This lecture describes community structure and the food web

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Biology 205
Ecology and Adaptation
Lecture 13
community 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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