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III) Connell and the experimental revolution
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
Width of “bar”
represents
strength of
importance
III) Connell and the experimental revolution
3) Importance of predation in determining zonation
a) System / Pattern:
Robert Paine 1966, 1974
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
i) Rocky intertidal in Pacific Northwest (Olympic Peninsula)
2
III) Connell and the experimental revolution
3) Importance of predation in determining zonation
a) System / Pattern (cont’d):
Robert Paine 1966, 1974
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
mussels
gooseneck barnacles
acorn barnacles,
tunicates, sponges,
anemones
pink corraline algae
Rocky Intertidal Zonation
Pisaster
3
d) Test:
e) Results:
i) Removed Pisaster from lower intertidal at two sites
i) Over several years, Mytilus distribution extended
down into lower intertidal zone
ii) Replicate “control” area at each site with no
removals
ii) Issue with design:
ii) Where Mytilus extended into lower intertidal zone,
species diversity declined… another story
3) Importance of predation in determining zonation
Robert Paine 1966, 1974
without within-site
replication of removal and control, how
distinguish treatment and area effects????
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
4
g) Postscript:
i) After experiment ended, Paine quit removing Pisaster, but
cont’d to sample sites:
3) Importance of predation in determining zonation
time
low
high
Lower limit
of
Mytilus
Tatoosh site
Mukkaw site
a) At one site, lower limit moved back up as Pisaster reinvaded
b) At other site, it did not!!!
Robert Paine 1966, 1974
removals
c) Mussels larger at Mukaw by end of experiment
g) Postscript:
ii) Two important implications:
3) Importance of predation in determining zonation
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
5
III) Connell and the experimental revolution
a) Upper limits determined by physical factors?
Underwood and Jernakoff 1981, Oecologia
4) Exceptions to the paradigm (of upper and lower limits)
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
Upper limits determined by physical factors?
4) Exceptions to the paradigm (of upper and lower limits)
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
6
III) Connell and the experimental revolution
Upper limits determined by physical factors?
4) Exceptions to the paradigm (of upper and lower limits)
e) Test:
- full cage (with roof) provides shade and excludes grazers
- roof only provides shade only
- cage with no roof (“fence”) only excludes grazers
- open is control grazers
shade
roof
only
fence open
full
cage
III) Connell and the experimental revolution
Upper limits determined by physical factors?
4) Exceptions to the paradigm (of upper and lower limits)
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
grazers
shade
roof
only
fence open
full
cage no
yes no
yes no
no no
yes
algae response algal reproduction
interaction
7
III) Connell and the experimental revolution
Upper limits determined by physical factors?
4) Exceptions to the paradigm (of upper and lower limits)
f) Conclusions:
- upper limit not set by limited settlement
- upper limit set by biotic interaction!!
- upper limit of reproduction set by interaction between grazers and
physical stress (physical factors effect grazer effect)
grazers
shade
roof
only
fence open
full
cage no
yes no
yes no
no no
yes
algae algal reproduction
interaction
III) Connell and the experimental revolution
Lower limits determined by biological factors?
4) Exceptions to the paradigm (of upper and lower limits)
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!!!!
8
IV) Horizontal patterns of distribution and abundance
a) Background: Have focused on vertical zonation
what about horizontal gradients?
CHARACTERISTIC EXPOSED SHORE PROTECTED SHORE
Dominated by Mussels (Mytilus) Fucoid algae
Free Space Rare (<10%) Common (40-90%)
Predators/Grazers Uncommon (16-80/m2) Common (108-450/m2)
Barnacle Cover Low Low
b) System: barnacles, mussels, algae in New England
rocky intertidal
1. Variation in relative importance of ecological
processes - Bruce Menge, 1976, Ecology
c) Patterns: Along a gradient from exposed to protected
sites…
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
9
IV) Horizontal patterns of distribution and abundance
f) Results: Exposed Shores
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.
Time
Mussels
Algae (Fucus)
Control
(no manipulation)
Time
Open
(-Fucus)
Mussels
Barnacles
Time
Cage
(-Fucus, -predators)
Mussels
Barnacles
Cage
(-Fucus,
-predators,
-mussels)
Time
Barnacles
Time
Mussels
Barnacles
Fucus
Cage
(-Predators,
-grazers)
- note pattern is similar to that in protected shores.
IV) Horizontal patterns of distribution and abundance
f) Results: Protected Shores
Time
Mussels
Algae (Fucus)
Control
(no manipulation)
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.
Time
Barnacles
Open
(-Fucus)
Mussels
`
Cage
(-Fucus,
-predators,
-mussels)
Time
Barnacles
Time
Mussels
Barnacles
Fucus
Cage
(-Predators,
-grazers)
Time
Cage
(-Fucus, -predators)
Mussels
Barnacles
10
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
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?
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 (is this because the predators and grazers are
snails sea stars?)
IV) Horizontal patterns of distribution and abundance
h) More generally:
Importance to
community
organization
Environmental harshness
Benign Severe
Physical
Processes Competition Predation
Low
High
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.
General paradigm of
community organization in
rocky intertidal
(see Connell 1975, Menge and
Sutherland 1976, Menge 1976,
Lubchenco and Menge 1978,
Underwood and Denley 1984)
11
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
Littorina abundance
Low High
Low
High
Littorina abundance
Low Higher
Low
High
Tidepools Rock (emergent)
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:
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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
13
f) Results:
i) Enteromorpha favored algae of littorines (in pools and on rock)
ii) Patterns from pools…
2) Alternative stable states - Lubchenco, J. 1978 Ecology
Time
Low
High Chondrus
Enteromorpha
Control
(littorines common)
Time
Low
High
Chondrus
Enteromorpha
Littorine addition
(rare before)
Time
Low
High
Chondrus
Enteromorpha
Littorine removal
(common before)
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