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Food Webs
Chapter 17
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Figure 17_01
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Food Webs
Chapter 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
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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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Figure 17_06
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Figure 17_07
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Figure 17_08
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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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Keystone Species
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Figure 17_10
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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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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
• 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
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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 lesspreferred 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
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