Succession

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Succession

• Chapter 18

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Succession Mt. St. Helens

• Mt. St. Helens erupts 832 am on May 18, 1980.
• Blast zone felled trees over a 600-km2 area.
• Avalanche of mud destroyed everything in its
  path.

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Succession Mt. St. Helens

• Succession of area
• Using new information, found evidence of 400
  prehistoric avalanches.
• Scientists learned how important stochastic
  (chance) events are in recovery.
• Traditional dogma area should be slow to
  recovery.

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Succession Mt. St. Helens

• Truth Some species were quick to colonize the
  area.
• Most seeds were blown in by wind.
• Dead wood was a bonanza for wood-boring insects,
  and woodpeckers and finches.
• Most lakes were covered in ice, protecting the
  inhabitants.
• Algae in lakes bloom and more frogs.

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Succession Mt. St. Helens

• 120 new lakes were created.
• Salamanders thrived especially in the new lakes.
• Blast zone converted area into a meadow habitat.
• Favorable for gophers, which then built new
  tunnels.
• Salamanders used tunnels to get to new lakes.

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Succession Mt. St. Helens

• Succession can be speeded up by chance events or
  indirect interactions.

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Development of Communities

• Succession gradual changes in a community that
  may be predictable and orderly.
• Primary succession when plants invade an area in
  which no plants have grown before.
• Often plants must build up soil, so the process
  can take hundreds of years.

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Development of Communities

• Secondary succession a modification of
  longer-term primary succession. It does not
  occur on bare ground, but on partially cleared
  land.

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Development of Communities

• Frank Egler, studies of secondary succession at
  Alton Forest (1950s).
• Most species already existed in the ground (seed
  bank or old roots).
• Rate of root regeneration or seed germination
  governed order in which species appeared.

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Development of Communities

• Eventually, larger, slower growing trees would
  outcompete smaller pioneer species.
• Egler called his theory the initial floristic
  composition model.

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Development of Communities

• Frederic Clements, father of successional theory
  (1916, 1936) emphasized succession as a
  deterministic phenomenon, with a community
  proceeding to some distinct end point or climax.
• Each unit in succession was called a sere or
  seral stage.
• Initial seral stage is termed the pioneer seral
  stage.

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Development of Communities

• A disturbance could return a later stage to an
  earlier one.
• The community progresses toward a climax stage.
• Climax community for any given region was
  dictated by climate and soil conditions.

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Development of Communities

• Succession governed by abiotic disturbances is
  termed allogenic.
• Succession dominated by biotic disturbances, such
  as herbivores eating later seral species, is
  called autogenic.

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Development of Communities

• Key assumption each invading species made the
  environment a little different, so that it
  becomes more suitable for K-selected species,
  which invade and outcompete earlier residents.
  This process is known as facilitation.

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Succession and Alaskan Glaciers

• Succession following the retreat of the glaciers
  fit the Clements idea of facilitation.
• Over 200 years, glaciers in the Northern
  Hemisphere have undergone dramatic retreats.

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• Ecological sequence of succession in Glacier Bay.

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Facilitation

• As glaciers retreat, they leave tills and
  moraines, which are deposits of stones and
  pulverized rock, respectively, and serve as soil.
• Bare soil has low nitrogen content and organic
  matter.

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Facilitation

• In the early stages, area is first colonized by a
  black crust of blue-green algae, lichens,
  liverworts, horsetail, and the occasional river
  beauty.

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Facilitation

• Blue-green algae is a nitrogen fixer, thus soil
  nitrogen levels increase
• Organic matter is still minimal.
• A few seeds and seedlings of willow, Dryas,
  alders, and spruce, occur, but they are rare in
  the community

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Facilitation

• After about 40 years, the nitrogen- fixing Dryas
  drummondi comes to dominate the landscape.
• Soil nitrogen has increased, and so has the soil
  depth and litter fall.

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Facilitation

• After about 60 years, the nitrogen-fixing alder
  forms dense, close thickets.

• Excess nitrogen produced by nitrogen-fixing
  bacteria, and not used by the alder, accumulates
  in the soil.
• Level of soil nitrogen increases dramatically, as
  does litter fall.

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Facilitation

• After about 75-100 years, spruce trees begin to
  overtop the alders, shading them out.
• Litter fall is still high.
• Large amount of needles turn the soils acidic.
• Shade causes competitive exclusion of many of the
  original understory species.
• Western hemlock and mountain hemlock begin to
  occur.

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Facilitation

• After 200 years, a mixture of spruce and hemlock
  results the climax community.

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Facilitation

• Spruce-hemlock is the climax community, if the
  soil is well-drained.
• If soil is poorly drained, the forest is invaded
  by Sphagnum mosses, which accumulate water and
  further acidify the soil.
• Most trees die due to lack of soil oxygen.
• Some lodgepole pine are found.
• This climax community is called a muskeg bog.

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Facilitation

• Evidence of facilitation
• Ecologists suggested that each species
  facilitated the entry into the community of the
  next species.

