Transcript Notes
Food Webs Chapter 17 1 Figure 17_01 2 Food Webs Chapter 17 3 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. 4 Food Web Complexity 5 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. 6 Figure 17_05 7 Figure 17_06 8 Figure 17_07 9 Figure 17_08 10 Keystone Species • If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities. 11 Keystone Species 12 Figure 17_10 13 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. 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 • 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. 16 Consumers’ Effects on Local Diversity 17 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. 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 lesspreferred algae. • Algal diversity decreased. 19 Consumers’ Effects on Local Diversity 20 Fish as River Keystone Species 21 22 Keystone Species: Summation • Power : Keystone species exert strong effects on their community structure, despite low biomass. 23 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. 24 Exotic Predators 25 Exotic Predators 26 Exotic Predators • Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes. – Dissolved oxygen concentrations significantly decreased. – Cultural eutrophication. 27 Mutulaistic Keystone Species • Cleaner fish in Red Sea coral reef communities… 28 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. 29 Homo sapiens as Keystone Species 30 31