Competitive Interactions

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Competition

• I. Background Information
• 1) competition - Any interaction that is mutually
  detrimental to both participants, occurring
  between species that share limited resources.

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2) Types of Competition

• A) Intraspecific - between individuals of the
  same species
• B) Interspecific - between individuals of
  different species

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3) Mechanisms of Competition

• A) Exploitative competition by a group that
  reduces a resource to a point that adversely
  affects other organisms
• indirectly reduces abundance of other species
• depends on how effectively each competitor uses
  the resource

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3) Mechanisms of Competition

• B) Interference competition in which access to a
  resource is limited by the presence of a
  competitor
• direct or aggressive displacement

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4) Resource Partitioning

• Division and differential utilization of
  resources by different species within a community
• needed for coexistence
• comprise bulk of competition studies
• A) Overlap and partitioning
• - evaluated by diet, habitat, and time when
  organism is active

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B) Resource Overlap
Indicates possibility of competitive
effect Argument over theory and interpretation
of the implications of high resource overlap
A) high overlap High overlap in existing
resources suggests that the population size is
kept down by other forces. If species have
competed historically, the evolutionary
divergence should have occurred
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B) Resource Overlap
Indicates possibility of competitive
effect Argument over theory and interpretation
of the implications of high resource overlap
B) Low overlap When there is little overlap in
resource use by similar species this may reflect
that past competition has favored one of the two
species (notion of the ghost of competition,
MacArthurs Warblers
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II. History

• 1) Origins of Theory
• Shaped over 30 years
• Focus on isolating mechanisms in 1930s and
  1940s
• Gause (1930s)
• Russian biologist, studied lab interactions of 2
  Paramecium spp.
• Lotka Volterra
• American and Italian, independently arrived at
  mathematical expressions for resource use
• Mayr, Lack Huxley

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II. History

• 2) Importance of ecological compatibility between
  species drew favor in 1950s
• More interested in mechanism
• Hutchinson (1959) Why are there so many kinds of
  animals?
• Pushed along by competitionists paradigm

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III. Competitionists paradigm

• Competition is the dominant ecological
  interaction
• Very prevalent idea
• Competition and its paradigm came under attack in
  mid-70s
• new formulas, statistical reevaluation, field
  studies

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4) Modern view of competition

• 6 main propositions (Schoener 1982)
• A) Species too similar cannot coexist for long
• One will competitively exclude other
• Gause principle
• B) Species coexisting have sufficient differences
  in ecological niche or use in resources

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• C) Interspecific competition is a strong
  evolutionary force
• Selects for adaptations that result in species
  differing in use of resources
• Grant Grant (1980)
• D) Geographic distributions of species are often
  determined by competitive pressures
• Species too ecologically similar are separated
  geographically
• Competitive pressures determine how many and
  which species coexist within a community, given
  time

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• ) Species may compete by interference or
  exploitative mechanisms
• Interference unlikely to evolve if resources not
  sufficiently scarce
• ) Studies of species with high level of resource
  overlap should indicate interspecific competition
• Ex. Introduction of a new species with closely
  related resource requirements should depress
  naturally established population in observable
  ways

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III. Criteria for identifying competition in
natural communities

• Reynoldson and Bellamy (1971)
• ) Competition should explain the distribution
  and/or relative abundance of the two potentially
  competing species.
• ) It is necessary to show that the competing
  species are utilizing a common resource which may
  provide the basis of competition.

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• ) There should be evidence that intraspecific
  competition is occurring by the performance of
  natural species populations.
• Relating of fecundity, survival, growth rate
• Assumes intraspecific comp. occurring, if
  persistent interspecific comp. is occurring

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• ) Separate field manipulations of resource and
  populations
• ) Results of removing or introducing a competing
  species should be consistent with the competition
  hypothesis.

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B) Resource Overlap studies

• i) high resource overlap
• Baker and Ross (1981)
• 8 cyprinid spp. in a Mississippi Stream
• High spatial overlap
• Found in the same reach types or channel units
  (i.e., macrohabitat)

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Microhabitat distinction by water column position
and use of aquatic vegetation
After Baker and Ross 1981, From Allan, 1995
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B) Resource Overlap studies

• ii) low resource overlap
• Moyle and Senanayake (1984)
• Small rainforest stream
• Highly structured w/ minimal overlap based of
  fish morphology, habitat use, and diet.

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Niche segregation in a tropical fish community in
Sri Lanka
After Moyle and Senanayake 1984, From Allan 1995
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C) Exploitative Competition

• i) Intraspecific exploitative competition
• - Caddisfly grazer Helicopsyche borealis,
  dominant grazer in spring -fed stream
• - field observations and experimental
  manipulations of population density
• Feminella and Resh (1990)

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H. borealis influence on food supply
from Feminella and Resh 1990
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H. borealis larval influence on pupation -
intraspecific competition for food resources
from Feminella and Resh 1990
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• ii) Interspecific exploitative competition
• Snail (Elimia clavaeformis) and caddis (Neophylax
  etnieri)
• Competition for periphyton
• 6 streams w/ snail and 6 streams w/out
• Periphyton biomass greater and caddis
• ? 2X larger in streams w/out snails

(Hill 1992)
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Stream Surveys snails, periphyton, and caddis
size
After Hill (1992) , from Allan (1995)
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D) Interference Competition

• Larvae of midge (Blepharicera) and black flies
  (Simuliidae) compete for space on stone surfaces.
• Strong inverse relationship between spp.
  densities
• Simuliids nipped at midges in reach and
  disrupted feeding
• Midges sign. smaller when reared w/ simuliids

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Simulium
Blepharocera
After Dudley et al. 1990, from Allan, 1995