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)

Rockweed or Fucus (FUC)

Red Algae (RED)

Soft Brown Kelps (SBR)

Green Algae (ULV)

Barnacles (BAR)

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 mid- intertidal

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

Figure 6.26

RSP works for plants too!

• 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

No cage, control

Inside cage

Barnacle cover

(months)

No cage, control

Inside cage

Barnacle cover

(months)

Quiz 10: Are barnacles controlled by predation, competition or physical factors?

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