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Competition

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the barnacle Balanus is less tolerant of desiccation but can displace Chthamalus ... Barnacle competition. Competition Among Terrestrial Animals ... – PowerPoint PPT presentation

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Title: Competition


1
Competition
  • Chapter 12

2
Classifications of Species Interactions
  • Based on either mechanism or effects
  • Mechanism
  • Competition Predation Herbivory Parasitism
    Disease Mutualism
  • Effects
  • ,-, or 0 effect (Odum) too simple, no time
    frame, fails to include indirect effects
  • Mechanism approach used in Krebs text

3
What is competition?
  • Competition is any use or defense of a resource
    by one individual that reduces the availability
    of that resource to other individuals.
  • Competition among individuals may be
  • intraspecific (within species)
  • interspecific (between species)

4
Competition may occur through exploitation or
interference.
  • Exploitation competition occurs when individuals
    compete indirectly through their mutual effects
    on shared resources
  • a common mechanism, typical of examples weve
    considered thus far
  • Interference competition occurs when individuals
    defend resources through antagonistic behaviors
  • less common, requires that resources can be
    profitably defended

5
Exploitation Competition vs. Interference
Competition
6
Consumers compete for resources.
  • A resource is any substance or factor that is
    consumed by an organism and that supports
    increased population growth as its availability
    in the environment increases
  • a resource is consumed
  • a consumer uses a resource for its own
    maintenance and growth
  • when resource availability is reduced, the result
    is reduced population growth

7
What is a resource?
  • Resources include foods that are eaten, but also
    include
  • open space for sessile organisms
  • hiding places and other safe sites
  • Conditions are not resources
  • for example, temperature is not a resource
  • it may affect growth and reproduction, but is not
    consumed by organisms

8
Limiting Resources
  • The potential of a resource to limit a population
    depends on availability relative to demand
  • this concept is embodied in Liebigs law of the
    minimum
  • each population increases until the supply of
    some limiting resource becomes depleted
  • this law applies strictly to resources that do
    not interact to determine population growth rate

9
The Theory of Competition and Coexistence
  • Mathematical models of competition were developed
    by A.J. Lotka and G.F. Gause, based on the
    logistic equation
  • 1/NdN/dt rK-N/K
  • where r exponential rate of increase
  • N population size
  • K carrying capacity
  • t time

10
Per Capita Growth Rate as a function of K
(logistic equation)
11
Volterras model
  • The Italian ecologist Vito Volterra incorporated
    interspecific competition into the logistic
    equation as follows
  • 1/N1dN1/dt r1(K1-N1-a1,2N2)/K1
  • where subscripts 1 and 2 indicate the species
  • a1,2 is the competition coefficient -
  • effect of species 2 on 1

12
The Competition Coefficient
  • The competition coefficient in Volterras model
    expresses the degree to which each individual of
    species 2 uses the resources of individuals of
    species 1
  • the extent to which this happens determines
  • effect of species 2 on species 1s rate of growth
  • effect of species 2 on the equilibrium population
    size of species 1 under interspecific competition

13
More on Competition Models
  • Two models are needed to describe the joint
    effects of species upon one another.
  • For two species to coexist, each population must
    reach a stable size greater than zero
  • dN1/N1dt 0 when
  • N1 K1 - a1,2N2
  • dN2/N2dt 0 when
  • N2 K2 - a2,1N1

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17
Coexistence
  • At equilibrium, each competing species reduces
    the carrying capacity of the environment for the
    other species.
  • Coexistence is most likely when
  • the coefficients of interspecific competition are
    relatively weak (less than 1)
  • this occurs when competitors share resources
    incompletely by partitioning resources among
    themselves

18
Effects of Interspecific competition
19
The Competitive Exclusion Principle - Background
  • Russian ecologist G.F. Gause conducted early
    experimental studies of competition
  • he grew Paramecium aurelia and P. caudatum alone
    and in mixture in nutritive media
  • each species grew well alone, but in mixture only
    P. aurelia persisted
  • similar experiments conducted on a wide variety
    of species have tended to show the same thing -
    one species persists and the other dies out,
    usually within 30-70 generations

