by Dr J Floor Anthoni (2009)
The general idea of urchin barrens is that
they are abnormal and that sea urchins have multiplied, while creating
barren areas, because snapper and crayfish populations (their predators)
have been decimated by people. So sea urchins and their barrens are a bad
thing. However, nothing could be further from the truth, as this page shows.
Dr Anthoni discovered that the barren zone is created by large storms,
and that urchins may or may not establish themselves there. Storm barrens
are highly productive and diverse and a sign of good environmental health.
Storm barrens were discovered by Dr Anthoni in an extensive habitat
survey done in 1993, work that has not been disproved. But please note
that mainstream scientists are not supportive of this idea (yet).
Barren zones or areas are places where large seaweeds cannot grow because
storms remove them. We must not confuse them with places where there is
not enough light or where there is surface wave action.
Introduction Every ocean shore in the world is subjected to the forces of waves,
which is the main sorting factor for what species live where along a depth
profile, as the wave's force rapidly declines with depth. In addition there
is the influence of light, or lack thereof. As one goes deeper, light intensity
and quality diminish until eventually plants can no longer grow. This depends
on the orientation of the coast (sunny or shaded side) and the turbidity
of the water, and of course the light sensitivity of the main habitat-forming
seaweeds. The resulting habitat zoning is also subjected to unusual events
like major storms (hurricanes) that may cause major damage only once every
After one such major event, a massive plankton bloom at the end of 1992,
the kelp forest in a large area died and was successively removed by a
tropical cyclone (hurricane) in January 1993. After the storm we did a
preliminary survey and found good reasons to follow it up with a more thorough
one in the (southern) winter of 1993 (See Hauraki
Marine Survey 1993). Because we looked at many factors, we also made
many discoveries, such as the full extent of the kelpbed death. For every
site we recorded habitat boundaries along a vertical transect down the
shore. This led to the first record of actually measured habitat boundaries
(they were previously guessed at).
For some time we suspected that the depth of the sand bottom restricts
waves as wave theory predicts, such that the maximum strength of the worst
storm waves is proportional to the depth of the sand bottom. Hurricane
waves are typically spaced over 400m apart, extending to a depth of 200m.
When they arrive at the continental shelf, they are hindered by the sand
bottom which becomes shallower as the waves approach land, draining their
energy accordingly. So, by sorting the transects by their bottom depth,
we could place them in order of worst wave exposure, and a consistent zoning
From left to right wave exposure from exposed to sheltered.
The top two zones are taken by stringy seaweeds which give way abruptly
to a barren zone, bordering the kelp forest. The kelp forest is bordered
by the sand bottom with occasionally a narrow fringe of sponges.
Above right a three-dimensional view with typical shore
profiles plotted. Please note that the urchin barrens (pink) end when the
bottom is shallower than 15m, and that a tough seaweed like the featherweed
plumosum) takes over. Note that this diagram could be extended further
into the sheltered shallows, but this was not the purpose of our survey.
We also didn't record zoning on shaded shores. Note also that exposed shores
become steeper as they become deeper.
without urchins To our surprise, we discovered that some barrens were not populated
by urchins. Some of these were grazed by the strong grazing Cook's turban
snail (Cookia sulcata) and some by the rainbow paua (Haliotis
iris) but there were also some that had no grazers at all. Instead
the barrens (that are devoid of kelps), were populated by colourful communities
of turfing calcareous algae (pink turf), sponges and anemones. When we
plotted these against the depth of the sand bottom, they followed a straight
line. Note that sites 24 and 22 were located in very strong currents, known
to erode the sand bottom as they shear past headlands (which moves their
positions to the right in the diagram). These barrens without urchins vindicated
our hypothesis that the barrens are indeed caused by storms.
Because these discoveries were made in 1993 and have since not been
proved wrong by mainstream science, we will introduce the word storm
barrens for the general case. Urchin barrens are a special case
when urchins are their main grazers.
Rules for storm barrens Because storm barrens are caused by physical factors, and then modified
by biological factors (their grazers), they follow strict rules. These
rules help to understand the underwater environment.
Here in New Zealand the top of the kelp forest is located at one quarter
of the depth of the sand bottom. Worldwide, this boundary depends
on the strength of the local kelp (or coral).
Where storm barrens are grazed, this boundary may extend further down,
and when the coast slopes gently while no deeper than 20m, there may not
be a kelp forest at all due to grazing.
The maximum strength of storm waves is limited by a gently sloping continental
shelf, but where such a shelf does not exist (as in Niue and the Kermadec
Islands), storm waves arrive with maximum strength, capable of changing
all zones into a single barren zone extending from the surface down to
When the sand bottom is shallower than 15m, there cannot be a barren
zone (in NZ). This explains why there are no storm barrens along New
Zealand's west coast where the sand bottom is 5-10m deep at the bottom
of a rocky shore.
Storm barrens are usually contiguous zones but can be patchy as
they are created first on rocks extending out above their surroundings.
They are likely to be patchy in border cases.
Storm barrens are absent on sheltered coasts, regardless of their depths.
Vertical storm barrens are difficult for major grazers like sea
urchins and snails, reason for their absence.
These rules are strict and exceptions should not be found.
