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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