Download Species Interactions and Community Structure

Food Webs
Chapter 17
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Outline

Community Webs


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

Winemiller described feeding relations among
tropical freshwater fish.

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

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) NE
Germany.

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.

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

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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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

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

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

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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.

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.

Predatory fish decrease algal densities.
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Low predator density increased midge production.

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

Power : Keystone species exert strong
effects on their community structure, despite
low biomass.
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Exotic Predators

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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Exotic Predators

Kaufman pointed out changes in Lake
Victoria fish community coincide with other
ecosystem changes.


Dissolved oxygen concentrations significantly
decreased.
Cultural eutrophication.
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Seed Dispersal Mutualists as Keystone
Species

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.
linepithema humile
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Fig. 17.21
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