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Hard Substrate Communities
Hard substrate = rocks, old shells,
pilings, jetties, piers, docks
Intertidal – perhaps best know marine
Tides = periodic rise and fall of sea
• Caused by gravitational attraction of sun
and moon and centrifugal force generated
by the rotation of the earth-moon system
around the sun
• Periodicity of 24 hours, 50 minutes
• gravitational attraction: sun and moon pull
at the Earth’s oceans, causing a tidal
• the tidal influence of the Moon is about
twice that of the Sun
• Two classifications of tide based on the
orientation of the Sun and Moon.
• Spring tide : Sun, Moon and Earth are all
in a line (full or new moon), Sun and
Moon act together to create one large
tidal bulge, and causes the greatest tidal
differences because the.
• Neap tide: Sun and Moon are at right
angles from each other (quarter moon
phase), when the, smaller tidal bulge.
• distance of Moon: 50K km difference.
perigee (nearest), the gravitational pull is
~40% greater than at apogee (farthest).
• Approximately two high tides and two low
tides each day
• when it is high tide at one coastal
location, it is low tide along a different
coast a quarter of the way around the
• Bay of Fundy, eastern Minas Basin, on the
- Moon near perigee @ spring tide, HT up
to 16 meters above LT.
• The geogr. location, shape of the
shoreline, water depth key reasons for
tidal variations.
Result of the tides –
• Exposure in the air for periods every day
– Greater temperature differences
– Desiccation/water loss – time of exposure
changes as tide advances
– Intense solar radiation
• Mechanical effect
• Creates a “splash zone”
• Mixing – chemicals, dissolved gases,
suspended particles
Salinity changes
• Rainfall, freshwater runoff
• Tidal pools – evaporation exceeds
Mytilus californianus
Nucella (Thais)
Starfish - Pisaster
Sea Anemones
Sea Urchins
Adaptations – Organisms
1. Minimize water loss/desiccation during
Hide out in most crack, crevice, under algae
Tolerate tissue desiccation, rehydrate at
high tide
Structural behavioral modifications – close
up, clamp down, mucus, gills in cavity
Adaptations – Organisms
2. Minimize thermal stress
Large size, surface/volume ratio
Light/heat reflection
Evaporative cooling
Adaptations – Organisms
3. Avoid destruction in waves
Attach to substrate
Thicker shells
Clamp to substrate
4. Salinity stress
Conspicuous horizontal banding, or
zonation of organisms
Stephenson & Stephenson 1949
universal scheme to describe
• Supralittoral – splash zone
– Littorina (periwinkle), encrusting lichens and
• Midlittoral – barnacle and mussels zone
• Infralittoral – algae and more diverse mix
of animals
• Sublittoral – subtidal zone
1. Period of submergence – “critical tidal
level” (Doty 1945)
2. Desiccation – sets upper limits to
north vs south-facing slopes
3. Thermal stress
4. Sunlight
Joseph Connell 1961
Coast of Scotland:
Chthamalus stellatus
Balanus balanoides
Thais lapillus (now
Robert Paine 1966
• Observed structure of the food web in rocky
intertidal from Washington to Costa Rica
Washington – predatory starfish Pisaster and
Nucella, the drill – lots of diversity
Costa Rica – no starfish, the drill Nucella is the
top predator – less species diversity
What was going on??
Went back to Washington and experimented
Paine’s experiments:
• Removed starfish from rocks and observed
• With Pisaster ~ 15 species
• Without ~ 8 species (plots dominated by
Mytilus, the best competitor for space)
• Pisaster - “Keystone species” – indirectly
controls the structure of whole community
Connell in California (1970)
• Balanus glandula (like Chthamalus)
• Balanus cariosus
• Refuge in space (Balanus glandula)
• Refuge in size (Balanus cariosus)
Paul Dayton 1971
Space is key – processes involved in getting,
keeping, and using space
• Physical stress – desiccation, etc.
• Physical disturbance – waves, drift logs
• Predation – biological disturbance
• Competition
• Natural death
Dayton’s experiments:
• Fucus – whip effect on barnacles vs protection
from desiccation
– Treatment = removal of Fucus
• Limpets – eat algae; disturb and/or eat
– Treatment = “dog dishes” to exclude limpets
• Thais – drills mussels and barnacles
– Treatment = cages to exclude
• Pisaster – eats mussels and barnacles, etc.
– Treatment = removal
Dayton’s conclusions:
• Sessile marine organisms compete for
• Three levels of “biological disturbance”
– Grazing by limpets
– Predation by Thais/Nucella
– Predation by Pisaster
• Pattern in nature = mosaic of disturbance
and recovery; disturbance maintains
patchiness, keeps diversity high
Impact of Connell, Paine, and
Dayton’s work on field of ecology
• Shift in thinking about how communities are
Earlier view – competitive dominant has
broadest niche, diversity the result of
specialization to avoid competition
New view – dominants often less abundant,
inferior competitors can have broader niche,
predators and disturbance create diversity by
preventing competition
Rocky Intertidal Paradigm
1. Upper limits of distribution are primarily determined by
physical factors
Lower limits of distribution are primarily determined by
biological interactions (competition for space and
Predation or physical disturbance can prevent
competitive exclusion (enhancing species diversity) by
keeping superior competitors at low densities
In benign physical environments, predation is most
important in determining community composition
In harsh physical environments, predation intensity is
reduced and competition is most important in
determining community composition
Exceptions and Limitations of RIP
1. Upper limits of distribution are primarily determined by
physical factors
2. Lower limits of distribution are primarily determined by
biological factors (competition for space and predation)
Starvation restricts upper reaches
Larval settlement determines the limits of distribution
shorter period of inundation means reduced probability of
settlement in upper reaches
-- larvae chose where they settle
-- adult larval interactions affect settlement patterns
Exceptions and Limitations of RIP
4. In benign physical environments, predation is most
important in determining community composition
5. In harsh physical environments, predation intensity is
reduced and competition is most important in
determining community composition
Assumes prey have a greater physical tolerance than predator,
can find refuges in space
Variations in recruitment determine the level of competition
Importance of predator selectivity and competitive hierarchy
In same environment, both predation and competition are
important, both for different life stages