Community Ecology

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Community Ecology

• Brooker Chapter 58

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Community Ecology

• Community Biodiversity
• Time, Area, and Productivity
• Estimating diversity
• Community Development
• Succession

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The Nature of Communities

• Communities vary in size
• Lack precise boundaries
• Rarely completely isolated
• Communities can be nested within communities
• A rotting log within a forest

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Community structure and functioning is complex
Connections to the size of the acorn crop
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Community Biodiversity

• Species richness
• Number of species within a community
• Species diversity
• Relative importance of each species within a
  community
• Diversity Indexes

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Species richness

• Number of species in each community
• Number of species of most taxa varies according
  to geographic range
• Increasing from polar to temperate to maximum in
  tropical areas
• Increases by topographical variation
• Reduced by peninsular effect (distance from the
  main body of land)

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• Mountains contain a large diversity of habitats
  hence, they support a greater biodiversity.

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Three hypotheses for polar-equatorial gradient

• Time hypothesis
• Temperate regions have more recently recovered
  from glaciations
• Resident species have not evolved to exploit
  vacant niches or species have not migrated back
  into now unglaciated areas
• Support more worms in comparable unglaciated
  lakes than glaciated
• Drawback limited applicability to marine
  organisms

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Glacial History of North America

• Pleistocene Ice Age began 110,00 ybp
• Glacial maximum 12,000 ybp
• Glacial melt raises sea levels and creates lakes
  and small seas

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• Area hypothesis
• Larger areas have more species because they can
  support larger populations and a greater range of
  habitats
• Support significant relationship between insect
  diversity and host tree range (species area
  effect)
• Problem there are not more species in Asia,
  tundra is largest biome but low richness, open
  ocean with largest volume has fewer species than
  tropical surface waters

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World Biomes
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3. Productivity hypothesis

• Greater production of plants results in greater
  overall species richness
• Support plants grow better where it is warm and
  wet and species richness in trees can be
  predicted by the evapotranspiration rate
• Problems some tropical seas have low
  productivity but high richness, sub-Antarctic
  Ocean has high productivity but low species
  richness

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• Why doesnt the species richness of trees
  increase in mountainous areas of the west as it
  does for birds?

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• Tropical rain forests are more species rich than
  northern regions of the world
• What are some factors that might account for this?

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Species Abundance

• Species richness is a useful measure (and can be
  misused)
• Species abundance tells us another dimension of
  community diversity
• Its take into account the population size of each
  species
• Identifies dominant species

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Species Richness and Abundance of a Swamp Forest
on Marylands Eastern Shore
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Forest Biodiversity

• Old growth forest in the Shenandoah Mountains of
  Virginia
• 50,000 trees represented by 10 species.
• Managed forest, recently clear cut
• 45,000 trees are maple and birch
• Only 1/10th of the forest is represented by the
  remaining 8 species

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Which forest is more diverse?

• Where is the diversity is more observable?
• The forest is more ecologically stable
• Each tree species provides food and shelter for
  different bird, animal, and insect species

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• Species diversity has two components
• Species richness how many different species are
  present in a habitat
• Relative abundance total number of individuals
  of each species present
• Diversity Indices
• Index 1 Species/ square root individuals
  emphasizes species richness
• Index 2 takes into account the relative
  abundance

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Calculating species diversity

• Shannon diversity index measures the diversity
  in a community
• pi proportion of individuals belonging to
  species i
• ln natural logarithm
• S is a summation sign

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• For a hypothetical community of 5 species and 100
  total individuals
• Values range for real communities falls between
  1.5 and 3.5
• Higher the value, the greater the diversity

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Table 58.1
http//www.afandpa.org/Template.cfm?sectionForest
ry
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Species richness and community stability

