11

Food Webs

Chapter 17

22

Outline

Community Webs Complexity and Structure

Keystone Species 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:

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.

Attacked by 14 species of parasitoid wasps. Predator specialization

Distinguished weak and strong interactions. Determination of keystone species.

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Keystone Species

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: 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 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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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.

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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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.

Seed Dispersal Mutualists as Keystone Species

linepithema humile

3131Winged adult and worker of Solenopsis geminata fire ants.

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Fig. 17.21

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