Food Webs Chapter 21

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• Food Webs (Chapter 21)
• Types of food webs
• General patterns
• Trophic cascades

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• Food Web a summary of the feeding relationships
  in a community

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• Trophic level position in the food chain
• basal (lowest level)
• intermediate act as predators and prey
• top predators have no predators
• Node a species or group of species (guild)
• Link shows who eats whom
• Omnivory feeding on more than one trophic level

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• Disclaimer
• All food webs are simplified some species and
  relationships are left out
• In the real world there are no top predators
• Most food webs exclude decomposers

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Decomposers
Soil nutrients
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Animal 2
Animal 1
Plant
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• Links can be distinguished by their linkage
  strength
• strong interaction absence would cause a
  pronounced change in the community
• weak interaction absence would not have a major
  effect on the community

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• Keystone Predator one that has a dominant
  effect on community composition
• reduce competition among prey species
• allow poorer competitors to persist
• Top predators arent always keystone predators
• abundance
• feeding patterns

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• The number of links per species tends to be
  constant, regardless of the number of species,
  but
• Stable environments have more links per species
  than unstable environments

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• Stability in Food Webs
• A community is stable if
• it doesnt change much in response to
  environmental changes, or
• it returns to its previous state after a
  disturbance
• Early models suggested that larger food webs are
  unstable

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• Current view more diverse food webs are more
  stable
• more species weaker interactions on average
• species influenced by many weak interactions have
  more stable populations than species with few
  strong interactions
• Field research suggests most interactions in
  nature are weak.

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• Trophic Cascades
• Herbivores can limit plant populations, but
• In most places, plants are abundant and
  herbivores dont limit plant growth
• Sowhy is the world green?

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• HSS hypothesis for why the world is green
  (Hairston, Smith, Slobodkin, 1960)

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Carnivores
Herbivores
Producers
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Carnivores

• No predators
• Limited only by food

Herbivores
Producers
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Carnivores

• No predators
• Limited only by food
• Limited by predation

Herbivores
Producers
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Carnivores

• No predators
• Limited only by food
• Limited by predation

Herbivores
Producers
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Carnivores

• No predators
• Limited only by food
• Limited by predation
• Limited by competition and resources

Herbivores
Producers
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• Trophic cascade influence of producers or
  consumers on species that are two or more trophic
  levels away
• Top-down Control influence of predators on the
  relative abundance of lower trophic levels
• Bottom-up Control influence of producers on the
  relative abundance of higher trophic levels

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• Effects of trophic cascades usually alternate by
  trophic level

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No otters
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x
x
Fewer fish, shellfish
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Algae
Fish
Predators
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2 Carnivores
Carnivores
1 Carnivores
increase production
Herbivores
Herbivores
Producers
Producers
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Carnivores
Herbivores
decrease production
Herbivores
Producers
Producers
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