Interspecific Competition Experiments

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Interspecific CompetitionExperiments
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Relative Competition Index

• P is Performance, usually expressed as dry mass
  or growth rate
• RCI Pmonoculture Pmixture
  ____________________
• Pmonoculture

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Absolute Competition Index

• ACI Pmonoculture Pmixture

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Log Response Ratio

• LRR Pmixture ln ________
• Pmonoculture

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Relative Efficiency Index

• Connolly (1987)
• REI RGRA RGRB
• Relative Growth Rates of the two species in
  mixture to predict dynamics of mixture

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Drawbacks to Indices

• collapse information into one number
• might be useful to look at overall performance
• strongly influenced by initial size of organisms
• assume linear responses to competitor density

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Types of Experiments

• Field
• realism multiple species, typical environmental
  conditions
• sacrifice control cant manipulate densities
  effectively, usually presence/absence only
• Laboratory/Greenhouse
• sacrifice realism cant include more than a few
  species
• control can manipulate a wide range of
  densities
• Hybrid
• artificial habitats brought into field conditions

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Classes of Experimental Designs
substitution
additive
response surface designs
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Substitution or Replacement Series Designs

• most commonly used (Gibson et al. 1999)
• frequency dependence
• substitution (animals) or replacement series
  (plants - DeWit series)
• total density constant, relative proportion of
  hypothesized competitors varied
• confound effects of variation in intraspecific
  and interspecific competition (Inouye 2001)
• results depend upon the arbitrary single-species
  densities chosen for mixture endpoints (Connolly
  1997)

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Additive Design

• testing density dependence
• hold density of target species constant, vary
  density of putative competitor
• doesnt address intraspecific competition
• results depend on chosen density of target
  species (Inouye 2001)

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Response Surface Designs

• vary density of both species
• can describe intraspecific and interspecific
  competition
• can detect asymmetric competition
• drawback is that it might be overkill (Cousens
  1991)

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Response Surface Designs
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Asymmetric Competition

• Intraspecific asymmetry species differ in the
  intensity of intraspecific competition
• Interspecific - species A has larger effects on B
  than B has on A
• may be due to inherent size differences
• largest individuals have disproportionate
  negative effects on their smaller neighbors

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Nonadditive Competition

• extrapolate to more than 2 species
• competition coefficients may not simply be a
  function of a pair of species
• may depend on ecological context and
  identity/density of other species in community
• higher-order interactions (indirect effects of
  other species)

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Regulating Factors

• intrinsic within/between species - biotic
• extrinsic imposed from outside - abiotic

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Density Dependence

• density-independent not influenced by population
  density
• density-dependent are influenced by population
  density

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Density-Independent Factors

• extrinsic abiotic
• weather
• drought
• volcanoes
• floods
• landslides

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Density-Dependent Factors

• tend to be biotic intrinsic
• resource competition
• intraspecific (within species)
• when resources become limiting, intensity of
  competition increases
• quick, healthy, and strong individuals will
  prevail
• territoriality can control access to resources

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Density-Dependent Factors

• interspecific (between species)
• competition ( -, -)
• predator-prey
• mutualism (,)
• commensalism (, 0)
• parasitism (,-)