Competition
Chapter 13
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Modes of Competition
• Interference:
–
• Intraspecific:
–
• Interspecific:
–
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Bart’s hay field…
http://www.gardenseeker.com/herbs/images/alfalfa_sprouted.jpg
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Intraspecific Competition Among
Herbaceous Plants
• Plants compete for…
• Competition is more intense at …
• Self-Thinning (-3/2 or -1/2 slope rule)
¿How does this explain influence spacing in
plants?
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Niches
• Niche: Summarizes environmental factors that
influence growth, survival, and reproduction
of a species.
• 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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Niches
• Hutchinson defined niche as:
– n-dimensional hyper-volume
• n equates the number of environmental factors
important to survival and reproduction of a species.
– Fundamental niche - hypervolume
– Realized niche includes interactions such as
competition that may restrict environments where
a species may live.
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Feeding Niches of Galapagos Finches
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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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Mathematical and Laboratory Models
• 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
• In general, LV predicts coexistence of two
species when, for both species, interspecific
competition is weaker than intraspecific
competition.
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Paramecia Lab Experiments
What happened
when grown
together?
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Paramecia Lab Experiments
• 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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Competition and Niches
• 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
• Tansley suggested interspecific competition
restricts realized niche of each of two species
of bedstraw (Galium spp.) to a narrower range
of soil types.
• Can you think of other variables/resources
that could be partitioned?
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Competition and Niches of Small
Rodents
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Competition and Niches of Small
Rodents
• 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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Character Displacement
• 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
• 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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