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12
Competition
Chapter 12 Competition
CONCEPT 12.1 Competition occurs
between individuals of two species that
share the use of a resource that limits
their growth, survival, or reproduction.
CONCEPT 12.2 Competition, whether
direct or indirect, can limit the distributions
and abundances of competing species.
Chapter 12 Competition
CONCEPT 12.3 Competing species are
more likely to coexist when they use
resources in different ways.
CONCEPT 12.4 The outcome of
competition can be altered by
environmental conditions, species,
interactions, disturbance, and evolution.
Figure 12.2 Competition Decreases Growth in a Carnivorous Plant
Introduction
Competition: An interaction between
individuals in which each is harmed by
their shared use of a limiting resource.
Interspecific competition: Between
members of different species
Intraspecific competition: Between
individuals of a single species
CONCEPT 12.1
Competition occurs between individuals of
two species that share the use of a
resource that limits their growth, survival,
or reproduction.
Concept 12.1
Competition for Resources
Resources: Features of the environment
required for growth, survival, or
reproduction, and that can be consumed
to the point of depletion.
Concept 12.1
Competition for Resources
Examples of resources:
• Food
• Water in terrestrial habitats
• Light for plants
• Space, especially for sessile
organisms
• For mobile animals, space for refuge,
nesting, etc.
Figure 12.3 Space Can Be a Limiting Resource
Concept 12.1
Competition for Resources
Species are also influenced by features
of the environment that are not
consumed, such as temperature, pH,
salinity.
These factors are not consumed and are
not considered to be resources.
Concept 12.1
Competition for Resources
Competition reduces availability of
resources.
Experiments with 2 diatom species by
Tilman et al. (1981) showed that when
each species was grown alone, a
stable population size was reached.
When grown together, they competed for
silica, and one species drove the other
to extinction.
Figure 12.4 Competing Organisms Can Deplete Resources (Part 1)
Figure 12.4 Competing Organisms Can Deplete Resources (Part 2)
Figure 12.4 Competing Organisms Can Deplete Resources (Part 3)
Concept 12.1
Competition for Resources
Competition can intensify when
resources are scarce.
Competition among plants should
increase in nutrient-poor soils.
Wilson and Tilman (1993) studied grass
plants that were transplanted into
fertilized and unfertilized plots.
Concept 12.1
Competition for Resources
Each plot type had 3 treatments:
1. Neighbors left intact (belowground and
aboveground competition).
2. Neighbor roots left intact but neighbor
shoots tied back to reduce shading
(belowground competition).
3. Neighbor roots and shoots both removed
(no competition).
Concept 12.1
Competition for Resources
Belowground competition (treatment 2)
was most intense in nitrogen-limited
plots.
Aboveground competition (treatment 1
minus treatment 2) for light increased
when light levels were low.
Figure 12.5 Resource Availability Affects the Intensity of Competition
CONCEPT 12.2
Competition, whether direct or indirect, can
limit the distributions and abundances of
competing species.
Concept 12.2
General Features of Competition
Exploitation competition: Species
compete indirectly; individuals reduce
the availability of a resource as they use
it.
Examples: The pitcher plants (Brewer),
and the 2 diatom species (Tilman et al.).
Concept 12.2
General Features of Competition
Interference competition: Species
compete directly for access to a
resource.
Individuals may perform antagonistic
actions (e.g., when two predators fight
over a prey item, or voles aggressively
exclude other voles from preferred
habitat).
Concept 12.2
General Features of Competition
Interference competition in sessile
species:
The acorn barnacle crushes or smothers
nearby individuals of another barnacle
species as it grows and directly
excludes the other species from portions
of the rocky intertidal zone.
Concept 12.2
General Features of Competition
Interference competition in plants:
Individuals of one species grow on or
shade other species, reducing their
access to light, for example kudzu.
Allelopathy: Plants of one species
release toxins that harm other species.
Figure 12.6 Interference Competition in Plants
Concept 12.2
General Features of Competition
For a resource in short supply,
competition will reduce the amount
available to each species.
The effects of competition are often
unequal, or asymmetrical, and one
species is harmed more than the other.
Example: When one species drives
another to extinction.
Concept 12.2
General Features of Competition
There is a continuum in how strongly
each competitor affects the other.
The ends of this continuum represent
amensalism:
–/0 interactions; individuals of one
species are harmed while individuals of
the other species are not affected at all.
Figure 12.7 A Continuum of Competitive Effects
Concept 12.2
General Features of Competition
Competition can influence species
distributions.
Connell (1961) examined factors that
influence the distribution, survival, and
reproduction of two barnacle species on
the coast of Scotland.
Concept 12.2
General Features of Competition
Distribution of the drifting larvae of the 2
species overlapped.
Adult distributions did not overlap:
• Chthamalus were found only near the
top of the intertidal zone.
• Semibalanus were found throughout
the rest of the intertidal zone.
Figure 12.9 Squeezed Out by Competition
Concept 12.2
General Features of Competition
Using removal experiments, Connell
found that Semibalanus excluded
Chthamalus from all but the top of the
zone.
Semibalanus smothered, removed, or
crushed the other species.
However, Semibalanus dried out and
survived poorly at the top of the intertidal
zone.
Concept 12.2
General Features of Competition
A “natural experiment” is a situation in
nature that is similar in effect to a
controlled removal experiment.
Patterson (1980, 1981) studied chipmunk
species in mountain forests and found:
When a species lived alone on a
mountain, it occupied a wider range of
habitats than when it lived with a
competitor species.
Figure 12.10 A Natural Experiment on Competition between Chipmunk Species
CONCEPT 12.3
Competing species are more likely to
coexist when they use resources in
different ways.
Concept 12.3
Competitive Exclusion
If the ecological requirements of
competing species—the ecological
niches—are very similar, the superior
competitor may drive the other species
to extinction.
Concept 12.3
Competitive Exclusion
In the 1930s, Gause did experiments on
competition using 3 species of
Paramecium.
Populations of all 3 species reached a
stable carrying capacity when grown
alone.
When paired, some species drove others
to extinction.
Figure 12.11 Competition in Paramecium (Part 1)
Figure 12.11 Competition in Paramecium (Part 2)
Concept 12.3
Competitive Exclusion
P. aurelia drove P. caudatum to
extinction; both feed on yeast cells
floating at the top of the growth medium.
P. caudatum and P. bursaria were able to
coexist, but the carrying capacity of both
species was lowered.
P. caudatum ate floating bacteria, while
P. bursaria ate yeast cells that settled to
the bottom.
Concept 12.3
Competitive Exclusion
Experiments on these and many other
species led to the competitive
exclusion principle:
Two species that use a limiting resource
in the same way cannot coexist
indefinitely.
Concept 12.3
Competitive Exclusion
Resource partitioning: Species using a
limited resource in different ways.
Stomp et al. (2004) studied two
cyanobacteria species in the Baltic Sea.
BS1 absorbs green wavelengths most
efficiently; BS2 absorbs red most
efficiently.
Concept 12.3
Competitive Exclusion
Each species could survive when grown
alone in either wavelength.
When grown together, one drove the
other to extinction, depending on the
wavelength used.
Under white light (all wavelengths) they
both persisted.
Figure 12.12 Do Cyanobacteria Partition Their Use of Light?
Concept 12.3
Competitive Exclusion
The Lotka–Volterra competition model:
  N1   N 2  
dN1

