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Biology 2483A Final: Unit 5 - Communities (Autosaved) cause internet

Course Code
BIOL 2483A
Hugh Henry
Study Guide

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Unit 5: Communities
Chapter 16: The Nature of Communities
We have mostly looked at two-way relationships when in reality, species experience multiple interactions that
shape the communities in which they live
What are Communities?
Communities are groups of interacting species that occur together at the same place and time
In practical terms, ecologists usually define communities based on physical or biological characteristics
oA physically defined community might encompass all the species in a sand dune, a mountain stream, or
a desert
oA biologically defined community might include all the species associated with a kelp forest, a
freshwater bog, or a coral reef. This approach emphasizes the importance of an abundant species, such
as trees
Ecologists often define a community arbitrarily based on questions they are asking
oE.g. a study of marine invertebrates in seagrasses might restrict the definition of the community to that
interaction, and not include mussel-eating birds, etc.
Counting all the species in a community is difficult to impossible, especially if small or unknown species are
Ecologists usually consider a subset of species when they define and study communities.
oSubsets of species can be defined by
Taxonomic affinity: e.g. all bird species in a community
Guild: group of species that use the same resources
Functional group: species that function in similar ways, but do not necessarily use the same
Food Webs
Food webs organize species based on trophic or energetic interactions
Trophic levels:
oPrimary producers: autotrophs e.g. plants and algae
oPrimary consumers: herbivores
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oSecondary consumers: carnivores
oTertiary consumers: carnivores
Food webs tell little about the strength of interactions or their importance in their community
oSome species span two trophic levels and some change feeding status as they mature
oSome species are omnivores, so feed on more than one trophic level
Food webs do not include nontrophic interactions (horizontal interactions such as competition)
Interaction webs more accurately describe both the trophic (vertical) and non-trophic (horizontal) interactions
than a traditional food web
Community structure
Community A has low species evenness; B has high species evenness (based on yellow mushroom)
Community structure: set of characteristics that shape communities
oSpecies richness: the number of species in a community
oSpecies evenness: relative abundances compared with one another
oSpecies diversity: combines species richness and species evenness
The most commonly used species diversity index is the Shannon index:
Pi: proportion of individuals in the ith species & S: # of species in the community
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oShannon index shows the difference in species evenness among communities
Biodiversity describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the
interconnectedness of the components
oGenetic diversity affects the viability of populations
oWhich in turn affects species diversity in a community
oWhich influences the diversity of communities at larger scales
Graphical representations of species diversity can give an explicit view or commonness or rarity
oRank abundance curves plot the proportional abundance of each species (pi) relative to the others in
rank order
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