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Lecture 11: Competition

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Introduction to Communities and Ecosystems

What is a community?
What is an ecosystem?
What is the history of these ideas?
What questions do community ecologists ask?

History of the Community and Ecosystem Concepts

- a schism in ecology

What is a community?

  • assemblage of species that co-occur
  • coexisting species can interact
    • consumer/resource
    • interspecific competition
  • ecologists study subsets of all species that co-occur
    • guild
    • life-form
    • taxon
    • place/habitat

Spectum of ideas about communities


Distinct boundaries
Coincident ranges
Coevolution common
Diffuse boundaries
Independant ranges
no coevolution
Led to ecosystem concept
(Tansley 1935)
More commonly
accepted by
community ecologists


Common questions asked by community ecology:

“Why are there so many kinds of animals?”

  • How do species interact with each other?
  • What are the patterns of species numbers and relative abundances? (diversity)
  • What controls number and relative abundance of species?
  • How are species in a community linked via trophic interactions?

Modes of Interaction Between Species

Species A
Species B






  • Resources
  • Competition
    • Types
      • Intraspecific and Interspecific
      • Interference and Exploitation
  • The Niche Concept
  • Finding Competition in Nature
  • Competition and Niches
    • Character displacement
  • Mathematical Models of Competition
    • Intraspecific competition
    • Interspecific competition 


  • Limited Resources (Leibig)
    • Limited by single resource that is most scarce
  • Shared Resources (Niche)           --------- >           COMPETITION

Types of Competition

  • Interference:
    • Direct aggressive interaction between individuals
  • Exploitation:
    • Consumption and depletion of resources

Exploitation (indirect)

Interference (direct)



  • Intraspecific:
    • Competition with members of own species (conspecifics)
  • Interspecific:
    • Competition between individuals of two species (heterospecifics)


  • Results in decrease in:
    • Survivorship
    • Growth
    • Reproduction
  • Ultimately affects:
    • Distribution
    • Evolution

Types of Competition - Similar requirements = similar niches

Ecological Niche

  • Origin, early part of this century
    • J. Grinnell (1917, 1924)
    • Ornithologist
    • An organism’s “home” or where it lives
    • Potential distribution in the absence of any interspecific interactions (competition, predation)
  • Charles Elton (1920’s)
  • Likened species to members of a human community
  • Across communities there are similar roles
    • Doctor, Lawyer, etc.
  • Each species has a role
  • Some roles exist across multiple communities
    • Dominant producer
    • Top vertebrate carnivore

Ecological Niche

  • G. F. Gause (1934)
    • 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.

Competitive Exclusion Principle

If competitive exclusion principle is true, then how do species coexist in nature?

  • G. E. Hutchinson (1957)
    • relate fitness or reproductive success (performance) to quantitative variables related to resources (space, etc.)
  • 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 the potential environments where a species may live

Competition for Space

Competition affects fitness -

Barnacles show physical separation of niches

Other organisms use slightly different resources

e.g. Galapagos finches

Character displacement -

  • allopatric populations
  • sympatric populations

Criteria for demonstrating character displacement

  1. sympatric vs. allopatric character differences
  2. differences must be heritable
  3. differences must NOT be due to founder effect
  4. character must have known effect on resource use
  5. competition must be demonstrated and must be correlated with character similarity
  6. differences in character must not be due to differences in resource availability

Interspecific Competition

  • Tom Park (1954)
    • Tribolium beetles
  • Gause’s
    • competitive
    • exclusion principle
  • No two species that occupy the same niche can coexist



Coexist? How?

Lotka-Volterra Model

  • Intraspecific Competition (Logistic Model)
    • dN/dt = rN (K-N)/K
  • Interspecific Competition (Lotka-Volterra model)
    • dN1/dt = r1N1 (K1-N1-α12N2 )/K1

intraspecific competition

intraspecific competition

α = competition coefficient

dN1/dt = r1N1 (K1-N1-α12N2 )/K1

if α12>1?

if α12<1?

Under what conditions will species coexist?

Community Structure Part I

What is community structure?
How is species abundance defined?
How is species diversity defined?

Preston’s distribution of commonness and rarity - most species are of intermediate abundance

Effect of sample size on lognormal distribution

Causal Mechanisms for lognormal distribution

  • Random (statistical expectation)
  • Niche partitioning (competition)

Utility of lognormal distribution -

Species Diversity -

  • Species Richness -
  • Species Evenness -

Quantification of Species Diversity

Shannon-Weiner Index

H’ = Σ pi loge pi


  • Can’t tell if differences due to richness or evenness
  • Can’t compare unequal sample sizes use rarefaction

Rank abundance curves -

plot relative abundance vs. abundance rank (most to least)

What controls Species Diversity?

Depends on Scale

  • Spatial: Local versus Regional
  • Temporal: Individual, Populations, Interactions, Evolutionary


  • Chance extinction
  • Competition
  • Predation (consumer/resource)
  • Habitat Selection (niche)


  • Dispersal
  • Evolution
  • Mass extinction

Local Scale Controls

1. Environmental Complexity complexity diversity

complexity             diversity


So, must know something about species requirements to predict environmental influence

Example: Robert McArthur’s forest birds

What kind of local control is involved in this example?


Example: the paradox of the plankton

Must think about their physiology and Leibig’s Law

What kind of local control is involved in this example?

Example: Terrestrial plant diversity

What kind of local control is involved in this example?

So, if organisms are competing for a limited resource, what happens if competition is relieved by adding a resource?

Example: Terrestrial Plants

2. Temporal complexity (variability)

Intermediate Disturbance Hypothesis

Example – Rocky Intertidal

Example- Prairie Dogs

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . factpetersen. (2008, March 03). Lecture 11: Competition. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License