Transcript Species Interactions and Community Structure

Food Webs
Chapter 17
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Outline
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Community Webs
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Keystone Species
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Complexity and Structure
Effects on Diversity
Exotic Predators
Mutualistic Keystones
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Fig. 17.2
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Food Web Complexity
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Winemiller described feeding relations among
tropical freshwater fish.
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Represented food webs in various ways:
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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 Web
Structure
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Paine suggested feeding activities of a few
species may have a dominant influence on
community structure.
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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) NE
Germany.
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Attacked by fly Giraudiella inclusa.
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Attacked by 14 species of
parasitoid wasps.
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Predator specialization
Distinguished weak and strong
interactions.
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Determination of keystone species.
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Keystone Species
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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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Food Web Structure and Species
Diversity
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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.
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Removal of starfish (top predator) caused decline in
diversity 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
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Lubchenko proposed to resolve the effect
herbivores have on plant diversity, you need
to know:
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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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Lubchenko studied influence of intertidal snail
(Littorina littorea) on structure of an algal
community.
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Snails fed on green (Enteromorpha spp.) and red
(Chondrus crispus) algae.
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Under normal conditions, Enteromorpha out-competes
Chondrus in tide pools, and Littornia prefers
Enteromorpha.
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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.
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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
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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.
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Algal diversity decreased.
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Fig. 17.9
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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.
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Predatory fish decrease algal densities.
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Low predator density increased midge production.
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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
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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.
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Nile Perch (Lates nilotica) exotic fish predator in
Lake Victoria.
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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.
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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.
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Seed-dispersing ants bury seeds in sites safe from
predators and fire.
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Argentine ants have displaced many native ant species that
disperse large seeds.
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Substantial reductions in seedling recruitment by plants
producing large seeds.
linepithema humile
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Fig. 17.21
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