 r1 N1 1 
dt
K1


  N 2   N1  
dN 2

 r2 N 2 1 
dt
K2


Concept 12.3
Competitive Exclusion
N1 = population density of species 1
r1 = intrinsic rate of increase of species 1
K1 = carrying capacity of species 1
α and β = competition coefficients—
constants that describe the effect of one
species on the other
Concept 12.3
Competitive Exclusion
Density of species 2 (N2) does not
change in size when:
N2  K2  N1
These two equations describe straight
lines; N2 is a function of N1 (zero
population growth isoclines).
Concept 12.3
Competitive Exclusion
Zero population growth isoclines: The
population does not increase or
decrease in size for any combination of
N1 and N2 that lies on these lines.
Zero growth isoclines can determine the
conditions under which each species will
increase or decrease.
Figure 12.13 Graphical Analyses of Competition
Concept 12.3
Competitive Exclusion
This graphical approach can be used to
predict the end result of competition
between species.
The N1 and N2 isoclines are plotted
together. There are four possible ways
the isoclines can be arranged relative to
each other.
Concept 12.3
Competitive Exclusion
When the isoclines do not cross (A and
B), competitive exclusion results.
Depending on which isocline is above the
other, either species 1 or species 2
always drives the other to extinction.
Figure 12.14 Outcome of Competition in the Lotka–Volterra Competition Model (Part 1)
Concept 12.3
Competitive Exclusion
In (C), competitive exclusion occurs, but
which species wins depends on how
densities change over time.
In only one case (D), the two species
coexist, although competition still has an
effect—the equilibrium density of each
species is lower than its carrying
capacity.
Figure 12.14 Outcome of Competition in the Lotka–Volterra Competition Model (Part 2)
CONCEPT 12.4
The outcome of competition can be altered
by environmental conditions, species
interactions, disturbance, and evolution.
Figure 12.15 Herbivores Can Alter the Outcome of Competition
Concept 12.4
Altering the Outcome of Competition
If herbivores prefer to feed on the
superior competitor, it reduces the
growth, survival, or reproduction of that
species.
The same is true of predators, pathogens,
and mutualists: change in abundance of
such species can change the outcome
of competition among the species with
which they interact.
Concept 12.4
Altering the Outcome of Competition
Disturbances such as fires or storms can
kill or damage some individuals, while
creating opportunities for others.
Some species can persist in an area only
if such disturbances occur regularly.
Concept 12.4
Altering the Outcome of Competition
Forest plants that need sunlight are found
only where disturbance has opened the
tree canopy.
As trees recolonize and create shade,
these plants cannot persist in the patch.
Such species are called fugitive species
because they must disperse from one
place to another as conditions change.
Concept 12.4
Altering the Outcome of Competition
The brown alga called sea palm coexists
with mussels, a superior competitor, in
the rocky intertidal zone.
Large waves sometimes remove the
mussels, creating temporary openings
for the alga.
In low disturbance areas, competition with
mussels causes sea palm populations to
decline over time.
Figure 12.16 Population Decline in an Inferior Competitor
Concept 12.4
Altering the Outcome of Competition
Competition can cause evolutionary
change, and evolution can alter the
outcome of competition.
This interplay has been observed in many
studies.
Concept 12.4
Altering the Outcome of Competition
Natural selection can influence the
morphology of competing species and
result in character displacement.
The phenotypes of competing species
become more different over time.
Figure 12.18 Character Displacement
Concept 12.4
Altering the Outcome of Competition
In two species of Galápagos finches,
beak sizes, and hence sizes of the
seeds eaten, are different on islands
that have both species.
On islands with only one of the species,
beak sizes are similar.
Figure 12.19 Competition Shapes Beak Size