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Intraspecific Effects I.
So far, we have assumed that:•Individuals have no effects on each other•Resources are unlimited
Sometimes these assumptions are justified, but often they are not.
Intraspecific competition:
When individuals of the same species interfere with each other either directlyor through preemptive use of resources
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Characteristics of Intraspecific Competition
1. The ultimate effect is a decrease in eitherreproduction or survival (decrease in thecontribution to the next generation).
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Characteristics of Intraspecific Competition
2. The resource for which individuals arecompeting must be in short supply.
3. Competing individuals are inherentlyequivalent (e.g., circumstances, more than individual differences, determineswho “wins”).
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Characteristics of Intraspecific Competition
4. The effects of intra-specific competitionare density dependent.
Density dependence:The greater the number of individuals,the greater the probability any givenindividual will be adversely affected.
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Density dependence
Not all DD effects are a result of intra-specificcompetition.
Density dependence tends to regulate population size.
They can arise from other processes such asinter-specific competition.
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Scramble and Contest Competition
•These are the theoretical extremes originallyproposed by the ecologist A.J. Nicholsonin 1954.
•Scramble: All individuals get an equalshare of resources
•Contest: A few “winners” get all theresources, the remaining individualsget nothing.
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Scramble competition
(From Begon et al. 1996)
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Size-Symmetric Competition in Plants
•This is the equivalent of scramble competitionin animals
•Also known as two-sided competition
•Resources are utilized in proportion to anindividual plant’s size
Example: soil nutrients are usually taken upin a size-symmetric manner
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Contest Competition
(From Begon et al. 1996)
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Size-Asymmetric Competition in Plants
•Equivalent to contest competition in animals
•Also known as one-sided competition
•Resources are disproportionately used bylarger individuals
Example: light is often taken up in a size-asymmetric manner: the bigger plantgets most or all, smaller plant dies
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Are these extremes reasonable?
Probably not.
However, illustrate the range of possibleresponses by real individuals.
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Intraspecific Competition in Plants
•This is a form of intraspecific competition
•Survival is not decreased, but plant size,and reproductive output, are affected.
Law of constant final yield
•This relationship compares plots of differentdensities at a single instant of time; it isnot a dynamic process.
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Self-thinning in plants
As plants grow in size, density-dependentmortality may start to occur.
This is a dynamic process known as self-thinning
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The -4/3 Thinning Law
(From Silvertown and Charlesworth 2001)
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The -4/3 Thinning Law
(Silvertown and Charlesworth 2001)
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Size Hierarchies Under Crowded Conditions
In both plants and animals, dense populationstend to lead to many small individualsbut only a few large ones.
This is a result of intra-specific competitionand can occur even if all organismsstart out the same size initially.
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Size Distributions: Examples
(Silvertown and Charlesworth 2001)
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Size Distributions: Examples
(Vandermeer and Goldberg 2003)
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ReviewIntraspecific competition is characterized
by four conditions:
1. Decrease in demographic ratesWith increasing density
2. A critical resource must be inLimiting supply
3. Individuals are inherently equivalent4. The effects of intraspecific competition
are density dependent.
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Review
Density dependence tends to regulatepopulation size.
Two forms of intraspecific competitionare scramble competition and contest competition in plants
These are equivalent to size symmetric andsize asymmetric in plants
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Review
In plants, the Law of Constant Final Yielddescribes a pattern of decreasing sizeand reproduction, but no mortality,with increasing density
At a certain threshold, mortality will increase
This process is known as Self-Thinning
The slope of the relationship is generallydescribed by -3/2 or -4/3
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Review
Self-thinning usually is called the -3/2 thinninglaw or the -4/3 thinning law
Intraspecific competition in both plants and animals leads to size hierarchies