Population Ecology
• What kind of ‘competition’ are we
considering here?
• Exploitative – depression of shared
resources
– Examples? Food, nutrients
• Interference – actions reduce the
exploitation efficiencies of others
– Examples? Birds, ants, allelopathy
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• Another category of competition is
‘pre-emptive’ competition
• Irrespective of the type of
competition, the result should be a
lowering of all species in question,
however, they should be modeled
differently
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• One problem with ‘interference’
competition is modeling it..why?
• Because its very nature suggests it is
a density dependent phenomena
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• Competitive exclusion of species 1
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• Competitive exclusion of species 2
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• Coexistence in a stable equilibrium;
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• Competitive exclusion in an unstable
equilibrium
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• Competitive Exclusion
• The worst scenario for species 1 will
be if its own abundance is close to 0
(N1 ≈ 0) and the abundance of its
competitor is close to carrying
capacity (N2 ≈ K2)
Population Ecology
• If species 1 can achieve positive
growth, then it should be able to
invade (e.g. [dN1/dt][1/N1] > 0)
• Since r1 is always positive, it must be
that (K1 – αK2)/K1 > 0 or K1 /K2 > α
• For species 2 it is K2/K1 > β
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• These numbers help explain the
principle of competitive exclusion
• If 2 species are close in resource use
and body size, then α and β will be close
to 1
• 1/β > K1/K2 > α for stable coexistence
• 1/0.9 (1.1) > K1/K2 > 0.9
• 1/0.2 (5) K1/K2 > 0.2
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• Populations are
highly variable and
differences may be
the result of
different r and
starting population
size
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• It is
counterintuitive
that a small r is
more stable. Why?
Population Ecology
• Relationship
between pop(n)
variability and
risk of
extinction
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• Adding a competitor or predator
generally makes population dynamics
less stable
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• It is
counterintuitive
that a small r is
more stable. Why?
Population Ecology
• If the competitive or predatory
relationship is particularly strong, we
might see an ecological response
(change in resource use or range
shift) or an evolutionary response
(e.g. character displacement or
habitat shift)…more later
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• Parasitism and herbivory rarely
threaten a population extinction, but
both can significantly depress a
populations numbers
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• Other factors that can influence
population dynamics:
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• Empirical demonstrations of
competitive exclusion
• Most successful examples are
relatively simple cases in species-poor
systems
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• Growth of
Paramecium
separately and in
the presence of
one another
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• Given that one species is almost
always more competitive than others,
why don’t we see more extinctions?
Variable competitive landscape
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• Park studied two species of Tribolum
(flour beetles)
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• When conditions were altered to a
cooler climate with less humidity,
Tribolium confusum was the winner
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competitive coexistence
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• Similar results have been shown with
a series of fruit fly experiments
(Ayala 1970) where Drosophila
serrata would outcompeted D.
pseudoobscura at temperatures >25o
C but would win when temperatures
were <22o C
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competitive coexistence
• Competitive advantages vary with
changing environmental conditions
• Coexistence can be possible even
amongst strong competitors in a
variable (spatially or temporally)
environment
• Another option may be competitive
advantages occur at different stages
of the life cycle
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competitive coexistence
• Hutchinson (1953) pointed out…
periodicity of environmental fluctuation
ecologically similar species
generation time
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competitive coexistence
• Huston (1979) has shown that
introducing occasional episodes of
strong density-independent mortality
can almost indefinitely delay
competitive outcomes
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competitive coexistence
• Sale (1977, 1979) has examined the
influence of ‘chance’ into competitive
outcomes
• Some systems (e.g. corals) occupancy
is completely random
• Consequently, no one consistently wins
and all species are able to coexist
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competitive coexistence
• Fig 2.2
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• What about empirical demonstrations
of predator-prey extinctions
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• Huffaker’s (1958) classic mite study
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• simple predator-prey dynamics
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