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Food WebsChapter 17

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Figure 17_01

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Food WebsChapter 17

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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.

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Food Web Complexity

Fresh water fish community in Argentina

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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 parasitoid wasps.

– Predator specialization

– Distinguished weak and strong interactions.• Determination of keystone species.

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Figure 17_05

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What appears as competition in food webs may not always be…

Is this relationship mutualistic, commensalistic or exploitative?

Direct or indirect?

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Figure 17_07

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Figure 17_08

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What explains the greater diversity in the subtropical system?

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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.

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Keystone Species

• If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.

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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.

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Consumers’ Effects on Local Diversity

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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.• Under normal conditions, Enteromorpha out-competes

Chondrus 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, 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 - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae.• Algal diversity decreased.

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Consumers’ Effects on Local Diversity

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Fish as River Keystone Species

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Keystone Species: Summation

• Power : Keystone species exert strong effects on their community structure, despite low biomass.

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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.

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Exotic Predators

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Exotic Predators

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Exotic Predators

• Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes.– Dissolved oxygen concentrations significantly

decreased.– Cultural eutrophication.

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Mutulaistic Keystone Species

• Cleaner fish in Red Sea coral reef communities…

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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.

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Homo sapiens as Keystone Species

23 – 60 million animals killed yearly in Brazilian Amazon…Reducing bird and mammal biomass in some areas by 70-90%.

“We must not let a forest full of trees fool us into believing all is well.” Kent Redford

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