11 Food Webs Chapter 17

22 Figure 17_01

33 Food Webs Chapter 17

44 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.

55 Food Web Complexity

66 Strong Interactions and Food Web Structure Tscharntke studied food webs associated with wetland reeds (Phragmites australis). – Attacked by fly Giraudiella inclusa. Attacked by 14 species of parasitoid wasps. – Predator specialization – Distinguished weak and strong interactions. Determination of keystone species.

77 Figure 17_05

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

99 Figure 17_07

10 Figure 17_08

11 What explains the greater diversity in the subtropical system?

12 Figure 17_10

13 Food Web Structure and Species Diversity Paine found … – According to his hypothesis, higher proportion of predators produces higher … Removal of starfish (top predator) caused decline in diversity from 15 to 8 species.

14 Keystone Species If ________________________ reduce likelihood of ___________________, their activities would increase the number of species that could coexist in communities.

15 Consumers’ Effects on Local Diversity Lubchenko proposed to resolve the effect herbivores have on plant diversity, you need to know: – – –

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

18 Consumers’ Effects on Local Diversity When snails are present in high densities, Littorina grazes down Enteromorpha, releasing Chondrus from competition. – Green crabs (Carcinus maenus) prey on young snails, preventing juveniles from colonizing tide pools. – Populations of Carcinus are controlled by seagulls.

19 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.

20 Consumers’ Effects on Local Diversity

21 Fish as River Keystone Species

22

23 Keystone Species: Summation Power : Keystone species exert ___________ on their community structure, despite _________________.

24 Exotic Predators Exotic species have dramatic impacts on communities because,… – Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria. Fish fauna dramatically reduced.

25 Exotic Predators

26 Exotic Predators

27 Exotic Predators Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes. – Dissolved oxygen concentrations significantly decreased. – Cultural ______________________.

28 ______________ Keystone Species Cleaner fish in Red Sea coral reef communities…

29 _______________________________ 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.

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

31