Chapter 13
• The niche of an organism reflects its environmental requirements
• Competition for resources effects population densities
• Competition for resources alters the realized niche of an organism
Effects
Effect on
Type
Species 1
Species 2
Competition
Negative
Negative
Exploitation
Positive
Negative
Mutualism
Positive
Positive
Competition
• Competition occurs when two organisms have negative impacts on each other
• Prerequisites are:
– Similar resource use
– Limited resource availability
• Same rules apply for interspecific competition
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Evidence For Competition
How do we know when competition is occurring?
By definition, presence of one species should result in:
– decreased growth of both species, or
– decreased population density of both species
Evidence for competition is
– increased abundance of one species in the absence of the other
Intraspecific Competition
Interactions among individuals show the effect of resource limitation
– Self-thinning in plants
• Evidenced in density-biomass graphs
– Leafhopper interactions
• High rate of increase
• Clumped distributions
Plant size and Plant Density (Fig. 13.3)
Self-thinning- Relation between numbers and biomass (Fig. 13.4, 13.5)
• Model predicts that as biomass increases, numbers should decrease
• Experimental results with alfalfa support model
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Life history traits, density, and planthoppers (Fig. 13.6)
At higher densities, survival declines
At higher densities, longer time to maturity
At higher densities, reduced body size
Niches and Competition
Fundamental and realized niches
– Presence of competitors may reduce space of realized niche
– If resource use is identical, competitive exclusion may occur
• Gause – two species with identical niches will not coexist indefinitely
Potential outcomes
– Loss of one competitor
– Alteration in resource utilization patterns (character displacement)
Annual changes in G. fortis, evidence for changes in a population resulting from
intraspecific competition (Fig. 13.9, 13.10, 13.11)
• Beak size and seed hardness
Seed size and beak size in finches:
Evidence for character displacement (Fig. 13.8)
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Mathematical Models of Competition
Recall the logistic growth model
– Key characteristic is carrying capacity
– Presence of competitor reduces carrying capacity
– Introduction of term that reflects that effect allows mathematical expression
of competitive effect
Mathematical Models of Competition
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Initial equation:
– dN1 / dt = r1N1 ((K1-N1) / K1)
Effect of interspecific competition on population growth of each species (LotkaVolterra):
– dN1 / dt = r1N1 ((K1-N1-12N2) / K1)
– dN2 / dt = r2N2 ((K2-N2-  21N1) / K2)
• 12: Effect of individual of species 2 on rate of population growth of
species 1.
•  21: Effect of individual of species 1 on rate of population growth of
species 2.
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Mathematical Models of Competition
When does population stop growing?
N1 = (K1 – N1 – α12N2) = 0
N2 = (K2 – N2 – α21N2) = 0
Rearranging gives:
N1 = K1 – α12N2, and
N2 = K2 – α21N1
Formula for a straight line (Isoclines), indicates at what level population growth
stops
Model Predictions
• If K1 / α12 is greater than K2, then the effect of interspecific competition on K2 is
greater than the effect of intraspecific competition
• If K2 / α21 is less than K1, then the effect of intraspecific competition on K1 is
greater than the effect of interspecific competition
• In general, LV predicts coexistence of two species when, for both species,
interspecific competition is weaker than intraspecific competition.
Competition coefficients and species isoclines (Fig. 13.14)