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

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

5. 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

6. 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

9. Decomposers Soil nutrients

12. Animal 2 Animal 1 Plant

13. 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

14. 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

17. 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

18. 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

19. 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.

21. 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?

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

23. Carnivores Herbivores Producers

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

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

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

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

28. 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

29. Effects of trophic cascades usually alternate by trophic level

31. No otters

33. x x Fewer fish, shellfish

35. Algae Fish Predators

36. 2° Carnivores Carnivores 1° Carnivores increase production Herbivores Herbivores Producers Producers

37. Carnivores Herbivores decrease production Herbivores Producers Producers