Biology 3500: Ecology

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Lecture 11

• Biology 3500 Ecology
• Chapter 13 Community Structure

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Learning Objectives

• Define
• Community
• Guild
• Discuss
• Various species interactions
• Calculate
• H and J (note bring a calculator to test!)
• Recall
• The vertical zones within ponds, forests

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Community

• Strictly (ie for any test), all the species in
  an area
• May interact with eachothere
• A subset of the larger ecosystem
• Never contains abiotic factors
• Less formally sense, a collection of
  taxonomically similar species within an ecosystem
• Bird community
• Insect community
• Plant community

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Guild

• Like a community, a grouping of species
• Different from community, in that species are
  grouped by function or resource
• Birds feeding on insects would be an insect
  feeding guild
• Smaller plants in a forest might be an
  understory guild
• Detritovores are a guild

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Species Interactions

• Within a community, species effects on eachother
  may be
• Mutualism both species benefit
• Commensalism one benefits, other neutral
• Competition both are harmed
• Amensalism one harmed, other neutral

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Other Species Interactions

• These are all positive for one, negative for the
  other
• Predation one eats, others die
• Many prey eaten over lifetime
• Parasitism one eats, other lives for a while
• Only one host, usually not killed
• Parasitoidism one eats, other dies
• Only one host ever

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Dominance

• Those species who are most important
• What qualifies as most important?
• Amount of space
• Biomass
• Affect on the community
• The number of individuals

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Keystone Species

• Any organism whose affect on the community is
  tremendous, but whose numbers are small
• Typically NOT a dominant species
• Examples
• Similar to police, firefighters (few in number,
  but important to human community)
• Starfishes are few, but removing them causes
  massive change (an ecological collapse of
  structure) in their community

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Relative Abundance

• A simple measure of a species dominance
• Given as a percentage of the community
• Can be
• Percentage cover
• Biomass
• Number of individuals

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One Virginia Forest

• Sampling all trees having at least 10 cm dbh
• 24 species represented (shown in book, but not
  here)
• Who are the dominant species here?
• Note most species in the world are rare

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Second Virginia Forest

• Sampling all trees having at least 10 cm dbh
• 10 species represented
• Who are dominant species here?

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Questions

• Which forest had more species?
• Which had better eveness among the species?

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Richness vs. Diversity

• Both richness and diversity describe communities
• Species richness the number of species in a
  community
• Species diversity takes into account
• Number of species
• The evenness of their frequencies

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Shannon Index

• Attempts to quantify diversity of a community
• H -S (pi)(ln pi)
• Where pi is the proportion of individuals
  belonging to that species

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Evenness

• Evenness in a community is given by J
• J is calcluated as a ratio of H and Hmax
• Hmax is the most that diversity COULD be

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Example on Board

• Richness same in both
• Which is more even?
• Calculate H and J for both communities

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7th Inning Stretch
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Food Chain

• Food chains are simple descriptions of energy
  flow through an ecosystem
• Grass to grasshopper to sparrow to hawk

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Food Web

• Much more realistic
• Involve several food chains
• Many, many links!
• Q who is dominant, who is keystone?

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Forest Vertical Structure

• From the top
• Canopy
• Understory
• Shrub layer
• Herb layer
• Forest floor
• Important for decomposition

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Vertical Distribution of Birds
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Pond Vertical Structure

• Epilimnion
• Well-mixed water on top
• Metalimnion
• Has a thermocline (steep drop in temperature)
• Hypolimnion
• The most dense water here (Q what is the coldest
  it gets?)

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Pond Vegetation

• Emergent vegetation near the edge of the pond
• Floating plants
• Submerged plants

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Pond Zones by Lighting

• Photic zone extends down only as far as light
  can support photosynthesis
• Aphotic zone all areas deeper than that