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Facilitation

• Chapin et al. (1994) showed that facilitation in
  the Glacier Bay region occurs only during part of
  the process.
• Dryas and alders dramatically increase soil
  nitrogen levels. The increase facilitates the
  invasion of spruce.
• On the other hand, alder shades out Dryas.

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Facilitation

• Competition is evident after 50 years, where
  Sitka spruce begins to shade out alder.
• Facilitation, originally thought to fuel the
  entire process of succession, was only important
  in the establishment stages. Competition was
  important in the later phases of succession.

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Facilitation

• Decomposition of plant material (e.g. logs).
• Edwards and Heath (1963).
• Oak leaves in nylon bags in soil studied
  decomposition rates.
• Nylon bags had different size meshes, which could
  vary the size of decomposers entering the bags.

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Facilitation

• The larger the mesh, the more complete the
  decomposition.
• In small-meshed bags, microorganisms were unable
  to decompose the leaves.

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Facilitation

• In the soil, earthworms are most important in the
  initial decay process.
• Thus, on a small scale, facilitation occurs in
  the decomposition of plant material.

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Facilitation

• Teresa Turner (1983) - Facilitation in the marine
  intertidal zone off the Oregon coast.
• Algae facilitated the succession of surfgrass,
  because surfgrass seeds became attached to the
  algae species and could then grow.

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Inhibition

• Many types of succession do not show elements of
  facilitation.
• Possession of space is all important.
• Principle mechanism affecting succession is the
  inhibition of subsequent colonists.

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Inhibition

• Facelli and Facelli (1993).
• Removed litter of giant foxtail, Setaria faberii
  (an early successional species in old fields).
• Removal increased the biomass of a later
  species,daisy fleabane, Erigeron annuis.

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Inhibition

• Conclusion release of a phytotoxin from
  decomposing foxtail litter or physical
  obstruction by the litter may contribute to the
  inhibition of daisy fleabane.

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Inhibition

• Inhibition dominant method of succession in
  marine intertidal zones, where space is limited.
• Early successional species are at a great
  advantage in maintaining possession of valuable
  space.

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Inhibition

• Wayne Sousa (1979)
• Scraped rock faces clean and out put new rocks
  and concrete blocks.
• First colonists green algae Ulva and
  Enteromorpha.
• Later, replaced by large brown algae and finally
  by red algae.
• By removing Ulva from the substrate, brown and
  red algae were able to colonize more quickly.

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Inhibition

• How does this occur in nature?
• Ulva is subject to herbivory by the crab
  Pachygrapus and is also susceptible to drying
  out.
• The middle species, brown algae, is commonly
  overgrown by epiphytes.
• Red algae is not susceptible to either of the
  above.

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Tolerance Other Patterns of Succession

• Connell and Slatyer (1977) a continuum from
  inhibition to facilitation.
• Facilitation model each species makes the
  environment more suitable for the next.
• Inhibition model early colonists tend to prevent
  subsequent colonization by other species.

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Tolerance Other Patterns of Succession

• Tolerance model any species can start
  succession, but the eventual climax community is
  reached in a somewhat orderly fashion.
• Evidence Eglers (1954) work on floral
  succession.
• Many floral communities, most species are present
  at the outset. Whichever species germinated
  first or grew from roots, would start the
  successional sequence.
• Works only for secondary succession.

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Tolerance Other Patterns of Succession

• John Lawton (1987) Fourth model of succession,
  random colonization.
• Succession proceeds by chance alone.
• Governed by who arrives first, and happens to be
  present when favorable conditions prevail.
• Ex. Mt. St. Helens succession.

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Tolerance Other Patterns of Succession

• Secondary succession in the Piedmont Plateau,
  North Carolina.
• Different patterns could govern different seral
  stages.

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Tolerance Other Patterns of Succession

• Henry Oosting (1942) and Catherine Keever (1950).
• Old growth forest were replaced by agriculture.
• Fields were abandoned, and trees returned.
• Continuum of ages in the fields of the region
  used to study succession.

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Tolerance Other Patterns of Succession

• In most communities, there is a gradual
  overlapping of species over relatively long
  periods of time. In the Piedmont little
  overlapping occurred.

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Tolerance Other Patterns of Succession

• One year after fields were abandoned, there were
  35 species recorded all annual and perennial
  species. Two species dominated crabgrass and
  horseweed.
• Early seres, rotting horseweed inhibits the
  growth of aster inhibition model.

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Tolerance Other Patterns of Succession

• In the second year, same species present, but now
  aster and ragweed dominant. 26 new species were
  present.
• Aster stimulated the growth of broom sedge
  facilitation model.

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Tolerance Other Patterns of Succession

• In the third year, species richness declined. A
  third species, broom sedge, became dominant, and
  remained so for several years. During this time,
  seeds of pines and some hardwoods arrived via
  wind dispersal.
• Arrival of hardwood seeds via wind is by chance
  random-colonization model.

