27: Food Web Ecology

Communities are organized into food webs

• Food chain

  • a linear representation of how different species in a community feed on each other

  • limited use

• Food web

  • a complex and realistic representation of how species feed on each other in a community

• Gut content analysis

  • evaluation of food items in animal guts

• Stable isotope analysis

  • sample of tissue to determine C and N isotopes

  • C isotopes provide information about food resources

  • N isotopes provide information about trophic position

• Trophic levels

  • a level in a food chain or food web of ecosystem

  • Primary consumer: a species that eats producers

  • Secondary consumer: a species that eats primary consumers

  • Tertiary consumer: a species that eats secondary consumers

  • Omnivore: a species that feeds at several trophic levels

• Direct vs Indirect effects

  • Direct effect: an interaction between two species that does not involve other species

  • Indirect effect: an interaction between two species that involves one or more intermediate species

  • Trophic cascade: indirect effects in a community that are initiated by a predator

Thinking about food web interactions… Why is the world green?

• Why plants are able to maintain so many leaves?

• Why is the world green?

  • Green World Hypothesis

    • herbivores are kept in check by predators, parasites, disease

    • highlighted the importance of trophic interactions

Trophic controls

• Bottom-up control: when the abundances of trophic groups in nature are determined by the amount of energy available from the producers in a community

• Top-down control: when the abundance of trophic groups is determined by the existence of predators at the top of the food web

• Trophic cascades: indirect effects that propagate along the food web