• Eltons diversity-stability hypothesis
  disturbances in a diverse or species-rich
  community would be cushioned by large numbers of
  interacting species and would not produce as
  drastic an effect as it would on a less diverse
  community
• 11 year study examined species richness and
  stability in grassland plots
• Found year-to-year variation in plant community
  biomass lower in plots with greater species
  richness

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• Are stable communities more diverse than unstable
  communities?
• Intermediate-disturbance hypothesis
• Highest diversities are maintained in communities
  with intermediate levels of disturbance
• At high rates of disturbance, only r-selected
  species would survive low diversity
• At low rates of disturbance, K-selected species
  would outcompete others low diversity

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• Fall of a tree creates a light gap in the
  rainforest canopy
• Direct sunlight is able to reach the rainforest
  floor
• Light gap is rapidly colonized by r-selected
  species which are well adapted for rapid growth
• While these pioneering species grow rapidly, they
  are overtaken by hardier K-selected species which
  fill in the gap in the canopy
• This happens often but in any one area with
  intermediate frequency

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Fire Ecology Yellowstone fires of 1988 Fire has
been used to manage marshes and forests
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Community DevelopmentEcological succession

• A process of change that results from disturbance
• Transition in species composition over ecological
  time
• Primary succession succession on a newly
  exposed site that was not previously occupied by
  soil and vegetation
• Secondary succession succession on a site that
  has already supported life but that has undergone
  a disturbance, such as a fire, tornado,
  hurricane, or flood

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Succession

• Primary succession
• Occurs on essentially lifeless terrain
• Volcanic eruptions
• Retreat of glaciers
• May take hundreds to thousands of years
• The first group of organisms to appear
• Autotrophic bacteria
• Lichens and mosses (spores)
• Grasses, ferns, shrubs, trees

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• Primary succession on Mount St. Helens

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• Glacier Bay used a specific example of
  facilitation as a mechanism of succession
• Over the past 200 years, glaciers have retreated
  100 km
• Succession has followed a distinct pattern of
  vegetation

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Secondary Succession

• Clements emphasized that succession had a
  distinct end point (climax community)
• Each phase of succession called a sere or seral
  stage
• Disturbance might set the community back to an
  earlier seral stage
• It then proceeded toward climax
• Each colonizing species made the environment a
  little different
• Facilitation colonizing species changed the
  environment so that it becomes more suitable for
  the next species

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• Alternative hypotheses to facilitation
• Inhibition early colonists may exclude
  subsequent colonists
• What gets there first determines subsequent
  community structure
• Primary method of succession in marine intertidal
  zone early successional species at a great
  advantage in maintaining possession of valuable
  space by removing the early colonist Ulva,
  Gigartina was able to colonize more quickly

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• Tolerance any species can start the succession,
  but the eventual climax community is reached in a
  somewhat orderly fashion
• Proposed as the middle ground between
  facilitation and inhibition hypotheses
• Study showed that succession in plant communities
  is determined largely by species that already
  exist in the ground as buried seeds or old roots
• Whichever species germinates first, or
  regenerates from roots, initiates the succession
  sequence

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Summary of Succession

• Key distinction between 3 models is in the manner
  succession proceeds
• Facilitation species replacement facilitated by
  previous colonists
• Inhibition species replacement is inhibited by
  previous colonists
• Tolerance species replacement is unaffected by
  previous colonists
• Other factors may also influence succession

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Secondary Succession

• Existing community has been disturbed
• Soil is intact
• Old field succession
• What factors determine the course of succession?
• Disturbance colonizers organisms with
• a high reproductive rate,
• good at dispersal
• weedy plant species
• Inhibition by competition
• Facilitation species pave the way for others

http//bioweb.wku.edu/faculty/Ameier/oldfield1.htm

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Old Field Succession
70th-100th Pine to Hardwood transition
3rd-18th year Young Pine forest
1st year Horseweed Crabgrass pigweed
19th-30th year Mature pine Forest Understory of
Young hardwoods
100th year plus Climax Oak-hickory forest
2nd year Asters Crab grass