Intraspecific competition

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Transcript Intraspecific competition

IB Fall 2007 Tuesdays
@ 4 Workshop Series
Transfer Students Follow-up Meeting
Tuesday October 16, 2007
4:00 – 5:00 PM
135 Burrill Hall
OUR Ecological Footprint - 4
Species Interactions…
Ch 19 Competition
Objectives I
Define facilitation
Define competition
Types of competition
Types of resources
Liebig’s Law of the minimum
D-D regulation via intraspecific competition
Plant responses
Animal responses
Competition added to logistic growth models
Sp 1 aids Sp 2
An alternative to competition
Not altruism; no cost to facilitator
Understudied area
Competition: use or defense of a limiting
resource that reduces availability to
Types of competition:
• Exploitation vs. interference
• Intraspecific vs. interspecific
Competition may occur through exploitation
(indirect) of shared resources or (direct)
interference (individuals defend resources
chemical (interference) competition.
This plant distribution is caused by:
H1: Allelopathy
H2: Seed predation by small mammals
No resolution: need experiment considering
both simultaneously.
Types of Resources
• Plants
• abiotic
• biotic
• Animals
• abiotic
• biotic
For sessile animals, space is an important
For most plants, space is not considered a
Limiting Resource:
If resource is scarce relative to demand.
• Renewable resource:
constantly regenerated
e.g. prey, nutrients
• Non-renewable resource:
• occur in fixed amounts and can be
fully re-used
• e.g. space, hiding places
Liebig’s Law of the Minimum:
Populations are limited by the single
resource that is most scarce.
• A population increases until the supply of
the limiting resource is insufficient; then
growth stops.
• Applies to resources that do NOT interact
to determine population growth rate.
• How realistic is this ‘Law’?
***Do these results support the Law of the
Minimum? Explain.
Figure A
N and P act synergistically to promote
growth. More than 1 limiting resource…
Figure B
Law of constant yield: *** What are 2
Figure C
Intraspecific competition + densitydependent population regulation
• Negative responses:
Growth is….
Sexual maturity is….
Birth rate is …
Death rate is…
Density-dependent regulation (via
intraspecific competition) of growth.
***Summarize 2 conclusions.
Figure 1
Density-dependent regulation of time to
reach sexual maturity. ***Does age or
weight determine sexual maturity?
Figure 2
Density-dependent regulation of birth rate
Figure 3
Higher density (> competition) leads to
lower birth rate and (probably) lower
population growth.
Figure 4
Intraspecific competition contributes to
density-dependent birth and death rates;
hence to regulation of population size.
Logistic growth model
• dN/dt = r N(K - N)/K)
• As N approaches K,
intraspecific competition
• (K - N)/K) gets smaller.
• Population growth slows.
Logistic equation is based on intraspecific
competition depressing r, the per capita
population growth rate; population size
continues to increase until N = K.
Interspecific competition reduces equilibrium
level of population below carrying capacity.
At equilibrium, each competing species reduces K
for the other species.
*** How much is K of sp 1 reduced by one
individual of competing sp 2? By all
Figure 6
Lotka-Volterra addition of interspecific
competition to logistic equation:
• (dN/dt) = rN[(K-N)/K] becomes:
• (dN1/dt) = r1N1[(K1 -N1 - 1,2N2)/K1]
• 1,2 = competition coefficient
effect of each individual of sp 2 on sp 1
effect of sp 2 on sp 1’s growth rate
• Similarly, for species 2:
• (dN2/dt) = r2 N2[(K2 -N2 - 2,1N1)/K2]
***What is  (competition coefficient) for
the effect of 1 ind. of sp. 2 (N2) on sp. 1
(N1) or equivalence of sp. 2 for sp. 1?
Figure 7
Objectives II
• Niche concepts
• Interspecific competition
• ‘Ghost of competition past’: how
deduce today whether competition
in past?
• Classic experiments
Competitive exclusion hypothesis
• Field experimental studies: how
document that competition is
• Ecological role of species in
• Ranges of conditions and resource
qualities within which the species
• Often conceived as a multidimensional space (hypervolume)
From 1 to n niche axes --> hypervolume of
niche space
Niche metrics
Intraspecific competition: creates
selective pressure for broader resource use.
within pop.
Food size utilized
• Interspecific competition: may select for
narrower resource use.
within pop.
Food size utilized
Invasion by new species brings
niche overlap
may -->
reduce niche breadth
allow more species packed
into community.
Niche breadth decreases with interspecific
Without competing sp.
