competition
Interference Competition
A Golden eagle and a red fox face off over
a moose carcass.
B In a dramatic demonstration of
interference competition, the eagle attacks
the fox with its talons. After this attack, the
fox retreated, leaving the eagle to exploit
the carcass.
Stepped Art
Fig. 41.5, p. 694
Effects of Competition
• Species compete most intensely when a shared resource is
the main limiting factor for both
• Whenever two species require the same limited resource to
survive or reproduce, the better competitor will drive the less
competitive species to extinction in that habitat
• competitive exclusion
• Process whereby two species compete for a limiting
resource, and one drives the other to local extinction
Experiment: Competitive Exclusion
• Two Paramecium
species compete for the
same food (bacteria)
• Each species thrives
when grown alone
• When grown together,
P. aurelia drove P.
caudatum to extinction
Experiment: Competitive Exclusion
Experiment: Competitive Exclusion
Fig. 41.6.1, p. 695
Experiment: Competitive Exclusion
Fig. 41.6.2, p. 695
Experiment: Competitive Exclusion
Fig. 41.6.3, p. 695
0
4
8 12 16 20 24
Time (days)
0
P. aurelia alone
4
8 12 16 20 24
Time (days)
Relative
population density
P. caudatum alone
Relative
population density
Relative
population density
Experiment: Competitive Exclusion
0
Both species together
4
8 12 16 20 24
Time (days)
Stepped Art
Fig. 41.6, p. 695
ANIMATION: Competitive Exclusion
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Resource Partitioning
• Resource partitioning is an evolutionary process by which
species become adapted to use a shared limiting resource in
a way that minimizes competition
• Example: Three plant species growing in the same field
• resource partitioning
• Species become adapted in different ways to access
different portions of a limited resource
• Allows species with similar needs to coexist
Resource Partitioning
• Roots of each species
take up water and
mineral ions from a
different soil depth
• Reduces competition
among the species and
allows them to coexist
Character Displacement
• Directional selection occurs when species with similar
requirements share a habitat and compete for a limiting
resource, resulting in character displacement
• Example: Beak sizes in Galapagos finches
• character displacement
• Outcome of competition between two species
• Directional selection shifts the range of variation for one or
more traits in a direction that lessens competition for a
limiting resource
ANIMATION: Hairston's Experiment
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ANIMATION: Resource Partitioning
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41.5 Predation and Herbivory
• Predation and herbivory are short-term interactions in which
one species obtains nutrients and energy by feeding on
another
• predation
• One species (the predator) captures, kills, and eats
another species (the prey)
Predator and Prey Abundance
• The abundance of prey species in a community affects how
many predators it can support
• With some predators, such as web-spinning spiders, the
proportion of prey killed is constant
• Usually, the number of prey killed depends on the time it
takes predators to capture, eat, and digest prey
Predator and Prey Abundance (cont.)
• Predator and prey
populations may rise
and fall in cycles
• Example: Lynx and
snowshoe hare
populations rise and fall
over a ten-year cycle
Lynx and Snowshoe Hare
Coevolution of Predators and Prey
• Predator and prey exert selection pressure on one another
• Predators exert selection pressure that favors improved prey
defenses
• Improved prey defenses in turn exert selection pressure on
predators to improve capture skills, and so on
Defensive Adaptations
• Defensive adaptations of prey include hard or sharp parts that
make prey difficult to eat, and chemicals that taste bad or
sicken predators
• Other adaptations trick or startle an attacking predator
• Well-defended prey often have warning coloration that
predators learn to avoid, such as the black and yellow stripes
of stinging wasps and bees
Defensive Adaptations (cont.)
• In a type of mimicry, prey masquerade as a species that has
a defense that they lack
• Example: Some flies that can’t sting resemble stinging bees
or wasps
• mimicry
• A species evolves traits that make it more similar in
appearance to another species
Wasp and Mimic
Predator Adaptations
• Predator adaptations include sharp teeth and claws
• Predators and prey may be coevolved for speed
• Example: cheetah and gazelle
• Both predators and prey use camouflage (a form, patterning,
color, or behavior that allows them to blend into their
surroundings) to avoid detection
Camouflage in Prey and Predators
Coevolution of Herbivores and Plants
• With herbivory, the number and type of plants in a
community can influence the number and type of herbivores
present
• herbivory
• An animal feeds on plant parts
Herbivores and Plants (cont.)
• There are two types of defenses against herbivory:
• Some plants have adapted to withstand and recover
quickly from herbivory
• Other plants have traits such as spines, tough leaves, or
toxins that deter herbivory
• Plant defenses favor adaptations in herbivores
• Example: Koalas have special enzymes to break down
toxins in eucalyptus
ANIMATION: Predator-Prey Interactions
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