Chapter 17
• Food webs and species diversity may be closely linked
• Mutualistic relationships and species diversity may be linked
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Community Webs (Fig. 17.2)
Summerhayes and Elton studied feeding relations on Bear Island in High
Arctic.
– Primary producers were terrestrial plants and aquatic algae.
– Fed on by several kinds of terrestrial and aquatic invertebrates.
– Consumed by birds.
– Attacked by arctic foxes.
Arctic Food Web
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Food Web Complexity (Fig. 17.3)
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.
Strong Interactions and Food Web Structure
• Paine suggested feeding activities of a few species may have a dominant
influence on community structure.
– One approach would be energy flow
– Which species would be the most important given this criterion?
– Alternative criterion is degree of influence on community structure.
– What happens when we remove a species?
Strong Interactions and Food Web Structure (Fig. 17.5)
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Remove the blue tit, what happens?
Remove A. geminipuncta, what happens?
Remove A. gratus, what happens?
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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, Species Diversity
and Intertidal Organisms (Fig. 17.6)
As species richness increases, proportion of the web represented by predators
also increased.
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– Do all predators use similar resources?
Increased predator diversity produces increased predation pressure.
– Are all prey types equally affected?
Promotes higher diversity.
Paine and Intertidal Predators (Fig. 17.7)
Consumers’ Effects on Local Diversity
To understand 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.
Consumers’ Effects on Local Diversity (Fig. 17.8)
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.
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.
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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Keystone Species: Summation
Power : Keystone species exert strong effects on their community structure,
despite low biomass.
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Exotic Predators (Fig. 17.15)
Exotic species have dramatic impacts on communities because they were
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outside the evolutionary experience of local prey populations.
Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria.
Fish fauna dramatically reduced.
Exotic Predators
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Seed Dispersal Mutualists as Keystone Species (Fig. 17.18)
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.