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community structure and food web
This document discusses community structure and food webs. It provides examples of food webs in arctic and tropical freshwater communities and examines how food web complexity increases with community diversity. The concept of keystone species is introduced, which are species that have a significant influence on community structure despite low biomass. Examples are given of keystone species such as a predatory snail that increases algal diversity and fish that influence midge production. The effects of birds on herbivorous insects are also discussed. Exotic predators can drastically simplify food web structure by collapsing prey populations outside of their evolutionary experience.
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community structure and food web
- 1. 11 Biology 205 Ecology andAdaptation Lecture 13 community structure and food web Dr. Erik D. Davenport 1
- 2. 2 Species interactions andcommunity structure • The most important interactions in a community are: • Feeding relationships provide some of the most easily documented examples of interactions within community. • Food web: a summary of the most the feeding interactions within a community, is one of the most basic and revealing descriptions of a community structure. 2
- 3. 3 Community Webs • Afood web summarizes the feeding relationship in a community. • Summerhayes and Elton studied feeding relations on Bear Island in High Arctic. – Primary producers were terrestrial plants and aquatic algae. – Fed on by several kinds of terrestrial and aquatic invertebrates. – Consumed by birds. – Attacked by arctic foxes. 3
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- 5. 5 Food Web Complexity •The level of food web complexity increases dramatically with more diverse communities. • 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
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- 7. 7 Strong Interactions andFood Web Structure • Paine suggested feeding activities of a few species may have a dominant influence on community structure. He called these important trophic relations strong interactions. • Suggested criterion for strong interaction is based on the degree of influence on community structure. 7
- 8. 8 Strong Interactions andFood Web Structure • Tscharntke studied food webs associated with wetland reeds (Phragmites australis). – Attacked by fly Giraudiella inclusa. – Attacked by 14 species of parasites wasps. • Predator specialization – Distinguished weak and strong interactions. • Determination of keystone species. 8
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- 10. 10 Keystone Species • Keystonespecies: the species has the most significant influence on community structure. • The feeing activities of few keystone species may control the structure of communities. • If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.( some predator may increase species diversity). 10
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- 14. 14 Consumers’ Effects onLocal 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. But the green algae is the preferred food source for the snails. – Under normal conditions, Enteromorpha (green algae) out- competes Chondrus (red algae) in tide pools, and Littornia prefers Enteromorpha. – In the absence of snails, Chondrus is competitively displaced. 14
- 15. 15 Consumers’ Effects onLocal Diversity • When snails are present in high densities, Littorina grazes down Enteromorpha (green algae) , 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. 15
- 16. 16 Consumers’ Effects onLocal Diversity • Low snail density - Enteromorpha dominates tide pool. • Medium snail density - Competitive exclusion eliminated, and algal diversity increased. • High snail density - Littorina grazes down Enteromorpha (green algae) , releasing Chondrus from competition. – Algal diversity decreased. Which algae will out- compete???? 16
- 17. 17 Consumers’ Effects onLocal Diversity 17
- 18. 18 Fish as RiverKeystone Species • Power investigated whether California roach Hsperoleucas symmetricus and steelhead trout Oncorhhyncus mykiss significantly influence food web structure. – Low predator density increased midge production. – Increased feeding pressure on algal populations. – Thus, fish act as Keystone Species. 18
- 19. 19 Effects of Predationby Birds on Herbivory • Atlegrim studied influence of birds on herbivorous insects and insect-induced plant damage. – Insectivorous birds may act as keystone species via effects on herbivorous insects. – Larval insect densities is low when many insectivorous birds are feeding their young. 19
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- 21. 21 Keystone Species • Keystonespecies exert strong effects on their community structure, despite low biomass. 21
- 22. 22 Exotic Predators • Exoticpredator can collapse and simplify the structure of food web. • 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. 22
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