Community Equilibrium Models

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Community Equilibrium Models
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Equilibrium vs. non-equilibrium communities

• Equilibrium communities
• Species abundances remain constant over time due
  to biological processes such as competition,
  predation, herbivory, and mutualism
• Local vs. Global stability
• Local is stable within a specified range
• Global is stable over all ranges

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Non-equilibrium communities

• Non-equilibrium communities will not be stable
• Species are individualistic
• Stochasticity is high
• Density-dependent population regulation is weak
• Community is highly variable

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Equilibrium theories of community organization

• Classical competition theory
• Competition is the sole interaction determining
  community organization
• Predicts n species for n limiting resources
• Deterministic population growth rates
• Spatially homogeneous populations, minimal
  migration
• Stable equilibrium point for coexistence of
  competitors

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Equilibrium theories of community organization

• Competition-predation theory
• Predation also important, extension of
  competition theory
• Predicts n species to coexist on fewer than n
  resources, due to predation
• Competition-spatial patchiness theory
• Environment is patchy, each competitor does best
  in own patch, variety of patches at landscape
  scale

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Feeding relationships organize communities in
food webs.

• From an ecosystem perspective, species are
  usually combined into relatively few trophic
  (feeding) levels
• a food web analysis emphasizes the diversity of
  feeding relationships within an ecosystem

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There are different ways to portray food webs.

• Connectedness webs emphasize feeding
  relationships as links in a food web.
• Energy flow webs represent an ecosystem
  viewpoint, in which connections between species
  are quantified by flux of energy.
• Functional webs emphasize the importance of each
  population through its influence on growth rates
  of other populations.

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Three approaches to studying food webs
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Generalizations emerge from food web studies.

• Communities may be characterized by the number of
  species (richness), connectance (ration of actual
  to possible interactions) and number of feeding
  links per species
• the number of feeding links per species is
  independent (constant) of the species richness of
  the community
• the number of trophic levels and the number of
  guilds per trophic level increase with community
  diversity
• Proportions of species in trophic levels remain
  constant
• Omnivory may or may not be common

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How does food web structure affect community
stability?

• Robert Paine and others who have studied food
  webs in natural communities have stressed the
  importance of consumer-resource relationships in
  community organization
• populations of keystone predators are
  particularly important in maintaining community
  stability and diversity

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Keystone species

• Species defined as keystone species do not have
  high biomass but control community structure,
  typically through their feeding activities as
  predators
• Examples include sea stars, elephants, sea
  otters, and certain herbivores, kangaroo rats,
  salamanders
• Keystone species can only be elucidated through
  experimental investigation of a community
• Thus, the conservation concern over the loss of a
  single species is that the community could change
  irreversibly, even though that species is rare.

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Keystone predators and community diversity
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Dominant Species

• Numerical superiority
• Abundance or biomass
• Achieved by competitive ability
• Transitive
• Linear hierarchy of competitive abilities
• Leads to competitive exclusion
• Intransitive
• Circular network of competitive abilities
• Prevents competitive exclusion
• May also be regulated by predation

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Influence of Predators on Food Webs
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Experiments with dominant species

• Removal of dominant species may not change
  community structure
• Loss of dominant chestnut tree in eastern US
  forests led subdominants to expand. Forests did
  not change much in structure or composition.
• Compare this to the idea of the keystone species.

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Nonequilibrium communities

• Focus is on patches and disturbance
• Patchiness refers to spatial scale of a system
• Different communities have different patchiness

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5 main spatial scales

• Space occupied by a plant or sessile animal, or
  home range of a single mobile animal
• Local patch (many plants, animals)
• Region (many local patches, dispersal links
  patches)
• Closed system or region closed to immigration or
  emigration
• Biogeographical scale (different biomes)

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Scale influences pattern

• Smaller the scale, less equilibrium
• Most field studies at small scale, usually local
  patch scale
• Few studies at larger spatial scales due to
  logistics, time, cost, difficulty

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Disturbance

• Any discrete event that disrupts community
  structure and that changes the physical
  environment, resources, or substrate
• Severe events (hurricane, fire) or natural
  fluctuations (drought, frost)

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Definitions of measures of disturbance

• Distribution where
• Frequency how many per unit time
• Return Interval (Turnover time) inverse of
  frequency time expected between events
• Rotation period mean time to disturb study area
• Predictability variance in return interval
• Area or size how much
• Magnitude Intensity (physical force), severity
  (impact on community)
• Synergism (interactions)

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Hurricanes as disturbance

• Distribution subtropical, in US, Southeast, Gulf
  of Mexico, Northeast, Hawaii
• Frequency varies, one every 4 years in Florida,
  one every 50 years in NE
• Return interval 4-50 years
• Rotation period long for small areas
• Predictability low
• Area or size large
• Intensity (variable), Severity (variable)
• Synergism associated with floods, later with
  fires

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Role of disturbance

• Coral reef communities affected by hurricanes
• Cause death of established coral
• Varies by location within the reef
• Recruitment rates of coral highly variable
• Coral reefs always changing due to hurricanes and
  variable recruitment

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Theoretical Non-equilibrium models

• Communities may range from biotic-centered
  equilibrium communities to more-stochastic, less
  biotically driven non-equilibrium communities
• Four main models have been proposed that describe
  non-equilibrium communities

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Non-equilibrium models

• Fluctuating environment models
• Competition main biotic interaction but
  environmental fluctuations change competitive
  rankings
• Density-independent models
• Spatial patchiness and density-vagueness limit
  competitive interactions
• Directional changing environment models
• Environment changes mean value, life history of
  species, dispersal, and history determine change
• Slow competitive displacement models
• No time trend of succession due to low population
  density of constituent species chance and
  history play big role