ENVS 10 Lecture 11

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ENVS 10Lecture 11

• Ecological Succession and Disturbance Regimes

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Ecological Stability

• Ecological communities are relatively stable over
  long periods of time
• Climax community
• a relatively stable, long lasting community
• The final or last step in a series of predictable
  temporary stages (communities) leading up to the
  climax community

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Ecological Succession

• A series of predictable temporary communities or
  stages leading up to a climax community
• Each stage/temporary community is called a
  successional stage or seral stage
• Each step prepares the land for the next
  successional stage

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2 Types of Ecological Succession

• Primary succession
• A community of plants and animals develops where
  they did not exist previously
• Ex- tops of mountains, newly exposed volcanic
  rock, rocks exposed by glaciers
• Begins with substrate (soil) only.

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2 Types of Ecological Succession

• Secondary succession
• A community of organisms is disturbed by a
  natural or human-related event (i.e., fire,
  grading, etc.)
• The community returns to an earlier successional
  stage and continues in the predictable series
  towards the climax community

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Example of primary succession

• Yosemite Valley
• Step 1 Formation of the Valley
• Uplift creates the Sierras
• Water flowing down the mountains creates a
  V-shaped valley

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Primary succession (cont)

• Step 2 Formation of a lake
• During the last ice age, a glacier made its way
  down the valley, changing the shape from a V to
  a U
• Material picked up and transported down the
  valley in the glacier is deposited at the mouth
  of the valley when the glacier melts
• The deposited material functions as a dam at the
  mouth of the valley, trapping water from the
  melted glacier and river behind it

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Primary succession (cont)

• Step 3 Formation of a meadow
• The rivers leading into the lake in the bottom of
  Yosemite Valley brought sediment downstream
• Eventually, the deposits filled up the entire
  lake (providing the soil to this community)
• As the lake became more and more shallow, plants
  began to grow, resulting in the formation of a
  wet meadow
• This first community is called a pioneer community

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Primary succession (cont)

• Step 4 Meadow with riparian area
• As more and more sediment is deposited in the
  valley floor, the meadow becomes more and more
  dry
• The Merced River is formed as water is confined
  to a specific course
• More traditional (terrestrial) species of plants
  are established along the creek and in the meadow

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Primary succession (cont)

• Step 5 Conifer forest
• Douglas fir and other conifers begin to invade
  the meadow
• Eventually, the entire valley floor would be
  inhabited by conifer trees it would become a
  forest

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Example of Secondary Succession

• Redwood forest
• Assumption primary succession has already
  created a functioning redwood forest community
• Step 1 Disturbance
• A fire comes through the forest
• Many species in the understory are lost
• Some trees die and fall over
• Many trees live and continue to grow

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Example of Secondary Succession (cont)

• Step 2 Establishment of earlier successional
  stage
• The canopy of the redwood forest is opened up,
  allowing light to penetrate to the forest floor
• Smaller understory plants (i.e., huckleberry, tan
  oak, rhododendrons, etc.) begin to grow
• This creates a brushy patch in the forest

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Example of Secondary Succession (cont)

• Step 3 Re-establishment of Redwood forest
  (climax community)
• Redwoods begin to sprout in the open area created
  by the fire, under the brushy species
• Redwoods continue to grow, eventually becoming
  taller and more dominant than the brushy species
• Redwoods will eventually shade-out the other
  plants and re-establish a redwood forest

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Disturbance-adapted communities

• In nature, certain communities experience
  somewhat predictable events that cause
  disturbance and return a community to an earlier
  successional stage
• Ex- Flooding, fires, etc.
• Often, plants and animals in these communities
  are adapted to deal with these disturbances.
  This ensures their survival through time.

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Disturbance-adapted communities (cont)

• Example
• Redwoods adapted to fire
• Thick, spongy bark protects them from damage
• Limbs fall off as the tree gets taller,
  eliminating ladder fuel
• Growth patterns increase survival when trees are
  young
• Stump-sprouting ensures that the tree can grow
  again

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Disturbance-adapted communities (cont)

• Example
• California red-legged frog (California threatened
  species) adapted to seasonal drought
• California has hot, dry summers
• Red-legged frogs metomorphose just about the same
  time that the ponds and creeks are drying up
• Frogs hop out of the pond and seek refuge in
  forests, cracks in the soil, ground squirrel
  burrows, etc. to survive until the next rains

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Disturbance can be caused by humans

• Examples of disturbance
• Fire
• Fire suppression
• Development
• Agriculture
• Grazing
• Recreational use
• Pollution
• Habitat fragmentation

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Community Stability

• How much disturbance can a community take and
  still return to its original equilibrium?
• Depends on its stability
• Persistence resistance to alteration
• Constancy ability to maintain size
• Resilience ability of system to restore self

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For a given area, will the same climax community
be reached each time it is disturbed?

• Depends on how much the system is disturbed
• Depends on the type of disturbance
• System that returns can be incomplete (i.e.,
  missing important aspects of the original
  community)
• System that returns may be completely different
  than the original community

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What can we do to return the community to its
original state?

• Restoration the process of re-establishing the
  original structure and function to a
  community/ecosystem that has been disturbed
• Scientists work to repair the land. Work
  includes
• Planting native plants
• Removing non-native plants
• Re-establishing hydrology, soils, etc.