Today 1 More evidence for Connell's rule 2 Exceptions to Connell's rule 3 Change in determinants of community structure along environmental ...

Today…

1) More evidence for Connell’s rule

2) Exceptions to Connell’s rule

3) Change in determinants of community structure
   along environmental gradients

4) Alternative states of communities

5) Ecological succession (?)

III) Connell and the experimental revolution

                                                                Width of “bar”
                                                                represents
                                                                strength of
                                                                importance

Consequences:
 1) “Connell’s rule”: upper limits set by physical processes, lower limits set
    by species interactions

 2) The dawn of appreciation and exploration of experimental field ecology

III) Connell and the experimental revolution
 3) Importance of predation in determining zonation
    Robert Paine 1966, 1974
 a) System / Pattern:
  i) Rocky intertidal in Pacific Northwest (Olympic Peninsula)
  ii) Mytilus californianus (M) - California mussel
   - dominant in mid-intertidal
     - why not higher? Assumed desiccation
     - why not lower? Hmmm…

   - lower limit remarkably stable
   - mussels can migrate, and settle below adult distribution
     - settlement may not be so important

III) Connell and the experimental revolution
  3) Importance of predation in determining zonation
     Robert Paine 1966, 1974
  a) System / Pattern (cont’d):
    iii) Pisaster ochraceus (P) - Ochre star
      - main predator on mussels
      - occurs mainly in lower intertidal
             - upper limit maybe set by desiccation?
  b) General hypothesis:
     i) Lower limit of Mytilus set by predation by Pisaster
  c) Specific hypothesis:
     i) In areas where Pisaster is removed, Mytilus
         distribution will expand lower

Rocky Intertidal Zonation

       mussels

   gooseneck barnacles

    acorn barnacles,
    tunicates, sponges,
    anemones

    pink corraline algae

           Pisaster

3) Importance of predation in determining zonation
   Robert Paine 1966, 1974
d) Test:
  i) Removed Pisaster from lower intertidal at two sites
  ii) Replicate “control” area at each site with no
      removals
  ii) Issue with design: without within-site
     replication of removal and control, how
     distinguish treatment and area effects????
e) Results:
   i) Over several years, Mytilus distribution extended
      down into lower intertidal zone
   ii) Where Mytilus extended into lower intertidal zone,
       species diversity declined… another story

3) Importance of predation in determining zonation
  Robert Paine 1966, 1974

f) Conclusions:
  i) Predation sets lower limit of mussels
  ii) Supports general paradigm that biotic interactions
      set lower limits of distribution in intertidal

3) Importance of predation in determining zonation
     Robert Paine 1966, 1974
 g) Postscript:
     i) After experiment ended, Paine quit removing Pisaster, but
         cont’d to sample sites:
              high

Lower limit
    of                                            Tatoosh site
  Mytilus

              low
                                                   Mukkaw site
                      removals
                                  time
     a) At one site, lower limit moved back up as Pisaster reinvaded
     b) At other site, it did not!!! WHY???
     c) Mussels larger at Mukaw by end of experiment

3) Importance of predation in determining zonation

g) Postscript:
   ii) Two important implications:

   a) Experimental design: site-site variability can mask experimental
        results --> more replication at the scale of sites

   b) Patterns: Distributions can be the result of temporary
       environmental conditions (in this case the reduction of
       Pisaster) referred to as “History” or “Legacy” Effects often
       resulting from episodic events

   - mussels move or recruit to lower intertidal, grow and escape
      predation by their greater size

   - Another example, southern California species that recruit to and
       remain in central California during episodic El Niños

III) Connell and the experimental revolution
 4) Exceptions to the paradigm (of upper and lower limits)
  a) Upper limits determined by physical factors?
    Underwood and Jernakoff 1981, Oecologia

    a) System: Grazing limpet and foliose macroalgae in intertidal
         of Australia.
    b) Pattern: Grazer occurs in zone above the alga that it feeds on.

                mid
                  lower

III) Connell and the experimental revolution
 4) Exceptions to the paradigm (of upper and lower limits)
   Upper limits determined by physical factors?
  c) General (alternative) hypotheses:
  - grazing determines upper limit of foliose algae
  - physical factors determine upper limit of algae
  - both grazing and physical factors…
  - anything else - e.g., spores don’t settle above upper limit of algae

  d) Specific hypotheses:
  - areas cleared and caged from grazers in mid-intertidal will become
      colonized by foliose algae
  - areas shaded will become colonized by foliose algae
  - areas both cleared of grazers and shaded will become colonized by
      algae

