INTRODUCTION

1

Food Webs

Chapter 17

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Outline

•Community Webs

Complexity and Structure

•Keystone Species

Effects on Diversity

•Exotic Predators

•Mutualistic Keystones

3

Community Webs

•Summerhayes and Elton studied feedingrelations on Bear Island in High Arctic.

Primary producers were terrestrial plantsand aquatic algae.

Fed on by several kinds of terrestrialand aquatic invertebrates.

Consumed by birds.

–Attacked by arctic foxes.

4

Arctic Food Web

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5

Food Web Complexity

•Winemiller described feeding relationsamong tropical freshwater fish.

Represented food webs in various ways:

Only included common species.

Top-predator sink.

Excluded weakest trophic links.

6

Strong Interactions and Food Web Structure

•Paine suggested feeding activities of a fewspecies may have a dominant influence oncommunity structure.

Suggested criterion for strong interactionis degree of influence on communitystructure.

7

Strong Interactions and Food Web Structure

•Tscharntke studied food webs associatedwith wetland reeds (Phragmites australis).

Attacked by fly Giraudiella inclusa.

Attacked by 14 species of parasitoidwasps.

Predator specialization

Distinguished weak and stronginteractions.

Determination of keystone species.

8

Keystone Species

•If keystone species reduce likelihood ofcompetitive exclusion, their activities wouldincrease the number of species that couldcoexist in communities.

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Keystone Species

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Food Web Structure and Species Diversity

•Paine found as number of species inintertidal food webs increased, proportion ofthe web represented by predators alsoincreased.

According to his hypothesis, higherproportion of predators produces higherpredation pressure on prey populations, inturn promoting higher diversity.

Removal of starfish (top predator)caused decline in diversity from 15 to 8species.

11

Consumers’ Effects on Local Diversity

•Lubchenko proposed to resolve the effectherbivores have on plant diversity, you needto know:

Herbivore food preference.

Competitive relationships between plantspecies in the local community.

Variance in feeding preferences andcompetitive relationships acrossenvironments.

12

Consumers’ Effects on Local Diversity

•Lubchenko studied influence of intertidalsnail (Littorina littorea) on structure of analgal community.

Snails fed on green (Enteromorpha spp.)and red (Chondrus crispus) algae.

Under normal conditions, Enteromorphaout-competes Chondrus in tide pools,and Littornia prefers Enteromorpha.

In the absence of snails, Chondrus iscompetitively 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 onyoung snails, preventing juveniles fromcolonizing tide pools.

Populations of Carcinus are controlled byseagulls.

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Consumers’ Effects on Local Diversity

Low snail density - Enteromorphadominates tide pool.

Medium snail density - Competitiveexclusion eliminated, and algal diversityincreased.

High snail density - Feeding requirementsare high enough that snails eat preferredalgae and less-preferred algae.

Algal diversity decreased.

16

Fish as River Keystone Species

•Power investigated whether California roachHsperoleucas symmetricus and steelheadtrout Oncorhhyncus mykiss significantlyinfluence food web structure.

Predatory fish decrease algal densities.

Low predator density increased midgeproduction.

Increased feeding pressure on algalpopulations.

–Thus, fish act as KeystoneSpecies.

17

Fish as River Keystone Species

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18

Effects of Predation by Birds on Herbivory

•Atlegrim studied influence of birds onherbivorous insects and insect-induced plantdamage.

Insectivorous birds may act as keystonespecies via effects on herbivorous insects.

Larval insect densities peak when manyinsectivorous birds are feeding theiryoung.

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Effects of Predation by Birds on Herbivory

•Birds reduce densities of insect larvaefeeding on dominant understory shrub.

Higher densities of insect larvae translateddirectly into higher levels of damage toshrubs.

•Marquis and Whelan found higher densitiesof herbivorous insects on tress from whichinsects were excluded.

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Keystone Species: Summation

•Power : Keystone species exert strongeffects on their community structure, despitelow biomass.

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Exotic Predators

•Exotic species have dramatic impacts oncommunities because they were outside theevolutionary experience of local preypopulations.

Nile Perch (Lates nilotica) exotic fishpredator 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 LakeVictoria fish community coincide with otherecosystem changes.

Dissolved oxygen concentrationssignificantly decreased.

Cultural eutrophication.

25

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 sitessafe from predators and fire.

Argentine ants have displaced manynative ant species that disperse largeseeds.

Substantial reductions in seedlingrecruitment by plants producing largeseeds.

26

Review

•Community Webs

Complexity and Structure

•Keystone Species

Effects on Diversity

•Exotic Predators

•Mutualistic Keystones

27

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