Transcript 17Molles5e
Species Interactions and Community
Structure
Chapter 17
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Outline
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Community Webs
Complexity and Structure
Indirect Interactions
Keystone Species
Effects on Diversity
Mutualistic Keystones
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Community Webs
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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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Indirect Interactions
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Indirect interactions are the effects of one
species on another through a third species
Examples:
Trophic cascades (Chapter 18)
Indirect commensalism
Apparent competition
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Indirect Commensalism
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One species indirectly benefits another
species (through a third species) while it is
neither helped or harmed
Example: Martinsen, Driebe, Whitham
(1998)
Beavers fell cottonwood trees which then
produce stump sprouts
Beetles prefer consuming high nutrition
sprout leaves
Beetles grow larger, faster and utilize
defensive compounds found in leaves
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Apparent Competition
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Negative effects between two competitors
who share a predator or herbivore
One species may facilitate the presence or
increase the abundance of the predator
which suppresses the second species
Orrock, Witter, Reichman (2008)
Exotic plant Brassica nigra sheltered
mammals which increased herbivory on
native bunchgrass Nassella pulchra
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Indirect Interactions
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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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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 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
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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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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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Mutualistic Keystones
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Power : Keystone species exert strong
effects on their community structure, despite
low biomass.
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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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Review
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Community Webs
Complexity and Structure
Indirect Interactions
Keystone Species
Effects on Diversity
Mutualistic Keystones
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