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Food Webs
Chapter 17
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Outline
Community Webs
Complexity and Structure
Keystone Species
Effects on Diversity
Exotic Predators
Mutualistic Keystones
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Fig. 17.2
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Food Web Complexity
Winemiller described feeding relations amongtropical freshwater fish.
Represented food webs in various ways:
Only included common species.
Top-predator sink.
Excluded weakest trophic links.
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Fig. 17.3b
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Strong Interactions and Food WebStructure
Paine suggested feeding activities of a fewspecies may have a dominant influence oncommunity structure.
Suggested criterion for strong interaction isdegree of influence on community structure.
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Strong Interactions and Food WebStructure
Tscharntke studied food websassociated with wetland reeds(Phragmites australis) NEGermany.
Attacked by fly Giraudiella inclusa.
Attacked by 14 species ofparasitoid wasps.
Predator specialization
Distinguished weak and stronginteractions.
Determination of keystone species.
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Keystone Species
If keystone species reduce likelihood ofcompetitive exclusion, their activities wouldincrease the number of species that couldcoexist in communities.
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Food Web Structure and SpeciesDiversity
Paine found as number of species in intertidalfood webs increased, proportion of the webrepresented by predators also increased.
According to his hypothesis, higher proportion ofpredators produces higher predation pressure onprey populations, in turn promoting higherdiversity.
Removal of starfish (top predator) caused decline indiversity from 15 to 8 species.
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Fig. 17.6
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Keystone Species
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Fig. 17.7
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Consumers’ Effects on Local Diversity
Lubchenko proposed to resolve the effectherbivores have on plant diversity, you needto know:
Herbivore food preference.
Competitive relationships between plant speciesin the local community.
Variance in feeding preferences and competitiverelationships across environments.
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Consumers’ Effects on Local Diversity
Lubchenko studied influence of intertidal snail(Littorina littorea) on structure of an algalcommunity.
Snails fed on green (Enteromorpha spp.) and red(Chondrus crispus) algae.
Under normal conditions, Enteromorpha out-competesChondrus in tide pools, and Littornia prefersEnteromorpha.
In the absence of snails, Chondrus is competitivelydisplaced.
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Consumers’ Effects on Local Diversity
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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 youngsnails, preventing juveniles from colonizing tidepools.
Populations of Carcinus are controlled byseagulls.
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Consumers’ Effects on Local Diversity
Low snail density - Enteromorpha dominates tidepool.
Medium snail density - Competitive exclusioneliminated, and algal diversity increased.
High snail density - Feeding requirements arehigh enough that snails eat preferred algae andless-preferred algae.
Algal diversity decreased.
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Fig. 17.9
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Fish as River Keystone Species
Power investigated whether California roachHsperoleucas symmetricus and steelheadtrout Oncorhhyncus mykiss significantlyinfluence food web structure.
Predatory fish decrease algal densities.
Low predator density increased midge production.
Increased feeding pressure on algal populations.
Thus, fish act as Keystone Species.
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Fig. 17.10a
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Fig. 17.10b
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Fish as River Keystone Species
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Keystone Species: Summation
Power : Keystone species exert strongeffects on their community structure, despitelow biomass.
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Exotic Predators
Exotic species have dramatic impacts oncommunities because they were outside theevolutionary experience of local preypopulations.
Nile Perch (Lates nilotica) exotic fish predator inLake Victoria.
Fish fauna dramatically reduced.
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Exotic Predators
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Exotic Predators
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Exotic Predators
Kaufman pointed out changes in LakeVictoria fish community coincide with otherecosystem changes.
Dissolved oxygen concentrations significantlydecreased.
Cultural eutrophication.
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Christian observed native ants disperse 30% ofshrubland seeds in fynbos of South Africa.
Seed-dispersing ants bury seeds in sites safe frompredators and fire.
Argentine ants have displaced many native ant species thatdisperse large seeds.
Substantial reductions in seedling recruitment by plantsproducing large seeds.
Seed Dispersal Mutualists as KeystoneSpecies
linepithema humile
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Winged adult and worker of Solenopsis geminata fire ants.
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Fig. 17.21
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