Transcript Factors affecting Rocky Intertidal Zonation Patterns

Factors affecting Rocky Intertidal Zonation Patterns
• Intertidal Zones
– Immersion in seawater
– Emmersion in Air
– Tidal Patterns
• Biobands of intertidal algae and Animals
– Diversity determined in part by the presence of a keystone
predator
– Upper end of species range determined by abiotic tolerances
• Temperature tolerance
• Desiccation tolerance
– Lower end of species range determined by biotic interactions
• Predation
• competition
Nearshore Patterns - Biobands
Biobands are species
assemblages with a
characteristic color and
across-shore elevation.
Splash Zone Lichen (VER)
Barnacles (BAR)
Rockweed or Fucus (FUC)
Green Algae (ULV)
Red Algae (RED)
Soft Brown Kelps (SBR)
Tides
• The gravitational forces of the moon and sun pull on the
earths surface water
• Since the moon is much closer to us, it has a much greater
effect, and in effect it pulls an ellipse of water around the
earth as it moves in its own rotational path, creating a
“bulge” of water pointing towards wherever the moon is,
and a reciprical bulge on the other side of the earth as the
planet’s rotation adds a centrifugal effect
– Each site on the planetary surface gets a surge of water – a high
tide- every half lunar day, where the lunar period is 24h and 50
min, thus cycling through high – low – high every 12h 25 min
– Hence shores normally experience two high tides and two low
tides a day which are delayed by a fixed period of 50 min on
successive days
– Exception – northern Gulf of Mexico – mostly diurnal tides – one
low and one high tide a day
Tides
• The effect of the sun modifies this basic pattern by
superimposing a lesser gravitational pull with a different
periodicity
– Spring tides at new and full moons – The sun and moon are in
line producing a greater gravitational pull – get particularly
extreme high and low tides
– Neap tides at half moons the sun and moon are working against
each other so the tidal ranges are reduced
– Springs and neaps occur twice each lunar cycle, or roughly twice
a month (Fig. 12.3)
• A third periodicity effects tides, since they also vary with
the declination of the sun, which runs on a yearly cycle
• Declination is greatest at the equinoxes, so the most
extreme tides always come in March and September
Tides
• The actual degree to which tides rise and fall on any shore is highly
variable and must be computed for each location because of
differences in coastal topography
– Neap and spring tidal ranges for Great Britain is 3.5m (neaps) and 4.7 m
(springs)
– Bay of Fundy – Spring – 60 ft
– Cook Inlet – Spring – 55 ft
– Juneau AK – Spring – 28 ft
– Friday Harbor, WA – spring – 6 ft
– Coastal LA – Spring – 2 ft
• On a smaller scale, long, thin inlets pointing upstream from
prevailing winds can experience a “tidal bore” when the fast
incoming tide is increasingly channeled into a narrowing profile
– Bristol Channel
– Cook Inlet – Turnagain Arm
– St. Johns New Brunswick
• Tides are also modified from day to day by weather, particularly by
strong winds and the after-effects of storms at sea
Zonation Patterns
Freeze Tolerance at Bridget Cove, AK
Biological factors, Lower Limit:
5. Predation and competition
• Examples from studies by Joe Connell and
Robert Paine
Connell study
• Balanus and Chthamalus differ in
distribution on rocky shores. Adult Balanus
extend from mean low water to above mid
tide line. There is an abrupt transition to
Chthamalus which extends from the upper
limit of Balanus to the upper intertidal.
• Adult barnacles do not overlap
• Classic experiment of Connell:
Figures 6.16, 6.17
`
• Larval settlement generally overlapping
– therefore, distribution is not due to larval
settlement patterns but post-settlement mortality
• Transplant experiments suggest that Balanus
cannot withstand physical stress in the upper
intertidal
– transplanted barnacles die due to temperature or
desiccation stress
• Chthamalus lives well if transplanted down and
if Balanus is removed
• If Chthamalus transplanted down and Balanus is
not removed, Balanus overgrows and kills
Chthamalus
– Chthamalus experiences a refuge from Balanus
above Balanus’ distributional range
Balanus is the superior competitor for space and
competition sets the lower distributional limit
for Chthamalus
Paine study:
• Pisaster ochraceus lives from the lower to
mid tide region and preys on many species
(barnacles, chitons, snails etc)
• However, Pisaster prefers Mytilus
californicus
• It “clears” a path, leaving open space
behind
An exclusion experiment:
• Paine excluded Pisaster from an area near a
mussel bed
• The mussel bed grew larger, by 2 m, and it spread
down the intertidal
• Thus, Pisaster predation sets the lower
distribution of Mytilus
• Keystone predator – a predator which, by preying
on a competitive dominant species, limits the
competitive exclusion of other species by the
dominant
A keystone predator
• Outside cages, 15 sessile species co-exist on
the rocky shore
• Without Pisaster, species richness decreased
to 8 because mussels excluded other species
by overgrowth
– Mytilus californicus is the superior competitor for
space in the mid-intertidal
Overall pattern of sessile biota:
• Physical factors limit distribution up the
intertidal
• Biological factors often limit downward
distribution
– predation intensity decreases upward
(predators are often limited to mid-tide region
- a refuge exists above this limit)
– competition is most intense in the midintertidal
Rocky shore paradigm:
• Space is limiting and biota will be
dominated by a superior competitor for
space unless:
– disturbance/predation reduces the abundance
of the superior competitor
– or tolerance provides a refuge for inferior
competitors
• Works best in the mid-intertidal
RSP works for plants too!
Figure 6.26
• Disturbance may be equivalent to predation
• Disturbance is greatest in the lower intertidal
– waves sweep away or logs crush sessile animals
– the frequency of disturbance events may dictate
species composition
– disturbance increases diversity
Barnacle cover
No cage, control
Inside cage
(months)
Barnacle cover
No cage, control
Quiz 10: Are barnacles controlled by predation,
competition or physical factors?
Inside cage
(months)
Is the rocky-shore paradigm widespread?
• Low larval supply may prevent it from
working on some shores
– few barnacles on the coast of northern
California because currents move them
elsewhere
• It may or may not work in other marine
habitats
Next Weeks lecture on Laboratory Topics
• Thursday April 23, 2009 at 7 PM in Williams 208
• Labs Covered
– Limiting nutrients and competition
– Go Fish
– Oil Spills