zonation: paine to lubchenco

1

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


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



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

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



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



4

g) Postscript:

i) After experiment ended, Paine quit removing Pisaster, but

cont’d to sample sites:


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!!!


removals

c) Mussels larger at Mukaw by end of experiment

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

5


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


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

6




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




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




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


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!!!!

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…


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


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.


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


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?)


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


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)



c2) Patterns: spatial variation in species diversity

varies as a function of grazer density and habitat

type:

12



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



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…


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

zonation: paine to lubchenco

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