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27
Community
Interactions
Why Are Community Interactions
Important?
• As community interactions limit population
size, they shape the bodies and behaviors of
the interacting populations
• Coevolution
Why Are Community Interactions
Important?
• The major community interactions:
• These interactions can be classified according to
whether each of the species is harmed or helped by
the interaction
How Does the Ecological Niche
Influence Competition?
• Each species occupies a unique ecological
niche that encompasses all aspects of its way
of life
• An ecological niche includes all physical
environmental conditions necessary for
survival and reproduction of a given species
How Does the Ecological Niche
Influence Competition?
• The physical and environmental conditions can include
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Nesting or denning sites
Climate
Type of nutrients the species requires
Optimal temperature range
Amount of water needed
The pH and salinity of the water or soil
The degree of sun or shade it can tolerate
How Does the Ecological Niche
Influence Competition?
• The niche also encompasses the entire “role” that a
given species performs within an ecosystem
– What the species eat and the other species with which it
competes
• No two species ever occupy exactly the same
ecological niche within the same natural community,
just as no two organisms can occupy exactly the
same physical space at the same time (competitive
exclusion principle)
How Does the Ecological Niche
Influence Competition?
• Competition occurs whenever two organisms
attempt to use the same, limited resources
– Interspecific - The greater the overlap between the
ecological niches of the two species, the greater the
amount of competition between them
– Detrimental to all species involved
How Does the Ecological Niche
Influence Competition?
• The competitive exclusion principle
– Leads to the hypothesis that if a researcher forces two
species with very similar niches to compete for the same
limited resource, one will outcompete the other, and the
species that is well less adapted to the experimental
conditions will die out
population density
Figure 27-1 Competitive exclusion
P. aurelia
P. caudatum
200
150
100
50
0
0 2 4
6 8 10 12 14 16 18 20 22 24
days
population density
Grown in separate flasks
200
150
100
50
0
0 2 4
6 8 10 12 14 16 18 20 22 24
days
Grown in the same flask
How Does the Ecological Niche
Influence Competition?
• The competitive exclusion principle
– Gause repeated the experiment, replacing P.
caudatum with P. bursaria, which fed mostly on
bacteria that had settled to the bottom of the test
tube
• These two species of Paramecium were able to coexist
indefinitely because they preferred feeding in different
places, and thus occupied slightly different niches
How Does the Ecological Niche
Influence Competition?
• Adaptations reduce the overlap of ecological
niches among coexisting species
– Ecologist Robert MacArthur observed five species
of North American warbler
• These birds all hunt for insects and nest in the same
type of eastern spruce tree
How Does the Ecological Niche
Influence Competition?
• Adaptations reduce the overlap of ecological niches
among coexisting species (continued)
– MacArthur found that each species concentrates its search
for food in specific regions within spruce trees, employs
different hunting tactics, and nests at a slightly different
time
– By dividing up the resources provided by the spruce trees
they share, the warblers minimize the overlap of their
niches and reduce interspecific competition
How Does the Ecological Niche
Influence Competition?
• Adaptations reduce the overlap of ecological niches
among coexisting species (continued)
– When species with largely similar ecological niches coexist
and compete, each species occupies a smaller niche than it
would by itself, a phenomenon called resource
partitioning
Yellow-rumped
warbler
Bay-breasted
warbler
Cape May
warbler
Black-throated
green warbler
Blackburnian
warbler
How Does the Ecological Niche
Influence Competition?
• Competition among species may reduce the
population size and distribution of each
– Although natural selection can reduce niche overlap,
interspecific competition may still restrict the size and
distribution of competing populations
– Barnacles of the genus Chthamalus share rocky
ocean shores with barnacles of the genus Balamus
• Their niches overlap considerably
– Both live in the intertidal zone, an area of the shore that is
alternately covered and exposed by the tides
How Does the Ecological Niche
Influence Competition?
• Competition among species may reduce the
population size and distribution of each (continued)
– Chthamalus dominates the upper intertidal zone and
Balamus dominates the middle intertidal zone
– The middle habitat is suitable for both species
– When Connell scraped off Balamus, the Chthamalus
population increased, spreading downward into the
middle intertidal region where its competitor had been
scraped off
• This demonstrated that Chthamalus would occupy the
middle intertidal zone if it didn’t have to cope with
competition from Bamalus, a larger, faster-growing barnacle
How Does the Ecological Niche
Influence Competition?
