Ecological Community

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Ecological Community

• Common useage an assemblage of species that
  occur together in the same place.
• The coexisting species interact through
  exploitative and mutualistic interactions.
• But,
• there are two historical views of what a
  community actually represents.

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Holistic Community Concept

• 1. An organized, integrated ecological unit.
• Species are inextricably interconnected
• Species have evolved to enhance the
  interdependent functioning of the species.
• If true, communities would be discrete entities
  with recognizable boundaries.

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Individualistic Community Concept

• 1. Community structure and functioning can only
  reflect interactions of independent, individual
  species.
• No discrete organization above the species level
• Natural selection acts on the reproductive
  differential of individuals, not on the group.
• Each population in a community evolves because of
  the reproductive success of its individuals not
  to benefit the community as a whole.

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Distinct boundaries seen
Either 1. abrupt changes in physical
environment 2. one species dominates the
environment The edge of its range determines
distribution limits of other species.
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3. Plants may maintain an ecotone
chemically. E.g., coniferous forest Organic acids
released from decomposing needles thick layer
of Humus.
Ecotones less likely to develop
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Continuum concept
temperature
rainfall
suggests different evolutionary history
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Community dominants Great Smoky Mountains No
distinct ecotones
Present focus 1. extent of community
integration 2. its biological interactions 3. its
recovery from disturbance
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Development of Communities

• Succession gradual specific changes in a
  community that are sometimes predictable and
  orderly
• Primary succession when plants become
  established on substrates lacking organic matter.
• Secondary succession plants become established
  on substrates previously occupied by plants.

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• Early view succession was deterministic
• a predictable sequence of communities leading to
  a final stable community resistant to change.
• Each transitional community in succession was
  called a sere or seral stage
• First colonizers represent the pioneer stage
• However, disturbance can reset stages

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• Succession progresses toward a climax stage with
  K-selected species dominating the site.
• Climax community attained dictated by climate and
  edaphic factors.
• Early key assumption
• each invading species makes the environment more
  suitable for a replacement species.
• This process is known as facilitation

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Space for time
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chance
Disturbance chance
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Example of Primary Succession

• Glacial retreat around Glacier Bay across 200
  years
• Tills and moraines (deposits of stones and
  pulverized rock) left - serving as rudimentary
  soil low in nitrogen and organic matter

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• Pioneers a black crust of cyanobacteria
  lichens, liverworts, and horsetail.
• Cyanobacteria fix nitrogen modest increase in
  soil nitrogen but organic matter is minimal.
• Willow sp., Dryas, Alder sp., and spruce
  seedlings occur, but distribution is sporadic.
• Dryas drummondi (nitrogen fixation) dominates
  after about 40 years
• Progressive increase in soil nitrogen and organic
  matter.

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• Dense, closed thickets of nitrogen-fixing alder
  at ca. 60 years.
• Nitrogen surpluses accumulate in the soil and
  litter increases.
• Invasion of spruce seedlings

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• Spruce-needle litter turns the soils acidic
• Shading causes inhibition of alder and many
  original, understory species.
• Hemlock begins to flourish.
• Ca. 200 years
• 1) Well drained soils a climax of spruce and
  hemlock results
• 2) Poorly drained soils forest invaded by
  Sphagnum mosses, which accumulate water and
  further increase soil acidity
• Muskeg bog

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Seral stage
Spruce,
Dryas 7.0 5.3 7.3 2.8
Alder 8.8 21.8 6.8 277
Spruce 15.1 53.3 3.6 261
Pioneer 5.2 3.8 7.2 1.5
Western hemlock,
Soil depth (cm) Soil N (gm ) Soil pH
-2
Litterfall
Liverworts Lichens River beauty Epilobium
(rare) Horsetail Equisetum Cyanobacteria
Mountain avens, Dryas drummondi
Alder, Alnus sinuata
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200
40
60
Time (years) since retreat of glacier
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• Facilitation
• Dryas and alders increase soil nitrogen levels
• Facilitates the invasion of spruce
• But, alder shades out Dryas inhibition
  (competitive exclusion)
• Eventually spruce shades out alder inhibition
  (competitive exclusion)
• Facilitation, originally thought to control the
  entire process of succession, important in early
  stages.
• Competition was important in the later phases of
  succession

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Huerfano Butte, Pima County, Arizona
1902
1969