With competing sp..
Niches defined by two dimensions.
***Under what conditions do sp 1 + 2 1) have
overlapping niches? 2) compete?
Figure 8
‘Ghost of competition past’:
What evidence is used to deduce that
interspecific competition has occurred in
the past?
1) Niche separation via
resource partitioning
among related
1) Niche separation via resource partitioning
1) Reduced niche overlap: closely related
species use different parts of resource
gradients when sympatric.
Figure 9
2) Habitat shift: Use of habitat often changes
depending on presence or absence of closely
related species.
Expand habitat when other species absent.
Figure 10
3) Character displacement when in sympatry.
3) Character displacement: Morphological
traits and food selection of species shift
depending on presence or absence of
closely related species.
Figure 11
4) Competitive exclusion may occur when
new competitors are introduced.
Failure of species to coexist in lab cultures
led to the Competitive Exclusion Hypothesis:
2 species can’t coexist on the same limiting
Figure 12
Outcome of interspecific competition is
sometimes dependent on abiotic conditions.
29.1 °C
Figure 13
5) Competitive release: Densities of
organisms often increase when densities
of competing species are reduced.
Figure 14
The winner in competition:
• Can not always be predicted by solo
• Can change through time owing to
genetic responses to selection (rapid
• Can change depending on the presence
of other species (e.g. predators) =
apparent competition
• Observation: One predator feeds on many competing prey
species. Does it control diversity?
• Hypothesis: If predation controls competition among prey
and maintains prey species diversity.
• Prediction: When predator is removed, dominant
competitors exclude other species and reduce species
• Conclusion: The outcome of competition can be altered by
Figure 15
• ***What is the main conclusion?
Predatory newt
3 species
of toad/
frog tadpoles
Figure 16
Field Experiments: How can we
demonstrate competition is occurring?
The following slides are sample
test questions for you to work on
at home as ICA 5. See website
to print worksheet to use. It is due
on October 16.
Question 1: Buttercups
The figure on the next slide illustrates the distribution of two species of
buttercups along a transect across ridge (high land) and furrow (low
valley) grassland.
In one sentence summarize the results.
Provide two alternative hypotheses (If…then) for the observed
Draw or describe one complete experiment that would test both
What specific results from the experiment would provide support for
your hypothesis 1 above?
Figure for preceding ?
Sp 1 peaks on furrow (F)
Sp 2 peaks on ridge (R )
No. of
Distance along transect (m)
Question 2. Observation: zones of sp. B + C
do not overlap.
What are two alternative hypotheses /
Experimental design?
What explains zonation of barnacles?
• Results:
Together: Only B survives with B + C in B zone.
Only C survives with B + C in C zone.
Alone: C grows in B but B can’t grow in C zone.
• What are two conclusions?
Question 3: Galium sp. + Common garden
experiment to study
interspecific competition:
• Observation:
• 2 closely related sp. grow in
different habitats or soils.
• What are 2 altenative hypotheses
• H1:
• H2 :
• Describe the experimental
• What are 2 key results?
• What is main conclusion?
• Is competition symmetric
between the two species?
• Question 4. Desmodium sp. X 2
• What are two questions being addressed by this
• Summarize two key results.
• What is the main conclusion?
• Which species is the better
• Is competition symmetric
between the two species?
•Question 5: 2 grasses
Observation: An outcrop of serpentine rock is adjacent
to an outcrop of granite rock. Grass 1 on granite soil is
replaced abruptly by Grass 2 on serpentine soil.
• What are two hypotheses that explain their distribution?
• Question 1: Are the two grasses restricted to their
respective substrates by soil factors?
• ***Design an experiment to answer the ?
• Grass 1 grows well on granitic soil but fails to grow on serpentine
• Grass 2 grows equally well on serpentine and granitic soils.
• ***What is the main conclusion for grass 1? For grass 2?
***Design two experiments to determine
whether competition limits the distribution of
these two grasses.
• Experiment 1
• 1:1 mix on serpentine,
Grass 1 dies, Grass 2 thrives.
• 1:1 mix on granite,
Grass 2 dies, Grass 1 thrives.
• Experiment 2
• When Grass 2 is cleared from serpentine,
Grass 1 does not spread into area.
• When Grass 1 is cleared from granite,
Grass 2 spreads into area.
• Is competition symmetric or assymetric?
• What is the main conclusion about grass 1’s competitive
ability? Its tolerance of serpentine soil stress?
• What may be a general principle relating to competitive
ability vs. tolerance of stressful conditions?