• Competition within a species is a major factor
controlling population size
– Intraspecific competition is the most intense form of
competition because all of the members of the species
occupy the same niche
– Intraspecific competition is one of the main factors driving
evolution by natural selection
• Individuals that are better equipped to obtain scarce
resources are more likely to reproduce successfully, passing
their heritable traits to their offspring
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Although predators are often regarded as being
carnivores (animals that eat other animals),
ecologists sometimes include herbivores (animals
that eat plants) in this general category
Pika
Long-eared bat
Eagle owl
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Predators are generally less abundant than their prey
• In order to survive, predators must feed and prey
must avoid becoming food
• Predator and prey populations exert intense selective
pressure on one another, resulting in coevolution
– As prey become more difficult to catch, predators must
become more adept at hunting
– Some predators and prey have evolved counteracting
behaviors
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Some predators and prey have evolved counteracting
behaviors
– Bats use sonar to find their prey
• Some moths (a favorite prey of bats) have evolved ears that are
particularly sensitive to the pitches used by echolocating bats
• The moth takes evasive action after hearing the sound
• Some moths produce their own high frequency clicks to interfere
– The bats have evolved the ability to counter this defense by
switching the frequency of their sound pulses from the moth’s
sensitivity range or just following the moth’s clicks
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Predator and prey may engage in chemical warfare
– May be used to attack or defend
– Many plants synthesize toxic and distasteful chemicals
– As plants evolved these defensive toxins, certain insects
evolved increasingly efficient ways to detoxify or even use
these substances
– Nearly every toxic plant is eaten by at least one type of
insect
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Chemical warfare example:
– Monarch butterflies lay their eggs on milkweed
– When their larvae hatch, they consume this poisonous plant
– The caterpillars store the poison in their tissues as a defense
against their own predators
• The stored toxin is retained in the metamorphosed monarch
butterfly
• Viceroy butterflies use a similar strategy, storing a bitter
compound from willows (eaten by the larvae) in the tissues of
the adult
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Camouflage
– Both predators and prey have evolved colors,
patterns, and shapes that resemble their
surroundings
Sand dab fish adjust camouflage
to different backgrounds
A camouflaged horned lizard
Figure 27-6 Camouflage by resembling specific objects
Citrus swallowtail larva
Leafy sea dragon
Thorn treehoppers
Living rock cacti
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Camouflage
– Predators that ambush prey are aided by
camouflage
A camouflaged snow leopard
A camouflaged frogfish
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Some prey animals have evolved very
differently, exhibiting bright warning
coloration
• These animals may taste bad, inflict a
venomous sting or bite (as bees or coral
snakes do), or produce a big stink when
bothered
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Mimicry
– Mimicry refers to members of one species having
evolved to resemble another species
– Two or more distasteful species may each benefit
from a shared warning coloration pattern
(Müllerian mimicry)
• Predators need only experience one distasteful species
to learn to avoid all with that color pattern
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Müllerian mimicry
• Toxic monarch and viceroy
butterflies have similar wing
patterns; if a predator becomes
ill from eating one species, it
will avoid the other
Monarch (distasteful)
Viceroy (distasteful)
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Mimicry (continued)
– Once warning coloration evolved, there arose a
selective advantage for harmless animals to
resemble venomous ones, an adaptation called
Batesian mimicry
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Batesian mimicry
Bee (venomous)
Hoverfly (nonvenomous)
Coral snake (venomous) Scarlet king snake (nonvenomous)
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Startle coloration
– Used to deter predators
• These animals may have
spots that resemble the
eyes of a larger animal
• If a predator gets close,
the prey will flash its
eyespots, startling the
predator and allowing
the prey to escape
False-eyed frog
Swallowtail
caterpillar
Peacock moth
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• A sophisticated variation of startle coloration
– Snowberry flies, which are hunted by territorial
jumping spiders
• When a spider approaches, the fly spreads its wings
and moves them in a jerky dance
Jumping spider
(predator)
Snowberry fly (prey)
How Do Predator-Prey Interactions
Shape Evolutionary Adaptations?
• Aggressive mimicry
– Predators entice their prey to come close by
resembling something attractive
• For example, by using a rhythm of flashes that is unique to
each species, female fireflies attract males to mate
• The frogfish is not only camouflaged but exhibits aggressive
mimicry by dangling a wriggling lure that resembles a small
fish just above its mouth
What Are Parasitism and Mutualism?
• Parasites live in or on their prey, which are called hosts,
usually harming or weakening them but not immediately
killing them
– Some parasite-host relationships are symbiotic; that is, the
relationships involve a close, long-term physical association
between the participating species
– Parasites are generally much smaller and more numerous than
their hosts
• Examples include tapeworms, fleas, ticks, and many types of
disease-causing protists, bacteria, and viruses
What Are Parasitism and Mutualism?
• Mutualism refers to interactions between species in
which both benefit
• Many mutualistic relationships are symbiotic and involve a
close, long-term physical association between the
participating species
– For example, lichens receive support and protection from
fungus while obtaining food from the photosynthetic alga
Lichen
Clownfish
What Are Parasitism and Mutualism?
• Mutualistic interactions
– Many mutualistic relationships are not intimate
and extended, and so are not symbiotic
• For example, the relationship between plants and the
insects that pollinate them is not symbiotic
• The insects fertilize the plants by carrying plant sperm,
and benefit by sipping nectar and sometimes eating
pollen
How Do Keystone Species Influence
Community Structure?
• In some communities, a keystone species
plays a major role in determining community
structure
– A keystone species role is out of proportion to its
abundance in the community
– If a keystone species is removed from the
community, normal community interactions are
significantly altered and the relative abundance of
other species changes dramatically
How Do Keystone Species Influence
Community Structure?
• In the African savanna, the African elephant is a keystone
predator
• By grazing on small trees and bushes, elephants prevent the
encroachment of forests and help maintain the grassland
community, along with its diverse population of grazing
mammals and their predators
• Wolves and cougars
– Maintain deer populations 
Maintains vegetation populations 
Provides food, nesting, & shelter
African elephant
How Do Keystone Species Influence
Community Structure?
• Identifying a keystone species can be a difficult task
– Many have been recognized only after their loss has had
dramatic, unforeseen consequences
– The intricate and tangled web of community interactions is
beautifully illustrated by the plight of the northern sea
otter in the Aleutian Islands of southwestern Alaska
• Kelp forests flourished around islands in waters where the
otters were abundant
• No otters  increased sea urchins
 No kelp forest  eliminate diversity
Northern sea otter
Review
1. Which is more intense for niches: interspecific
or intraspecific competition? Why?
2. Define camouflage.
3. What is the difference between Mullerian and
Batesian mimicry?
4. What is a symbiotic relationship?
5. Define mutualism.
6. What is a keystone species?