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CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
41
Species
Interactions
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
In a given community, what types of interactions
could different organisms have with each
other?
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Concept 41.1: Interactions within a community
may help, harm, or have no effect on the species
involved
 Ecologists call relationships between species in a
community interspecific interactions
 Examples are competition, predation, herbivory,
symbiosis (parasitism, mutualism, and
commensalism), and facilitation
 Interspecific interactions can affect the survival and
reproduction of each species, and the effects can be
summarized as positive (), negative (−), or no
effect (0)
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Competition
 Interspecific competition (−/− interaction) occurs
when species compete for a resource that limits their
growth or survival
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Define competitive exclusion.
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Competitive Exclusion
 Strong competition can lead to competitive
exclusion, local elimination of a competing species
 The competitive exclusion principle states that two
species competing for the same limiting resources
cannot coexist in the same place
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How might two organisms occupying the same
niche affect competition?
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Ecological Niches and Natural Selection
 Evolution is evident in the concept of the ecological
niche, the specific set of biotic and abiotic resources
used by an organism
 An ecological niche can also be thought of as an
organism’s ecological role
 Ecologically similar species can coexist in a
community if there are one or more significant
differences in their niches
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 Resource partitioning is differentiation of
ecological niches, enabling similar species to
coexist in a community
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Figure 41.2
A. distichus perches on
fence posts and other
sunny surfaces.
A. insolitus usually
perches on shady
branches.
A. ricordii
A. insolitus
A. aliniger
A. distichus
A. christophei
A. cybotes
A. etheridgei
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 A species’ fundamental niche is the niche potentially
occupied by that species
 A species’ realized niche is the niche actually
occupied by that species
 As a result of competition, a species’ fundamental
niche may differ from its realized niche
 For example, the presence of one barnacle species
limits the realized niche of another species
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Character Displacement
 Character displacement is a tendency for
characteristics to be more divergent in sympatric
populations (geographically overlapping) of two
species than in allopatric (geographically separate)
populations of the same two species
 An example is variation in beak size between
populations of two species of Galápagos finches
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Predation
 Predation (/− interaction) refers to an interaction in
which one species, the predator, kills and eats the
other, the prey
 Some feeding adaptations of predators are claws,
teeth, stingers, and poison
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Describe some evolutionary adaptions prey might
have acquired to avoid being eaten?
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Figure 41.5
(a) Cryptic
coloration
Canyon
tree frog
(b) Aposematic
coloration
Poison
dart frog
(c) Batesian mimicry: A harmless species mimics
a harmful one.
Nonvenomous
(d) Müllerian mimicry: Two unpalatable
hawkmoth larva
species mimic each other.
Venomous green
parrot snake
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Cuckoo bee
Yellow jacket
Herbivory
 Herbivory (/− interaction) refers to an interaction
in which an herbivore eats parts of a plant or alga
 In addition to behavioral adaptations, some
herbivores may have chemical sensors or specialized
teeth or digestive systems
 Plant defenses include chemical toxins and
protective structures
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Differentiate between parasitism, mutualism, and
commensalism.
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Symbiosis
 Symbiosis is a relationship where two or more
species live in direct and intimate contact with one
another
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Parasitism
 In parasitism (/− interaction), one organism, the
parasite, derives nourishment from another
organism, its host, which is harmed in the process
 Parasites that live within the body of their host are
called endoparasites
 Parasites that live on the external surface of a host
are ectoparasites
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Mutualism
 Mutualistic symbiosis, or mutualism (/ interaction),
is an interspecific interaction that benefits both
species
 In some mutualisms, one species cannot survive
without the other
 In other mutualisms, both species can survive alone
 Mutualisms sometimes involve coevolution of related
adaptations in both species
Video: Clownfish and Anemone
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Commensalism
 In commensalism (/0 interaction), one species
benefits and the other is neither harmed nor helped
 Commensal interactions are hard to document in
nature because any close association likely affects
both species
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Facilitation
 Facilitation (/ or 0/) is an interaction in which
one species has positive effects on another species
without direct and intimate contact
 For example, the black rush makes the soil more
hospitable for other plant species
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Number of plant species
Figure 41.9
(a) Salt marsh with Juncus
(foreground)
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8
6
4
2
0
(b)
With
Juncus
Without
Juncus
Figure 41.UN03
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Concept 41.2: Diversity and trophic structure
characterize biological communities
 Two fundamental features of community structure
are species diversity and feeding relationships
 Sometimes a few species in a community exert
strong control on that community’s structure
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Species Diversity
 Species diversity of a community is the variety of
organisms that make up the community
 It has two components: species richness and
relative abundance
 Species richness is the number of different species
in the community
 Relative abundance is the proportion each species
represents of all individuals in the community
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Figure 41.10
A
B C
D
Community 1
A: 25% B: 25% C: 25% D: 25%
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Community 2
A: 80% B: 5% C: 5% D: 10%
 Two communities can have the same species
richness but a different relative abundance
 Diversity can be compared using a diversity index
 Widely used is the Shannon diversity index (H)
H  −(pA ln pA  pB ln pB  pC ln pC  …)
where A, B, C . . . are the species, p is the relative
abundance of each species, and ln is the natural
logarithm
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 Communities with higher diversity are
 More productive and more stable in their productivity
 Able to produce biomass (the total mass of all
individuals in a population) more consistently than
single species plots
 Better able to withstand and recover from
environmental stresses
 More resistant to invasive species, organisms that
become established outside their native range
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How would biodiversity affect the topic levels in
an ecosystem?