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The Competitive Exclusion Principle
  • Results from many studies were summarized by
    Gause and others in the competitive exclusion
    principle
  • two species cannot coexist indefinitely on the
    same limiting resource
  • although similar species exist, careful study
    usually reveals that they differ in their habitat
    or diet requirements

27
Niches
  • Fundamental Niche
  • Set of resources a species can use in the absence
    of competitors
  • Realized Niche
  • Actual usage of resources due to competition and
    other biotic factors

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30
Theory vs. Nature
  • Paradox of high species diversity in nature vs.
    expectations of low diversity from theory
  • Either competition is rare in nature
  • Competition occurred in the past, current species
    show adaptations that limit competition

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33
Field studies demonstrate the pervasiveness of
competition.
  • Sequential introductions of biological control
    agents provide evidence for competitive exclusion
    among species utilizing a limiting resource
  • various species of parasitoid wasps in the genus
    Aphytis have been introduced to control citrus
    scale in California citrus groves
  • the earliest introduction, A. chrysomphali, was
    readily displaced by A. lingnanensis
  • A. melinus subsequently displaced A. lingnanensis
    in all but the milder coastal areas

34
Competition among introduced scale insects
35
Tansleys Classic Study
  • British ecologist A.G. Tansley was the first to
    experimentally demonstrate competition between
    closely related species
  • he selected two species of the plant species
    Galium
  • G. saxatile is normally found on acid, peaty
    soils
  • G. sylvestre is normally found on limestone hills
    and pastures
  • these two species were grown alone and in mixture
    with the other species on both soil types in a
    common garden

36
Tansley and Competition
37
Tansleys Results
  • When grown alone in common garden experiments,
    each species performed better on its preferred
    soil, although each could grow on the other soil
    type.
  • When grown in mixture, each species overgrew and
    shaded the other on its preferred soil type.
  • Tansley concluded that each species was at a
    disadvantage in competition when grown on the
    other soil type this helped explain the observed
    distributions of the two species in nature.

38
Tansleys Conclusions
  • Tansleys conclusions have far-ranging
    implications for all competitive situations
  • the presence or absence of species can be
    determined by competition with other species
  • conditions of the environment affect the outcome
    of competition
  • competition may be felt very broadly throughout
    the community
  • the present segregation of species may have
    resulted from past competition

39
Competition among Aquatic Animals
  • Experimental studies of competition among
    intertidal barnacles by Joseph Connell have
    enhanced our understanding of how competition can
    affect spatial distributions of species
  • barnacles are not food-limited, but compete
    intensely for limited space on rocks in the
    intertidal zone
  • the barnacle Chthamalus is more tolerant of
    desiccation and thrives in the upper intertidal
    zone
  • the barnacle Balanus is less tolerant of
    desiccation but can displace Chthamalus in the
    lower intertidal zone

40
Barnacle competition
41
Competition Among Terrestrial Animals
  • Interference competition (through aggressive
    behavior) has been observed among species of
    voles (small mouse-like mammals) in the Rocky
    Mountain states
  • Microtus pennsylvanica and M. montanus inhabit
    wetter and drier habitats, respectively
  • when either species was removed by trapping from
    its preferred habitat, the other species
    gradually moved in
  • Exploitation competition is much more common,
    although its effects are more difficult to
    identify.

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43
To compete or not to compete, that is the question
  • Animals
  • r-selection (fast population growth, avoid
    competition)
  • K-selection (slow population growth, good
    competitor)
  • Plants (CSR model of Grimes)
  • Competitive (K), ruderal (weed or r), and
    stress-tolerant strategy
  • Related to tolerance of stress, intensity of
    disturbance

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Character Displacement
  • If character traits of two closely related
    species differ more in sympatric regions than in
    allopatric regions, this pattern may have arisen
    from strong selective pressure for divergence in
    sympatry, a process called character
    displacement
  • ecologists disagree on the prevalence of
    character displacement in nature
  • patterns consistent with the operation of
    character displacement have been observed among
    Darwins finches of the Galápagos Islands

46
Resource-utilization curves
47
Character Displacement
48
Competition in Darwins Finches
49
Diffuse Competition and Indirect Effects
  • Diffuse competition
  • Combined effects of many species on a given
    species
  • Direct vs. indirect competition
  • Exploitative is indirect, interference is direct
  • Apparent competition
  • Via shared enemies or via other specie on same
    trophic level

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