Presentation on theme: "Intraspecific Effects I."— Presentation transcript:
1 Intraspecific Effects I. So far, we have assumed that:Individuals have no effects on each otherResources are unlimitedSometimes these assumptions are justified,but often they are not.Intraspecific competition:When individuals of the same speciesinterfere with each other either directlyor through preemptive use of resources
2 Characteristics of Intraspecific Competition The ultimate effect is a decrease in eitherreproduction or survival (decrease in thecontribution to the next generation).
3 Characteristics of Intraspecific Competition The resource for which individuals arecompeting must be in short supply.Competing individuals are inherentlyequivalent (e.g., circumstances, morethan individual differences, determineswho “wins”).
4 Characteristics of Intraspecific Competition 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.
5 Density dependence Not all DD effects are a result of intra-specific competition.They can arise from other processes such asinter-specific competition.Density dependence tends to regulatepopulation size.
6 Scramble and Contest Competition These are the theoretical extremes originallyproposed by the ecologist A.J. Nicholsonin 1954.Scramble: All individuals get an equalshare of resourcesContest: A few “winners” get all theresources, the remaining individualsget nothing.
8 Size-Symmetric Competition in Plants This is the equivalent of scramble competitionin animalsAlso known as two-sided competitionResources are utilized in proportion to anindividual plant’s sizeExample: soil nutrients are usually taken upin a size-symmetric manner
10 Size-Asymmetric Competition in Plants Equivalent to contest competition in animalsAlso known as one-sided competitionResources are disproportionately used bylarger individualsExample: light is often taken up in a size-asymmetric manner: the bigger plantgets most or all, smaller plant dies
11 Are these extremes reasonable? Probably not.However, illustrate the range of possibleresponses by real individuals.
12 Intraspecific Competition in Plants Law of constant final yieldThis is a form of intraspecific competitionSurvival is not decreased, but plant size,and reproductive output, are affected.This relationship compares plots of differentdensities at a single instant of time; it isnot a dynamic process.
13 Self-thinning in plants As plants grow in size, density-dependentmortality may start to occur.This is a dynamic process known asself-thinning
14 The -4/3 Thinning Law(From Silvertown and Charlesworth 2001)
15 The -4/3 Thinning Law(Silvertown and Charlesworth 2001)
16 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.
17 Size Distributions: Examples (Silvertown and Charlesworth 2001)
18 Size Distributions: Examples (Vandermeer and Goldberg 2003)
19 Review Intraspecific competition is characterized by four conditions: Decrease in demographic ratesWith increasing densityA critical resource must be inLimiting supply3. Individuals are inherently equivalent4. The effects of intraspecific competitionare density dependent.
20 Review Density dependence tends to regulate population size. Two forms of intraspecific competitionare scramble competition andcontest competition in plantsThese are equivalent to size symmetric andsize asymmetric in plants
21 Review In plants, the Law of Constant Final Yield describes a pattern of decreasing sizeand reproduction, but no mortality,with increasing densityAt a certain threshold, mortality will increaseThis process is known as Self-ThinningThe slope of the relationship is generallydescribed by -3/2 or -4/3
22 Review Self-thinning usually is called the -3/2 thinning law or the -4/3 thinning lawIntraspecific competition in both plants andanimals leads to size hierarchies