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Food Webs

Food Webs. Chapter 17. Figure 17_01. Food Webs. Chapter 17. Food Web Complexity. Winemiller described feeding relations among tropical freshwater fish. Represented food webs in various ways: Only included common species. Top-predator sink. Excluded weakest trophic links.

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Food Webs

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  1. Food Webs Chapter 17

  2. Figure 17_01

  3. Food Webs Chapter 17

  4. Food Web Complexity • Winemiller described feeding relations among tropical freshwater fish. • Represented food webs in various ways: • Only included common species. • Top-predator sink. • Excluded weakest trophic links.

  5. Food Web Complexity Fresh water fish community in Argentina

  6. Strong Interactions and Food Web Structure • Tscharntke studied food webs associated with wetland reeds (Phragmitesaustralis). • Attacked by fly Giraudiellainclusa. • Attacked by 14 species of parasitoid wasps. • Predator specialization • Distinguished weak and strong interactions. • Determination of keystone species.

  7. Figure 17_05

  8. What appears as competition in food webs may not always be… Is this relationship mutualistic, commensalistic or exploitative? Direct or indirect?

  9. Figure 17_07

  10. Figure 17_08

  11. What explains the greater diversity in the subtropical system?

  12. Food Web Structure and Species Diversity • Paine found as number of species in intertidal food webs increased, proportion of the web represented by predators also increased. • According to his hypothesis, higher proportion of predators produces higher predation pressure on prey populations, in turn promoting higher diversity. • Removal of starfish (top predator) caused decline in diversity from 15 to 8 species.

  13. Keystone Species • If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.

  14. Consumers’ Effects on Local Diversity • Lubchenko proposed to resolve the effect herbivores have on plant diversity, you need to know: • Herbivore food preference. • Competitive relationships between plant species in the local community. • Variance in feeding preferences and competitive relationships across environments.

  15. Consumers’ Effects on Local Diversity

  16. Consumers’ Effects on Local Diversity • Lubchenko studied influence of intertidal snail (Littorina littorea) on structure of an algal community. • Snails fed on green (Enteromorpha spp.) and red (Chondrus crispus) algae. • Under normal conditions, Enteromorpha out-competes Chondrus in tide pools, and Littornia prefers Enteromorpha. • In the absence of snails, Chondrus is competitively displaced.

  17. Consumers’ Effects on Local Diversity • When snails are present in high densities, Littorinagrazes down Enteromorpha, releasingChondrusfrom competition. • Green crabs (Carcinusmaenus) prey on young snails, preventing juveniles from colonizing tide pools. • Populations of Carcinus are controlled by seagulls.

  18. Consumers’ Effects on Local Diversity • Low snail density - Enteromorpha dominates tide pool. • Medium snail density - Competitive exclusion eliminated, and algal diversity increased. • High snail density - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae. • Algal diversity decreased.

  19. Consumers’ Effects on Local Diversity

  20. Fish as River Keystone Species

  21. Keystone Species: Summation • Power:Keystone species exert strong effects on their community structure, despite low biomass.

  22. Exotic Predators • Exotic species have dramatic impacts on communities because they were outside the evolutionary experience of local prey populations. • Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria. • Fish fauna dramatically reduced.

  23. Exotic Predators

  24. Exotic Predators

  25. Exotic Predators • Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes. • Dissolved oxygen concentrations significantly decreased. • Cultural eutrophication.

  26. Mutulaistic Keystone Species • Cleaner fish in Red Sea coral reef communities…

  27. Seed Dispersal Mutualists as Keystone Species • Christian observed native ants disperse 30% of shrubland seeds in fynbos of South Africa. • Seed-dispersing ants bury seeds in sites safe from predators and fire. • Argentine ants have displaced many native ant species that disperse large seeds. • Substantial reductions in seedling recruitment by plants producing large seeds.

  28. Homo sapiens as Keystone Species 23 – 60 million animals killed yearly in Brazilian Amazon… Reducing bird and mammal biomass in some areas by 70-90%. “We must not let a forest full of trees fool us into believing all is well.” Kent Redford

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