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Trophic Structure
 Trophic structure is the feeding relationships
between organisms in a community
 It is a key factor in community dynamics
 Food chains link trophic levels from producers to
top carnivores
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Figure 41.13
Quaternary consumers:
carnivores
Tertiary consumers:
carnivores
Secondary consumers:
carnivores
Primary consumers:
herbivores and zooplankton
Primary producers:
plants and phytoplankton
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 A food web is a branching food chain with complex
trophic interactions
 Species may play a role at more than one trophic
level
Video: Shark Eating a Seal
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Figure 41.14
Humans
Smaller
toothed
whales
Baleen
whales
Crabeater
seals
Birds
Leopard
seals
Fishes
Sperm
whales
Elephant
seals
Squids
Carnivorous
plankton
Copepods
Krill
Phytoplankton
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What are some leadership roles
species can hold in a community?
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Species with a Large Impact
 Certain species have a very large impact on
community structure
 Such species are highly abundant or play a pivotal
role in community dynamics
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 Dominant species are those that are most
abundant or have the highest biomass
 One hypothesis suggests that dominant species
are most competitive in exploiting resources
 Another hypothesis is that they are most successful
at avoiding predators and disease
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Interpret this figure, for the sea star
Pisaster.
Number of species
present
Results
20
15
With Pisaster (control)
10
Without Pisaster (experimental)
5
0
1963’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73
Year
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 Keystone species exert strong control on a
community by their ecological roles, or niches
 In contrast to dominant species, they are not
necessarily abundant in a community
 Field studies of sea stars illustrate their role as a
keystone species in intertidal communities
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 Ecosystem engineers (or “foundation species”)
cause physical changes in the environment that
affect community structure
 For example, beaver dams can transform landscapes
on a very large scale
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Bottom-Up and Top-Down Controls
 The bottom-up model of community organization
proposes a unidirectional influence from lower to
higher trophic levels
 In this case, the presence or absence of mineral
nutrients determines community structure, including
the abundance of primary producers
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Describe a model that would best show the
biological control of organisms on each
trophic level.
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 The bottom-up model can be represented by the
equation
N
where
N  mineral nutrients
V  plants
H  herbivores
P  predators
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V
H
P
 The top-down model, also called the trophic
cascade model, proposes that control comes from
the trophic level above
 In this case, predators control herbivores, which in
turn control primary producers
N
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V
H
P
How are ecosystems repaired after disturbances?
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 Biomanipulation is used to improve water quality in
polluted lakes
 In a Finnish lake, blooms of cyanobacteria (primary
producers) occurred when zooplankton (primary
consumers) were eaten by large populations of roach
fish (secondary consumers)
 Removal of roach fish and addition of pike perch
(tertiary consumers) controlled roach populations,
allowing zooplankton populations to increase and
ending cyanobacterial blooms
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 Decades ago, most ecologists favored the view that
communities are in a state of equilibrium
 This view was supported by F. E. Clements, who
suggested that species in a climax community
function as an integrated unit
 Recent evidence of change has led to a
nonequilibrium model, which describes
communities as constantly changing after being
buffeted by disturbances
 A disturbance is an event that changes a
community, removes organisms from it, and alters
resource availability
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Describe some disturbances that may affect the
stability of an ecosystem.
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Characterizing Disturbance
 Fire is a significant disturbance in most terrestrial
ecosystems
 A high level of disturbance is the result of a high
intensity and high frequency of disturbance
 Low disturbance levels result from either low intensity
or low frequency of disturbance
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 The intermediate disturbance hypothesis
suggests that moderate levels of disturbance can
foster greater diversity than either high or low levels
of disturbance
 High levels of disturbance exclude many slowgrowing species
 Low levels of disturbance allow dominant species to
exclude less competitive species
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Ecological Succession
 Ecological succession is the sequence of
community and ecosystem changes after a
disturbance
 Primary succession occurs where no soil exists
when succession begins
 Secondary succession begins in an area where
soil remains after a disturbance
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Figure 41.19-1
Glacier
Bay
Alaska
0 5 10 15
Kilometers
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Figure 41.19-2
1941
1 Pioneer stage
Glacier
Bay
Alaska
0 5 10 15
Kilometers
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Figure 41.19-3
1941
1907
1 Pioneer stage
2 Dryas stage
Glacier
Bay
Alaska
0 5 10 15
Kilometers
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Figure 41.19-4
1941
1907
1 Pioneer stage
1860
Glacier
Bay
2 Dryas stage
Alaska
0 5 10 15
Kilometers
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3 Alder stage
Figure 41.19-5
1941
1907
1 Pioneer stage
1860
Glacier
Bay
2 Dryas stage
Alaska
1760
4 Spruce stage
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0 5 10 15
Kilometers
3 Alder stage
Human Disturbance
 Humans have the greatest impact on biological
communities worldwide
 Human disturbance to communities usually reduces
species diversity
 Trawling is a major human disturbance in marine
ecosystems
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