Download
slide1 n.
Skip this Video
Loading SlideShow in 5 Seconds..
Food Webs (Chapter 21) Types of food webs General patterns Trophic cascades PowerPoint Presentation
Download Presentation
Food Webs (Chapter 21) Types of food webs General patterns Trophic cascades

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

237 Views Download Presentation
Download Presentation

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

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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

  2. Food Web – a summary of the feeding relationships in a community

  3. 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

  4. 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

  5. Decomposers Soil nutrients

  6. Animal 2 Animal 1 Plant

  7. 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

  8. Keystone Predator – one that has a dominant effect on community composition • reduce competition among prey species • allow poorer competitors to persist • Top predators aren’t always keystone predators • abundance • feeding patterns

  9. 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

  10. Stability in Food Webs • A community is stable if • it doesn’t 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

  11. 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.

  12. Trophic Cascades • Herbivores can limit plant populations, but… • In most places, plants are abundant and herbivores don’t limit plant growth • So…why is the world green?

  13. HSS hypothesis for why the world is green (Hairston, Smith, & Slobodkin, 1960)

  14. Carnivores Herbivores Producers

  15. Carnivores • No predators • Limited only by food Herbivores Producers

  16. Carnivores • No predators • Limited only by food • Limited by predation Herbivores Producers

  17. Carnivores • No predators • Limited only by food • Limited by predation Herbivores Producers

  18. Carnivores • No predators • Limited only by food • Limited by predation • Limited by competition and resources Herbivores Producers

  19. 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

  20. Effects of trophic cascades usually alternate by trophic level

  21. No otters

  22. x x Fewer fish, shellfish

  23. Algae Fish Predators

  24. 2° Carnivores Carnivores 1° Carnivores increase production Herbivores Herbivores Producers Producers

  25. Carnivores Herbivores decrease production Herbivores Producers Producers