III) Connell and the experimental revolution
 4) Exceptions to the paradigm (of upper and lower limits)
   Upper limits determined by physical factors?
  e) Test:
  - full cage (with roof) provides shade and excludes grazers
  - roof only provides shade only
  - cage with no roof (“fence”) only excludes grazers
                                                        grazers
  - open is control

                                                    roof        full
                                                    only        cage
                                         shade
                                                     open       fence

III) Connell and the experimental revolution
 4) Exceptions to the paradigm (of upper and lower limits)
   Upper limits determined by physical factors?
  f) Results:
  - algae colonized the grazer exclusions (“fences”), but not the roof-only or
       the open plots ( grazers effects) any shade effects on abundance?
  - fences:
    - algal cover reached 100% but never lived long enough to reproduce
    - higher cover due to continuous recolonization by new spores
  - algae grew and survived to reproduce only in the (full cages - with roof)
  - algae never occurred in open plots
                                  algae response        algal reproduction
                  grazers

                roof   full          no     yes            no      yes
                only   cage                                                interaction
     shade
                open                  no    yes            no      no
                       fence

III) Connell and the experimental revolution
 4) Exceptions to the paradigm (of upper and lower limits)
    Upper limits determined by physical factors?
 f) Conclusions:
 - upper limit not set by limited settlement (rather, post-settlement
     mortality)
 - upper limit set by biotic interaction!!
 - upper limit of reproduction set by interaction between grazers and
     physical stress (physical factors effect grazer effect)
                                     algae         algal reproduction
                   grazers

              roof     full        no        yes     no     yes
              only     cage                                         interaction
     shade
              open                 no    yes         no      no
                       fence

III) Connell and the experimental revolution

 4) Exceptions to the paradigm (of upper and lower limits)

    Lower limits determined by biological factors?

  a) Intertidal organisms adapted to marine and terrestrial habitats
  b) Though most studies find that lower limit set by biotic
       interactions…
  c) Exceptions:
       - Littorina (snail) limited to very high intertidal and will die if
       submerged too long
       - Two macroalgae, Selvitia and Fucus, die if submerged too long
  d) Few studies have tested this!!!!

IV) Horizontal patterns of distribution and abundance
    1. Variation in relative importance of ecological
       processes - Bruce Menge, 1976, Ecology
    a) Background: Have focused on vertical zonation
       what about horizontal gradients?
    b) System: barnacles, mussels, algae in New England
       rocky intertidal
    c) Patterns: Along a gradient from exposed to protected
       sites…
 CHARACTERISTIC      EXPOSED SHORE         PROTECTED SHORE
 Dominated by        Mussels (Mytilus)     Fucoid algae
 Free Space          Rare (

IV) Horizontal patterns of distribution and abundance

   d) General hypotheses:
     i)   Competition and predation important in determining
          these patterns, but
     ii) Importance of C and P differ in exposed and protected
         sites

   e) Specific hypotheses (experimental design):
    Complicated design using cages and cage controls to assess effects of:

     i)   competition: barnacles, mussels, and algae
     ii) predation / grazing
     iii) exposure: importance and how it varied along gradient
     iv) all areas initially cleared

IV) Horizontal patterns of distribution and abundance

    f) Results: Exposed Shores - all about competition
                                                                       Cage               Cage
                                                                     (-Fucus,         (-Predators,
         Control               Open                 Cage
                                                                      -predators,       -grazers)
     (no manipulation)       (-Fucus)         (-Fucus, -predators)
                                                                      -mussels)
         Mussels           Barnacles                                                         Mussels
                                                Barnacles
                                    Mussels                               Barnacles   Barnacles
                                                          Mussels

           Algae (Fucus)                                                                  Fucus

         Time                 Time                  Time                Time             Time

 At exposed sites - same pattern for both Fucus and predator removals (cages)
                          and Fucus removals alone (open areas).
 a) barnacles colonize then are out-competed by mussels (no additional effect of
         predators: see open areas)
 b) If mussels are also removed then barnacles persist.
     - note pattern is similar to that in protected shores.

