Competition
Chapter 13
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Outline
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Intraspecific Competition
 Modes
Competitive Exclusion and Niches
Mathematical and Laboratory Models
 Lotka-Volterra
Competition and Niches
 Character Displacement
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Modes of Competition
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Interference:
 Direct aggressive interaction between
individuals.
Intraspecific:
 Competition with members of own
species.
Interspecific:
 Competition between individuals of two
species - reduces fitness of both.
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Intraspecific Competition Among
Herbaceous Plants
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Plant growth rates and weights have been
found to increase in low density populations.
 Competition for resources is more intense
at higher population densities.
 Usually leads to mortality among
competing plants.
– Self-Thinning
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Intraspecific Competition Among
Herbaceous Plants
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Intraspecific Competition Among Planthoppers
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Denno and Roderick attributed prevalence of
competition among Homoptera to their habit
of aggregating, rapid growth, and the mobile
nature of their food supply.
 Demonstrated intraspecific competition
within populations of planthopper
Prokelesisia marginata.
 Probably result of limited resources.
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Competitive Exclusion Principle
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Gause: Principle of Competitive Exclusion
 Two species with identical niches cannot
coexist indefinitely.
 One will be a better competitor and thus
have higher fitness and eventually
exclude the other.
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Feeding Niches of Galapagos Finches

Grant found differences in beak size
among ground finches translates directly
into diet.
 Size of seeds eaten can be estimated
by measuring beak depths.
 Individuals with deepest beaks fed on
hardest seeds.
 After 1977 drought, the remaining
seeds were very hard. Thus, mortality
was most heavy in birds with smaller
beaks.
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Feeding Niches of Galapagos Finches
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Mathematical and Laboratory Models
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Metz summarized models:
 Abstractions and simplifications, not
facsimiles of nature.
 Man-made construct; partly empirical and
partly deductive.
 Used to provide insights into natural
phenomena.
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Lotka Volterra
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Effect of interspecific competition on
population growth of each species:
dN1 / dt = rm1N1 ((K1-N1-12N2) / K1)
 dN2 / dt = rm2N2 ((K2-N2-  21N1) / K2)

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 12: Effect of individual of species 2 on
rate of pop. growth of species 1.
 21: Effect of individual of species 1 on
rate of pop. growth of species 2.
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Lotka Volterra
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In general, LV predicts coexistence of two
species when, for both species, interspecific
competition is weaker than intraspecific
competition.
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Lotka Volterra
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Predict population growth for the two species
will stop when:
 N1=K 1-  12 N2
and N2 = K 2-  21 N1
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Zero Growth Isoclines
 Above: Population decreasing
 Below: Population increasing
Coexistence of two species is only
possible when isoclines cross.
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Lotka Volterra
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Paramecia Lab Experiments
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Gause demonstrated resource limitation with
Paramecium caudatum and Paramecium
aurelia in presence of two different
concentrations of Bacillus pyocyaneus.
 When grown alone, carrying capacity
determined by intraspecific competition.
 When grown together, P. caudatum
quickly declined.
 Reduced resource supplies increased
competition.
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Paramecia Lab Experiments
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Flour Beetle Experiments
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Tribolium beetles infest stored grain
products.
 Park studied interspecific competition
between T. confusum and T. castaneum
under varied environmental conditions.
 Growing the two species together
suggested interspecific competition
restricts the realized niches of both
species to fewer environmental
conditions.
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Competition and Niches
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Competition can restrict species to their
realized niches.
 But if competitive interactions are strong
and pervasive enough, they may produce
an evolutionary response in the competitor
population.
 Changes fundamental niche.
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Niches and Competition Among Plants
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Tansley suggested interspecific competition
restricts realized niche of each of two
species of bedstraw (Galium spp.) to a
narrower range of soil types.
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Niche Overlap and
Competition Between Barnacles
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Connell discovered interspecific competition
in barnacles. Balanus plays a role in
determining lower limit of Chthamalus within
intertidal zone.
 Did not account for all observed patterns.
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Niche Overlap and
Competition Between Barnacles
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Competition and Niches of Small Rodents
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Brown studied competition among rodents in
Chihuahuan Desert.
 Predicted if competition among rodents is
mainly for food, then small granivorous
rodent populations would increase in
response to removal of larger granivorous
rodents.
 Insectivorous rodents would show little
or no response.
 Results supported hypothesis.
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Competition and Niches of Small Rodents
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Character Displacement
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Because degree of competition is assumed
to depend upon degree of niche overlap,
interspecific competition has been predicted
to lead to directional selection for reduced
niche overlap.
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Character Displacement
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Character Displacement
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Taper and Case: Necessary criteria:
 Morphological differences between
sympatric species are statistically greater
than differences between allopatric
populations.
 Differences between sympatric and
allopatric populations have genetic basis.
 Differences between sympatric and
allopatric populations evolved in place,
and are not derived from different founder
groups already differing in the character.
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Taper and Case: Characteristics
Variation in the character must have a
known effect on use of resources.
 Must be demonstrated competition for the
resource and competition must be directly
correlated with character similarity.
 Differences in character cannot be
explained by differences in resources
available to each of the populations.

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Review
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Intraspecific Competition
 Modes
Competitive Exclusion and Niches
Mathematical and Laboratory Models
 Lotka-Volterra
Competition and Niches
 Character Displacement
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