Community Structure – Shaking up old paradigms.
Bruno, Stachowicz and Bertness (2003) – The inclusion of facilitation into
ecological theory. TRENDS in Ecology and Evolution. 18(3)
Historically, negative factors have been considered, with the support of
experimental data, as the controlling forces in community structure. These
include:
 Competition
 Predation
 Abiotic Stress
The Role of Predation - Classic study by Robert Paine
The food web as described by Paine (1966)
2 spp.
2 spp.
1 spp.
3 spp.
The Role of Predation - Classic study by Robert Paine
When Pisaster was removed from some of the pools, the
number of species dropped from 18
The Role of Predation - Classic study by Robert Paine
When Pisaster was removed from some of the pools, the number of species
dropped from 18 to 3. What was the limiting factor?
 Within 3 months, the barnacle Balanus glandula
occupied 60%-80% of the space.
 One year – B. glandula was crowded out by mussels,
benthic algae, Mytus californianus and gooseneck
barnacles, Pollicipes polymerus.
 algae populations declined.
 Herbivorous chitons and limpets populations declined
for lack of space and food.
 Sponges were also crowded out.
 A nudibranch that feeds on sponges also left.
 Five years – pools dominated by the mussel M.
californianus and gooseneck barnacles, P. polymerus.
The Role of Predation - Classic study by Robert Paine
Bruno et al. (2003) “Mussel beds are home to hundreds of invertebrates that do
poorly in the presence of the mussels competitors.”
Does predation really control community structure?
Are there other forces at play in structuring communities?
Can a community dominated by just a few species really be
diverse?
Species with a Large Impact
• Dominant species have a very large impact on community
structure. (Also known as foundation species.)
• Such species are highly abundant or play a pivotal role in
community dynamics.
The Role of Predation - Classic study by Robert Paine (1966)
Species with a Large Impact
 Keystone species exert strong control on a community by
their ecological roles, or niches.
• They play a pivotal role. They are not necessarily
abundant in a community.
• Pisaster ochraceus
 Ecosystem engineers (or “foundation species”) cause
physical changes in the environment that affect community
structure.
 E.g. Mussel, Mytilus californianus.
Species with a Large Impact
Importance is greater
relative to abundance
Importance is greater
relative to abundance
Competition – Interference competition
Disturbance and diversity in an intertidal algal community
Gigartina stellata
Ulva sp.
(Sousa 1979)
Competition – Interference competition
Disturbance and diversity in an intertidal algal community
Intermediate Disturbance Hypothesis -abiotic stress
Intermediate Disturbance Hypothesis – foundation effect?
Species richness and abundance of invertebrates increased from early to middle successional stages, then
remained similar into the later stage. These changes are paralleled by those in the physical structure (biomass
and surface area) of the algae. Increased complexity of the algal physical structure probably influenced the
associated invertebrate community through several mechanisms:
(1) It decreased mortality caused by predation from fish and crabs.
(2) It reduced the severity of physical stresses, primarily wave shock.
(3) It increased the accumulation of those individuals and species transported passively by wave action.
(4) In mobile species, selection of algal substrates was largely based on physical aspects of algal structure.
Any apparent “structure” in this assemblage seems to be the result of the summation of individual responses
to changing algal physical structure during succession. No abiotic disturbance.