Communities of interacting species

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Communities of interacting species How do we
characterize them?
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How do we define a community?
Assemblages particular taxa within a community
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Ecotones are areas where environmental gradients
are steep, and thus, species composition changes
abruptly.
Think about what we mean by steep and shallow
environmental gradients. Does human perception
matter?
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Open community
Evidence from sampling lake sediments
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Ecotones can also be environmental gradients we
cant see!
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Food webs as an alternative way to characterize
communities

• Emphasize trophic and interaction connections
  among species in a community

Functional groups Descriptions of the feeding
role of each species (producers, 1 2 3
consumers) Trophic levels Tend to be numbered
levels from producers (1) up
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Simplified food web
Functional group
Trophic level
4 3 2 1
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Types of food webs
Connectedness
Energy flow
Functional or interaction
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Are all species equally important in functional
(interaction) food webs?
Keystone predator (species)
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Keystone predators and community structure
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Keystone species concept

• Some consumers can control community structure
  (and diversity)
• These consumers are called keystones because
  when they are removed the entire community
  reorganizes
• Need experimental removals (or additions) of
  particular species to test this

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Whats the difference between dominant species
and keystones?
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Changes in distribution of large N. American
mammals
Laliberte and Ripple (2004) BioScience
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Trophic cascades
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Maintenance of kelp forests by trophic cascade
Should killer whales also be considered keystones?
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Food web complexity and omnivory
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Bottom-up and Top-down control of community
structure
Relative importance of predation and production
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Top-down and bottom-up control of plankton
communities
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How do we measure the diversity of a community?
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Species richness Count of the number of species
in a community
Whats the richness of this community?
The rarest species counts the same as the most
abundant
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Species diversity Many different ways to
calculate, but all of them require knowing number
and relative abundance
Evenness Heterogeneity
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Species richness versus species diversity
Species diversity accounts for the relative
abundance of species (Simpsons,
Shannon-Weiner)
Evenness