Species Interactions and Community
Structure
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Community Webs
 Complexity and Structure
Indirect Interactions
Keystone Species
 Effects on Diversity
Mutualistic Keystones
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Figure 17_02
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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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Winemiller food web
Lowland stream in Venezuela; S > 88
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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
Tscharntke studied food webs associated
with wetland reeds (Phragmites australis).
 Attacked by fly Giraudiella inclusa.
 & 14 species of parasitoid wasps.
 Blue tit
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Figure 17_05
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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
 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
E.g., Martinsen et al. (1998)
 Beavers cut cottonwood trees - trees
produce stump sprouts

Beetles prefer sprout leaves
 Beetles grow larger, faster and use
defensive compounds in leaves
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Figure 17_06
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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 presence or
increased abundance of a predator which
suppresses the second species
Orrock et al. (2008)
 Exotic plant Brassica nigra shelters
mammals which increases 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
increase the number of species that coexist
in communities.
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Food Web Structure and Species Diversity
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Paine found: increased number of species in
intertidal food webs = proportion of
predators also increased.
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His hypothesis = higher proportion of
predators produces higher predation
pressure on prey populations, promoting
higher diversity.
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Keystone Species
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Paine experiment:
Remove top predator
Monitor for 2 years
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Consumer Effect on Local Diversity
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Lubchenko proposed to resolve the effect
herbivores have on plant diversity
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: influence of intertidal snail
(Littorina littorea) on algal community.
 In Lab: snails prefer green (Enteromorpha
spp.) over red (Chondrus crispus) algae.
 In field, Enteromorpha out-competes
Chondrus in tide pools.
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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, eat young snails, =
no juveniles in tide pools.
 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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Mary Power: do California roach
Hesperoleucas symmetricus and steelhead
trout Oncorhhyncus mykiss influence food
web structure?
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Fish as River Keystone Species
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Mary Power experiment
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Cages with or w/out
predator fish, allow insects
in
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Fish present = low algae
No fish = high algae
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Why?
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Mary Power predictions:
Predatory fish decrease algal
densities.
 Low predator density increases midge
production.
 Increased feeding pressure on algal
populations.
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Thus, fish act as Keystone
Species.
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Mutualistic Keystones
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Power : Keystone species exert strong
effects on 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.
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Seed-dispersing ants bury seeds in sites
safe from predators and fire.
 Argentine ants displaced native ant
species that disperse large seeds.
 = reductions in seedling recruitment by
plants producing large seeds.
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= Exotic species is mutualistic
keystone species
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Keystone Nutrient Recycler
Idea that a species that cycles
large amount of nutrients can
affect community
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Upstream Sura
43 fish species found here
Frugivores, insectivores, piscivores, herbivores
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Alfaro
Archocentrus
Astyanax
Terrestrial
insects
Astatheros
Aquatic
insects
Insects
Priapichthyes
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•Astyanax = tetra that excretes 10X P
of entire community
= Keystone Nutrient Recycler
5 common
species
Astyanax
Astatheros
Archocentrus
Priapichthys
Alfaro
P excreted SRP
81
0.3
0.2
2.2
8.6
40.5
0.1
0.1
1.1
3.4
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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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