Competition: Important concepts
Competitive Exclusion: one species may drive
another to extinction
Interspecific: among different species
Intraspecific: within the same species
• Intraspecific competition is most intense
Exploitative competition
Interference competition
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Territoriality
Allelopathy
Resource Partitioning: eliminates direct
competition
Competition
Competition: An interaction between
individuals in which each is harmed by
their shared use of a limiting resource.
Competition for Resources
Resources: Features of the environment
required for growth, survival, or
reproduction, and that can be consumed
to the point of depletion.
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.
Space Can Be a Limiting Resource
Interspecific Competition
Competitive Exclusion
Competitive exclusion principle:
Two species that use a limiting resource
in the same way cannot coexist
indefinitely.
When paired, some species drove others
to extinction.
Gause’s
Experiment
Species grown together
Paramecium caudatum
Paramecium aurelia
Forms of Competition
Exploitative competition
• Species have equal access to
resource; one is simply better at
exploiting it
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.
Competing Organisms Can Deplete Resources
Competing Organisms Can Deplete Resources
Competing Organisms Can Deplete Resources
Competition for Resources
Competition can intensify when
resources are scarce.
Forms of Competition
Interference competition
• One species prevents another
from using resource
 usually through aggressive
behavior
Intraspecific Competition
Intraspecific Competition
Territoriality
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.
Intraspecific Competition Allelopathy (sibling harming)
Eucalyptus oils harm
young saplings of
same species.
Can also affect other
species
Interference Competition in Plants: Kudzu grows fast and can smother other plants
General Features of Competition
Competition can influence species
distributions.
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.
Squeezed Out by Competition
Interference Competition
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.
A Natural Experiment on Competition between Chipmunk Species
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.
Competitive Exclusion
Resource partitioning: Species using a
limited resource in different ways.
Resource Partitioning
Apparent competitors
may have slightly
different niches
May use resources in a
different way or time
Minimizes competition
and allows coexistence
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.
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.
Cyanobacteria Partition Their Use of Light
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.
Connections in Nature: The Paradox of Diversity
Noncarnivorous plants are superior
competitors for light and soil nutrients.
But pitcher plants are tolerant of fire and
grow mostly when competitors are
absent or reduced (e.g., after a fire).
This strategy may allow them to persist
with their competitors.
Figure 12.21 Coexistence in a Nutrient-Poor Environment
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.
Population Decline in an Inferior Competitor Sea Palm
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.
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.
Character Displacement
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.
Competition Shapes Beak Size