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Tolerance Other Patterns of Succession

• Fifth year, pines became established, and a
  closed canopy by year 10.
• Pine seedlings have a difficult time surviving
  under the canopy but hardwoods thrive.
• By 100 years, there are equal numbers of
  hardwoods as pines.
• By 200 years, only scattered pines remain.

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Restoration Ecology

• How to restore a community, after the habitat has
  been altered restoration ecology.
• Restoration ecology is in its infancy.
• Some cleanup methods appear to cause more damage
  than good.
• Ex. Oil spill from tanker, Torrey Canyon. Cleanup
  methods caused more damage to indigenous biota
  than the oil did.

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Restoration Ecology

• Foundation of an economical and successful
  restoration program is a clear understanding of
  the environment, plants, animals, and people
  involved in it.

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Restoration Ecology

• Steps in a restoration program
• The knowledge of why a species or community
  disappeared in the first place.
• An understanding of the natural history of
  similar ecosystems.
• Test plots.

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Restoration Ecology

• Soil preparation solorization.
• Revegetation
• Advanced techniques pest control, irrigation,
  fertilizer.
• The reintroduction of animal components.

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Patterns in Species Richness During Succession

• Eugene Odum (1969) and Fahriki Bazzaz (1979)
  summarized general trends in succession.

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Patterns in Species Richness During Succession

• Species in early seral stages.
• Often wind dispersed, seeds can live for a long
  time, maximizing their chances for successful
  colonization.
• Acquire nutrients quickly, grow fast, and reach
  maturity at low biomass.

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Patterns in Species Richness During Succession

• Plant species richness generally increases as
  succession proceeds.
• Ex. Old fields in Minnesota
• The older the field, the greater the number of
  species (more time for species to colonize).

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Patterns in Species Richness During Succession

• Animals and succession
• Animals are simple followers of succession.
• Johnston and Odum (1956) examined the richness of
  bird communities.
• Grasslands 2 bird species.
• Grass shrub 7 bird species.
• 35-year old pine forest 10 bird species.
• Old pine forest (60-100 years) 18-20 bird
  species.
• Climax oak-hickory forests 19 bird species.

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Patterns in Species Richness During Succession

• Climax communities can experience a decrease in
  richness.
• Val Brown and G. Edwards-Jones plant species
  richness increased only in middle seres.
• Ex. At climax, a birch woodland, species richness
  drops dramatically.
• Ex. Insect richness also decreased dramatically.

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Patterns in Species Richness During Succession

• Animals can affect succession.
• Crab herbivory in intertidal areas.
• In Africa, elephants are prolific grazers,
  maintaining open savannas.
• In marine systems, fish grazing can deflect
  succession.

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Biotic Interactions Succession

• Walker and Chapin (1987) biotic processes that
  can affect succession.
• Mechanisms promoting seed dispersal are more
  important to primary succession, whereas buried
  seeds and surviving vegetative propagules are
  more important to secondary succession.
• Stochastic variation is more important in severe
  and low-resource environments.

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Biotic Interactions Succession

• Facilitation is more important in severe
  environments, in primary succession, and in early
  stages of community development, where low levels
  of nutrient, shade, and water may prevail.
• Competition is probably more widespread than
  facilitation, especially in more favorable
  environments.

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Biotic Interactions Succession

• Maximum potential growth is particularly
  important in favorable environments, in which
  resources are sufficient to promote rapid growth.
• Differences in longevity among plant species are
  more important in older communities.

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Biotic Interactions Succession

• Mycorrhizae may be especially important in severe
  environments, where plants need extra help to
  survive.
• Insect herbivores and pathogens are more
  important on mature vegetation.
• Mammalian herbivores are more important in early
  and midsuccessional communities.

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Summary

• Primary succession occurs when species invade an
  area in which no organisms are present, such as
  land unearthed after receding glaciers.
  Secondary succession is the change in species
  composition following a change in land usage,
  such as the reversion of old fields to forests
  after agriculture has stopped.

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Summary

• Early theories of succession viewed the entire
  process as facilitative, in which each species
  makes the environment more suitable for the next.

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Summary

• Later work on succession revealed the existence
  of inhibition, wherein early colonists actually
  prevented colonization by other species.

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Summary

• In 1977, Connell and Slatyer recognized the
  existence of a third type of succession, which
  they termed tolerance, which was intermediate
  between the other two. In this model, any
  species can start the succession, but the
  eventual climax community is reached in a
  somewhat orderly fashion.

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Summary

• In 1987, John Lawton proposed the
  random-colonization model of succession, which
  posits that there is no facilitation or
  competition and that succession proceeds by
  chance alone.

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Summary

• In succession, early communities are often
  species poor and later communities are species
  rich. However, this is not always the case.

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Summary

• Recently, it has been recognized that the path of
  succession can be modified by herbivory, disease,
  and other factors. Thus, there is no single
  universal cause of succession, which is a
  multifaceted process.