Community ecology

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					      Unit III: Community Ecology
        & larger-scale ecology
• A. Introduction: Determinants of community structure & species
  diversity (biodiversity)
• B. Spatial patterns in community structure: latitudinal gradients
  in biodiversity
• C. Temporal patterns in community structure: response of
  ecosystems to disturbance
    – Introduction to concepts ecosystem response to disturbance
    – Examples of succession: start thinking about mechanisms
    – Summarize observations: hypotheses on what causes
      successional changes
    – Ecosystem attributes as they change through time
           Ecological response:
         Two “types” of succession

• Primary succession [definition]:
• A substrate that does not have any living
  organisms or legacy of former living organisms
  Large disturbance [but below biosphere disturbance]

• Secondary succession [definition]:
• A substrate has been disturbed but contains a
  legacy of organisms that lived their before
  Smaller disturbance
         Succession applies to community structure and
                   all ecosystem processes
• Succession is the response to disturbances that affect
  single ecosystems or communities within them.
• Succession does not involve evolution.
• Succession is relatively predictable.
• Succession is causes by suites of ecological processes
  and can be predicted by ecological principles.
• Succession normally occurs at a demographic time scale,
  but the time scale in some ecosystems overlaps with long-
  term climate change (no simple equilibrium)

Large, rare disturbances

  start primary succession
         Mt. St. Helens

Before                    After
Raw devastation right after the eruption

  Nothing alive left here! All parent material (pre-soil)
Mt. St. Helens: a model system for studying
     primary & secondary succession
                                                        5 years later

“At first glance, this ash covered clear cut…appeared
completely lifeless following the eruption.”
Fireweed one of the first colonizing species at Mt.
St. Helens (any significance to this name??)
All living things dispersed from somewhere else:
Life history strategies?
Primary/secondary? succession on Mt. St. Helens

   “Blow down” forest 1983 (3 years after eruption).
Primary/secondary? succession on Mt. St. Helens

       “Blow down” forest 1989 - 6 years later
Primary/secondary? succession on Mt. St. Helens

      “Blow down” forest 1994 - 11 years later
First species to appear on Mt. St. Helens


                        Nitrogen fixing plants are
                        among the first to grow on
                        volcanic substrates-(think
                        about mechanisms)
Glaciers also start primary succession

  As this glacier in Alaska retreats, it leaves a bare
  surface for primary succession
Succession starts when the glacier retreats

                     Glacial morraines and till

                   Nothing left alive; no legacy of
                   “life” after a glacier has passed
                   over. “Till” is composed of large
                   gravel rocks = “parent material,
                   not “soil”.
 Global warming has caused many glaciers to
     retreat quickly in the past 100 years.

Same outcrop
               Succession has proceeded quickly in
               60 years here.
       Toboggan Glacier

1909                       2000
              Rapid succession
                                            If we could get in a
                                            time machine, we’d
                                            see all these
                                            changes over about
                    Parent material ages    500 years.
Glacier melting
                                            Instead of waiting
                                            around for 500 years,
                                            an ecologist works
                                            with a geologist to
                                            age different surfaces
  Pioneer species                           (in time since glacial
     establish                              retreat).
Different species replace each other over time   When does
                                                 change stop?
                  Hypotheses about succession
• Basic ecological questions:
• When can a population “invade” and establish when rare?
• Mathematically, can dNi/dt > 0, when Ni  0?
• How long can a population persist? What are the mechanisms of
• Mathematically, can Ni* > 0, Ni*  Ki for large T?
• Can a population resist invasion by other species?
  Mathematically, can Nj < 0 or Nj*  0 when Ni  Ki?
•   Where Ni and Nj are population densities of the ith and jth species
• When we know that the environment (abiotic and biotic) are
  changing with time, T.
•   We want to know what the steady state is--not only attributes of
    community structure and ecosystem processes, but also names of the
    A SERE = a sequence of plant communities in
            succession at one location
                                       seral stage
                         “pioneer”                    “climax”
Abundance of
each species

                          early           mid            late
                       successional   successional   successional


                                                         No change
                                                         after this point


 Community composition as a balance between the ability of new
 species to invade and established species to resist invasion.
 Think about attributes of species that will be successful as time passes.
           Mechanisms of succession
• Allogenic: Change comes from progression of physical
  (abiotic) processes:
   – Weathering of “parent material” (lava, ash, glacial till,
     talus, etc.) by wind and water
• Autogenic: Change comes from biotic processes
   – Soil is created by first living organisms interacting with
     weathered parent material.
   – includes life history traits or interspecific interactions
     as populations establish through time.
• Primary succession starts with allogenic, proceeds to
• Secondary succession starts with autogenic.
            Autogenic processes
• Facilitation: some species arrive earlier and
  make it easier for species to invade later.

• Inhibition: some species arrive earlier and
  make it harder for species to invade later.

