The Nature of Communities

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The Nature of Communities
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The Nature of Communities

• What Are Communities?
• Community Structure

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Introduction

• Two-way species interactions considered
• competition
• predation (and herbivory)
• parasitism
• commensalism
• mutualism
• In reality
• all species experience multiple interactions
• These interactions shape the communities in
  which they live.

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What Are Communities?
Communities are groups of interacting species
that occur together at the same place and time.

• Interactions among multiple species produce an
  entity that supercedes the sum of its parts.
• In practice, biological or physical guidelines
  used to define a community.
• 1. physically defined communities all species
• in a sand dune
• in Clear Creek
• in Clear Creek under the Lowell Blvd. overpass
• in Vogel Canyon of southeastern Colorado

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Figure 15.3 A Defining Communities
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• 2. Biologically defined communities all species
  associated with
• a kelp forest
• a maple forest
• a coral reef.
• Dominant species the basis for the community
  delineation
• kelp
• Sugar Maple
• coral

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Figure 15.3 B Defining Communities
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• Taxonomic identification of species problematic.
• Therefore, counting all species difficult to
  impossible.
• Therefore, ecologists usually consider a subset
  of species when they define and study
  communities.
• 1. Taxonomic affinity
• Aspidoscelis tesselata, A. sexlineata, and their
  arthropod prey (Orders).
• 2. Guildsgroups of species that use the same
  resources.
• 3. Functional groupspecies that function in
  similar ways, but do not necessarily use the same
  resources.

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Figure 15.4 Subsets of Species in Communities
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• 4. Food webs
• Ecological organization of species based on their
  trophic or energetic interactions.
• Functional (not taxonomic) classification.
• Trophic levels defined by groups of species that
  have similar ways of obtaining energy
• Producers
• Primary consumers
• Secondary consumers
• Tertiary consumers
• Food webs provide little information about the
  strength of interactions or their importance in
  the community.

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• Some species span two trophic levels, and some
  species change feeding status as they mature.
• Some species are omnivores, deriving energy from
  more than one trophic level.

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Figure 15.5 Four-Level Food and Interaction Webs
Interaction webs more accurately describe both
the trophic (vertical) and non-trophic
(horizontal) interactions in a traditional food
web.
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Concept Species richness and species diversity
are important descriptors of community structure.

• Community structure is the set of characteristics
  that shape communities.
• Species richnessthe number of species in a
  community.
• Species evennessrelative abundance of the
  various species.
• Species diversity combines species richness and
  species evenness.

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• Example Two communities with four species each
  (species richness is equal).
• In community A, proportion of yellow species is
  0.85 proportion of each of the other three
  species is 0.05.
• In community B, each species has the same
  proportional abundance 0.25. This community has
  higher diversity.

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• There are several quantitative species diversity
  indices. The one most commonly used is the
  Shannon index

pi proportion of individuals in the ith
species s number of species in the community
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• Oh! Oh! Species diversity is often used to mean
  the number of species in a community.
• Biodiversity describes the diversity of
  important ecological entities.
• These entities can span multiple spatial scales
  alleles
• species
• communities
• Implicit in biodiversity is the
  interconnectedness of all components of diversity.

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Figure 15.7 Biodiversity Considers Multiple
Spatial Scales
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• Species diversity indices
• Used to compare different communities.
• Rank abundance curves used to provide information
  on commonness or rarity of species.
• The proportional abundance of each species (pi)
  is plotted relative to the others in rank order
  (most abundant, next most abundant, etc.).

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• Relative abundances can suggest the types of
  species interactions that might occur.
• Example In Community A, the dominant species
  might have a strong negative effect on the three
  rare species.
• Experiments that add or remove species are used
  to explore such hypotheses.

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• An example.
• Two soil bacteria communities in pastures in
  Scotland.
• One pasture had been fertilized regularly.
• Bacteria species can be identified quickly using
  DNA sequencing of 16S ribosomal DNA. The bacteria
  can then be grouped using phylogenetic analysis.
• Species diversity and rank abundance curves were
  determined.
• McCaig et al. (1999) found 22 phylogenetic groups
  of bacteria.

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Figure 15.9 Bacterial Diversity in Pastures in
Scotland
Both pastures had very similar community
structure. A few species were abundant most
species were rare.
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Community Structure

• Species accumulation curvesspecies richness is
  plotted as a function of the total number of
  individuals that have been counted with each
  sample.
• These curves can help determine when most or all
  of the species in a community have been observed.

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Figure 15.10 When Are All the Species Sampled?
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Community Structure

• As additional samples are taken, additional
  species will tend to be sampled.
• At some point, the curve will reach a threshold
  at which no new species are added despite
  additional sampling.

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Community Structure

• Hughes et al. (2001) compared species
  accumulation curves for 5 different communities
• Temperate forest in Michigan.
• Tropical bird community in Costa Rica.
• Tropical moth community in Costa Rica.
• Bacterial community from a human mouth.
• Bacterial community from tropical soils.

The 5 communities varied significantly in the
amount of sampling effort necessary to determine
their species richness.
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• Species compositionthe identity of species
  present in the community.
• Two communities could have identical species
  diversity values, but have completely different
  species.
• The identity of species is critical to
  understanding community structure.