IV) Horizontal patterns of distribution and abundance
    f) Results: Protected Shores – predation precludes competition
                                                                     Cage               Cage
         Control            Open                  Cage             (-Fucus,         (-Predators,
     (no manipulation)    (-Fucus)          (-Fucus, -predators)    -predators,       -grazers)
                                                                    -mussels)
          Algae (Fucus)                                                                    Mussels
                                              Barnacles
                          Barnacles     `               Mussels         Barnacles   Barnacles

         Mussels              Mussels                                                   Fucus

        Time               Time                   Time                Time             Time
At protected sites - differences between cages with Fucus and predator removals and
                            Fucus removals (open areas).
a) barnacles colonize and persist in low numbers outside of cages
b) barnacles are out-competed in cages by mussels
c) mussel abundance is kept low by predators
d) barnacles persist in high number if you remove Fucus, mussels and predators.
Predator (only) removals - If you remove only predators (including grazers) algae and
barnacles colonize but get out-competed by mussels.

IV) Horizontal patterns of distribution and abundance
   g) Conclusions:
  Different processes are important at exposed and protected sites:

  a) at exposed sites, predation/grazing unimportant - competition is the primary
           organizing force in the system.
           1) Predators are generally uncommon
           2) Mussels are competitively dominant (over algae and barnacles)

  b) at protected sites, predation important in preventing competition
           1) with predation barnacles dominate if Fucus is removed
           2) without predation mussels out-compete barnacles and algae
           3) predation keeps competition from occurring with mussels (mussel
           abundance is kept low). What about competition between barnacles
           and Fucus? Fucus outcompetes barnacles to dominate area
  c) Importance of predation varies with exposure; at exposed sites predators
      are uncommon, their feeding ability is reduced because they have to spend
      more time hanging on and not feeding (because the predators and grazers
      are snails & sea stars?)

IV) Horizontal patterns of distribution and abundance
                                                                                      Physical
 h) More generally:                                        Predation
                                                                       Competition    Processes
                                                       High

  General paradigm of                      Importance to
  community organization in                community
  rocky intertidal                          organization

  (see Connell 1975, Menge and
                                                        Low
  Sutherland 1976, Menge 1976,
  Lubchenco and Menge 1978,                                Benign                         Severe
  Underwood and Denley 1984)                                  Environmental harshness
 A) In habitats with relatively benign physical environments - predation structures communities

 B) With increasing environmental harshness - predation efficiency is decreased and
   competition becomes a major process structuring communities

 C) With even greater environmental harshness - importance of competition decreases and
   physical processes become more important.

 D) Local escapes from predation (in benign environments) or physical stress (in harsh
   environments) cause patchiness in the community.

IV) Horizontal patterns of distribution and abundance
 2) Alternative stable states - Lubchenco, J. 1978 Ecology

  a) Background: Why might sites exhibit different stable
     communities in the absence of environmental differences?
  b) System: grazing snail and algae in New England rocky
     intertidal
  c1) Patterns: spatial variation in community structure:
Habitat     Littorina      Enteromorpha   Chondrus/Fucus    Diversity
Tidepools   common         rare              common         low
Tidepools   intermediate   intermediate      intermediate   high
Tidepools   rare           common            rare           low

Rock        common         rare              common         low
Rock        intermediate   rare              common         intermediate
Rock        rare           uncommon          common         high

IV) Horizontal patterns of distribution and abundance
 2) Alternative stable states - Lubchenco, J. 1978 Ecology

  c2) Patterns: spatial variation in species diversity
     varies as a function of grazer density and habitat
     type:

            Tidepools                  Rock (emergent)
     High                           High

      Low                           Low
        Low             High           Low           Higher
         Littorina abundance           Littorina abundance

IV) Horizontal patterns of distribution and abundance
 2) Alternative stable states - Lubchenco, J. 1978 Ecology

  d) Hypotheses:
    i) Littorina prefers to eat Enteromorpha
    ii) Enteromorpha out-competes other algae in tidepools (if no littorines)
    iii) Littorina can suppress competitive abilities of Enteromorpha in tidepools
    iv) Enteromorpha is competitively inferior on emergent rock surfaces

 Habitat       Littorina        Enteromorpha       Chondrus/Fucus         Diversity
 Tidepools     common           rare                  common              low
 Tidepools     intermediate     intermediate          intermediate        high
 Tidepools     rare             common                rare                low

 Rock          common           rare                   common             low
 Rock          intermediate     rare                   common             intermediate
 Rock          rare             uncommon               common             high

IV) Horizontal patterns of distribution and abundance
 2) Alternative stable states - Lubchenco, J. 1978 Ecology

  e) Design: Why might sites exhibit different stable
     communities in the absence of environmental differences?