• Tolerance: no interactions, some species
  tolerant early conditions, some species tolerate
  late conditions--may depend on life histories,
  but NOT on species interactions.
   Summary of autogenic models of succession
                                                         Figure 20.20
Primary succession

Secondary succession
                       Initial floristic composition     Floristic relay
   Floristic relay
      Facilitation            Tolerance                Inhibition
                   Clement’s “floristic relay”
Clements 1916

                                            Inhibition predominates

                Facilitation predominates
             Egler’s Initial Floristic Composition
Egler 1954      Tolerance model predominates

                                               All species are
                                               always present--they
                                               live out their lives
                    Glacier Bay, Alaska

Detailed example in Chapter 20
Primary succession:Using physically separate locations with different histories
to infer patterns and mechanisms of succession
   Dryas                   Alders

                                                Primary succession post-glacial
                                                Retreat in Glacier Bay, AK
                                                (Chapin et al. 1994)

  “pioneer” forbs                   Spruce
           Mechanisms of succession - Glacier Bay

Effect on spruce seedlings through time 
Ecosystem attributes change with time
                           Species richness and
                              diversity increase

                             Figure 20.3
    Primary succession: Early stages allogenic and
              then autogenic-facilitation
        Soil depth

                     C horizon = parent material

Soil development begins with “weathering” of parent material;
Proceeds with development of organic components = true soil
            Soil properties through time
N & water holding
capacity increases….               Figure 20.11

Phosphorus &       N limiting in early stages, P in later
pH decrease
Ecosystem attributes through time: Nitrogen

      Nitrogen fixers
Ecosystem attributes through time:
     Nitrogen v Phosphorus

                     Nitrogen fixing plant
                     species common in
                     early succession

                     Organic accumulation
                     results in increasingly
                     more nitrogen available
                     to plants

                     Phosphorus becomes
                     depleted with time bc of
Hurricanes can start primary succession or
         secondary succession

            Hurricane Katrina
Andrew’s damage in the Everglades
                  Old field succession
 Crop monoculture -- poor at resisting invasion and succession.
 Takes energy and chemical input to stop succession
 High disturbance system

Low diversity, but high productivity, which is what humans want!
Climax community Old growth forest
                Very resistant to invasion; is self
                Also highly vulnerable to

                Biodiversity might be low but
                includes species found nowhere

                            Spotted Owl
Species richness increases at first and then levels off

                                     Figure 20.5:
                                     Number of bird

Figure 20.4:
Number of
woody plant
Fire disturbance maintains grasslands in the
    central US & Canada and central Asia
Fire is an essential component to maintain the natural
structure of tall grass prairie communities.
Fire eliminates trees and cycles nutrients into the prairie
“sod” the mixture of grasses and wildflowers.
Humans like to use grassland biomes for their
Humans replace fire as a disturbance with agricultural
activities like plowing and harvesting.
                 Tall grass prairie
                                Big Bluestem

Tallgrass prairie has high
biodiversity of perennial
grasses and herbaceous plants
(wild flowers).
 Fire promotes biodiversity
 Fire disturbs the winners of
Fire makes new space and
releases C,N and other
nutrients used by the
     Bison & fire

 Fire and grazing are
 Effects of fire are
 patchy because of
 grazing by bison

Urine (N) deposition
Intermediate Disturbance Hypothesis
       Often applies well to succession
         Humans can cause both high and
            intermediate disturbance


      Late                                 Early
      Succession      Mid-Succession       Succession

                   Disturbance frequency
Algae species in a stream fit the IDH

 Algal species diversity H’

                              Figure 20.8
 Shorelines are constantly starting primary succession

Energy from waves is constantly
transporting and depositing
parent materials (sand)

New surfaces
Succession begins
 Plate 5.8

Dune succession
in the Algarve
region of Portugal

First plants

First plants
stabilize shifting
sand, add
organic matter.
Next set of
species can
                  Shoreline dune succession:
        Clements’ Floristic Relay fits most dune seres well
                                                     Endpoint depends
                                                     on regional climate
   r-selected life histories                   K-selected life histories
                  Question: do early species affect the
                  ability of later species to establish,
Disturbance! survive and reproduce?

    Short lifespan, small size,           Good competitors, resist
    high reproduction & high              invasion: long lifespan, low
    dispersal ability.                    reproduction, low range of
      Low biodiversity,                    Higher biodiversity,
      high productivity.                   high standing crop
       World famous ecologist studying plant
             succession at Mono Lake
                          Exposure of
                          new shoreline

                                                     500 yr former shoreline

Pioneer plants 10 yr.

                                          Studying seedling establishment
                                          on 50 year shoreline

Prof Toft with students
    Primary succession is commonly dominanted
    by facilitation in early stages.

                                       Pioneer “greasewood” facilitates
 50 year shoreline                     establishment of other species
 still dominated by                    by building dunes.

                                After weathering of dunes,
                                rabbitbrush can establish

Rabbitbrush can only establish after pH of
sand goes “down” to 8 or 9.
            Ecosystem development:
      shoreline dunes& old field succession
                                                    Species “turnover” high
                                                    in early stages
                                                    “Productivity” high
                                                    r-selected life histories

                                                     Biomass = “standing
                                                     crop” accumulates
                                                     through time
                                                     Diversity often highest
                                                     at intermediate time
                                                     K-selected life histories

Eventually competition for resources occurs at “climax” steady state
             Productivity vs. efficiency
Productivity increases       Biomass
at first, may level off or   continues to
decline                      increase

                                        Low “turnover” of
                                        biomass in later stages =
                   Summary of
            disturbance & succession
• Biodiversity increases with time, overall, in
• Moderate disturbance can provide the greatest
  biodiversity because it prevents competitive
• Succession’s endpoint depends on the climate
  ultimately (Biodiversity varies with the climate).
• The endpoint of succession is ecological stability
  because the community structure persists through
  time. [biodiversity and ecological stability are related]

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