Post on 11-Feb-2017
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Biology 205Ecology and Adaptation
Lecture 13community structure and food web
Dr. Erik D. Davenport
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Species interactions and community 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.
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Community Webs
• A food 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.
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Arctic Food Web
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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.
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Strong Interactions and Food 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.
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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 parasites wasps.• Predator specialization
– Distinguished weak and strong interactions.• Determination of keystone species.
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Keystone Species
• Keystone species: 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).
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Keystone Species
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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. 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.
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Consumers’ Effects on Local 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.
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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 - Littorina grazes down Enteromorpha (green algae) , releasing Chondrus from competition.– Algal diversity decreased. Which algae will out-
compete????
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Consumers’ Effects on Local Diversity
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Fish as River Keystone 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.
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Effects of Predation by 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.
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Keystone Species
• Keystone species exert strong effects on their community structure, despite low biomass.
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Exotic Predators
• Exotic predator 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.
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Exotic Predators
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Exotic Predators
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