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Food Webs
Chapter 17
1
Community Webs
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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
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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
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Paine suggested feeding activities of a few
species may have a dominant influence on
community structure.
 Suggested criterion for strong interaction
is degree of influence on community
structure.
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Strong Interactions and Food Web Structure
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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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Keystone Species
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Keystone species – species that, despite low
biomass, exert strong effects on the
structure of the community they inhabit
http://www.prairiedogs.org/keystone.html
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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Figure 53.15 Sea otters as keystone predators in the North Pacific
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Keystone Species
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Food Web Structure and Species Diversity
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Paine found as number of species in
intertidal food webs increased, proportion of
the foodweb 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
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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
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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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Fish as River Keystone Species
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Power investigated whether California roach
Hsperoleucas symmetricus and steelhead
trout Oncorhhyncus mykiss significantly
influence 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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Fish as River Keystone Species
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Effects of Predation by Birds on Herbivory
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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 peak when many
insectivorous birds are feeding their
young.
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Effects of Predation by Birds on Herbivory
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Birds reduce densities of insect larvae
feeding on dominant understory shrub.
 Higher densities of insect larvae translated
directly into higher levels of damage to
shrubs.
Marquis and Whelan found higher densities
of herbivorous insects on trees from which
predatory insects were excluded.
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Keystone Species: Summation
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Power : Keystone species exert strong
effects on their community structure, despite
low biomass.
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Exotic Predators
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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
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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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Seed Dispersal Mutualists as Keystone Species
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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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