   i) Assessed food preferences of littorines

   ii) manipulated density of littorines in pools and rock surfaces

2) Alternative stable states - Lubchenco, J. 1978 Ecology
   f) Results:
   i) Enteromorpha favored algae of littorines (in pools and on rock)
   ii) Patterns from pools…
              Control               Littorine addition                   Littorine removal
      (littorines common)              (rare before)                     (common before)
    High    Chondrus             High                             High
                                                                                         Enteromorpha
                                            Enteromorpha

                                                    Chondrus
           Enteromorpha
    Low                          Low                              Low                    Chondrus
              Time                         Time                              Time
Pools - results and conclusions
1) Enteromorpha can out-compete Chondrus, but
2) High densities of littorines can suppress effects of Enteromorpha
3) Intermediate densities of Littorina allow coexistence of most species
4) Littorines are a keystone species but maximum effect on diversity occurs at intermediate densities
Rock - results and conclusions
1) Enteromorpha competively inferior - but still favored prey
2) Fucus (mid) and Chondrus (low) are superior competitors
3) Littorines effect is to graze an already uncommon species (Enteromorpha and other ephemerals)
4) Predation on uncommon species speeds up competitive exclusion and acts to reduce species diversity

IV) Horizontal patterns of distribution and abundance
 2) Alternative stable states - Lubchenco, J. 1978 Ecology

  c2) Patterns: spatial variation in species diversity
     varies as a function of grazer density and habitat
     type:

            Tidepools                  Rock (emergent)
     High                           High

      Low                           Low
        Low             High           Low           Higher
         Littorina abundance           Littorina abundance

V) Maintenance of species diversity
 1. Ecological succession
 A) Definition: the sequential, predictable change in
    species composition over time following a disturbance
    - Primary succession – succession starts from a completely empty
    community (i.e. bare substratum) such as that following glaciations
    or a volcanic eruption
    - Secondary succession – when the majority of individuals are
    removed by a disturbance of lesser intensity, often leaving
    propagules (seeds, spores, larvae) only (e.g., flooding, forest fire)
    - Change in community will, given sufficient time, result in a climax
    community, in which the competitive dominants will prevail
 B) Why is there succession?
    i) Species differ in life history characteristics
    ii) Species cannot optimize all characters, so there appears to be
         trade-offs among characters that influence how a species
         responds to a disturbance

1. Ecological succession
C) Comparison of early and late successional species:
   Life History               Early Successional          Late Successional
   Character                    (“r-selected”)             (“K-selected”)
   Reproduction               semelparous (once)          iteroparous (multiple)
   Fecundity                  high                        low
   Dispersal ability          good-long                   poor-short
   Growth rate                fast                        slow
   Life span                  short                       long
   Generation time            short                       long
   Competitive ability        POOR                        GOOD
- not all species fit these categories but it is a useful general scheme
- Early species – good at dispersing to and colonizing newly disturbed sites,
   grow rapidly, reproduce and are out-competed.
- Late species – poor at dispersing to and colonizing newly disturbed sites,
   grow slowly and out-compete earlier species.

1. Ecological succession
D) Models of succession: (Connell and Slatyer 1977 American Naturalist)
i) Facilitation: early species modify the environment…

    - make it more suitable for later species

    - later species can’t colonize until environment modified

    - modified environment is often not so good for early species and
    they are outcompeted by latter species

Facilitation:

1. Ecological succession
 D) Models of succession: (Connell and Slatyer 1977 American Naturalist)

 ii) Inhibition: early species inhibit later species from colonizing…

     - later species colonize as early species die

     - as they colonize, later species out-compete earlier species

Inhibition:

1. Ecological succession
D) Models of succession: (Connell and Slatyer 1977 American Naturalist)
iii) Tolerance: no interactions (positive or negative) between earlier
     and later species…

    - earlier species are quick to colonize (arrive earlier)

    - later species are slow to colonize (arrive later)

    - later species “tolerate” earlier species and lower resource
       availability

Tolerance:

Upward Shifts
 Barnacles 2000

                                  Endocladia 1992

                  Silvetia 1992
                                     Boathouse

More upward shifts
  Barnacles 2000 (+0.5 meters)

             Endocladia 2000 (+0.6 meters)

                                              Endocladia 1992
                             Barnacles 1992

                                                 Occulto

Endocladia

Spring 1992                 Spring 1993

                           Silvetia

Spring 1995                   Fall 1999

Mesquite  Saguaro cactus

Endocladia  Silvetia