Other barren areas Barren areas are areas without the large habitat-forming seaweeds (macroalgae)
like kelps. But seaweeds need moisture and light, so when either is absent,
a barren area results. So it pays to distinguish these from storm barrens.
the intertidal rocky shore extends above low tide where seaweeds cannot
live. Not surprisingly, they are barren.
inside caves and archways there is not enough light for seaweeds, so they
on degraded shores the water may be too murky to allow plant life below
the storm barrens.
the water may become too unhealthy for plants to live or for spores to
settle, even though the amount of light remains adequate.
strong grazers may create barren patches here and there
diagram shows how loss of light due to murky water creates a barren area
where normally none would be expected. On left a healthy situation with
shallow stringy seaweeds (fucalia), an urchin-grazed storm barren,
a kelp forest (laminaria) and a deep reef habitat with sponges.
When the water is clear and healthy, there is standing room only in an
environment which is rich in variety.
But degrading water quality pulls the lower light boundary up, while
also diminishing density and diversity. Eventually the whole shore becomes
barren, devoid of the species one would have expected there. In the end
also crustose coralline algae (pink paint) disappear.
f033810: due to murky water, the kelp boundary has moved
up and these grey sponges are infested and dying. The whole barren area
extends from 6m down. Martins Bay. Note how the pink paint has become patchy.
Note also that one cannot take overview photos due to severely limited
f036804: the kelp forest is dying even though there is enough
light. There are no young plants (recruits) and there are no sea urchins.
This becomes a barren zone caused by degradation of water quality. Arid
f043124: a diver swims into Rikoriko cave at the Poor Knights,
where the barrens are densely populated but where seaweeds cannot grow.
Notice the large purple urchins. These barrens are caused by lack of light.
f020032: life inside an archway can be very rich because
the current brings food. But plants can't grow here. Notice the purple
urchin in the foreground. Apparently it can live from animal life. Poor
f030621: purple urchins have carved out a barren patch in
a sheltered place. This is not a storm barren. Poor Knights.
f029902: purple urchins (Centrostephanus rodgersi)
and their local barren. Notice the many sockets in the rocks, carved by
generations of urchins. Poor Knights.
f036012: strong grazers like sea urchins and large snails
can easily fall off steep rock faces, reason why they are easily bared.
This is a boundary case of a storm barren. Deadmans fingers Alcyonium
f033926: a storm cleared a high patch on a boulder and urchins
occupy its sheltered side, which has been cleared completely compared to
the exposed side. Note that urchins do not need to graze because they can
catch broken seaweeds. Mayor Island.
f007213: a storm cleared a high boulder and sea urchins are
grazing it further. Notice how they are penned into their patch by the
steep sides of the boulder. By staying closely together, they resist attacks
from predating starfish, whelks and fish.
Ecology of the
storm barrens The ecology of storm barrens is actually quite simple: what is not
removed by storms, stays and thrives. But this can have interesting twists,
illustrated here with photographs. Once an area is cleared from tall seaweeds,
it lets the light in. Immediately kelp 'seedlings' pop up but because these
are rare in the whole area dominated by a dense tough canopy, they are
also immediately eaten by mobile browsers like fish. It will take up to
4 years for kelp to effectively re-establish itself. This gives slow grazers
like sea urchins enough time to take over as they mature in about two years.
As the sea urchins grow, they more or less run out of food, so removing
a few helps the vitality of all. In any case, their grazing power is far
more than needed to keep the rock barren. Note that urchins can catch broken
seaweeds that float by, and many live entirely from this food source, without
ever moving (cryptic or hidden urchins).
However, with their five teeth, they cannot reach into cracks and crevices
and they cannot graze the pink paint that gives the rock a whitish cover.
soon an army of more suitable grazers arrives, and they specialise on the
patches that the urchins can't reach.
The main grazers of the storm barrens in NZ are:
green or common urchin (Evechinus chloroticus): the main tree feller
and clearer of the barrens, always hungry, prolific spawner, partly roaming,
partly homing to its socket.
purple urchin (Centrostephanus rodgersi): mainly outer islands,
strong grazer but sensitive to waves, patchy, homing.
f001319: urchins do the ground work by night while they shelter
inside their sockets by day. Large Cook's turban shells do some rough grazing
whereas the finer work is done by smaller catseyes, top shells and limpets.
f001932: a cluster of sea urchins at the sheltered side of
a protruding rock. The habitat is barren as far as the eye can see. Goat
Island marine reserve Waterfall Reef June 1995. Note that the kelp has
not yet invaded this habitat, contrary to what scientists claim!
f035721: detail of overlapping fronds of pink paint. The
whitish spots are dead because some organism once grew there (sponges,
kelp). Dark red patches are a different species.
f036235: the Cook's turban snail (Cookia sulcata)
is a powerful grazer but sea urchins need to do the clearing first. A variable
triplefin sits on its shell. The shell's rate of growth (and health) is
revealed by a band of bare shell.
f006828: a lonely sea urchin about to clear-fell the last
kelp tree on its patch. Once the plant is bleeding, other urchins are attracted.
f033937: a marblefish has also made use of a barren patch
by meticulously trimming a stand of sea lettuce, thereby encouraging it
to grow very densely. No other grazers allowed here! Mayor Island.
f007210: a lot of food in the form of sea lettuce is found
above the urchin zone but it is too risky to go there. This shows the subtle
difference between urchin barren (below) and storm barren (above). Mayor
f045604: the urchin barrens support a highly productive and
diverse community. In the photo the green sea urchin (Evechinus chloroticus),
top shells (Trochus viridis), a stellar limpet above (Cellana
stellifera), an unidentified limpet perhaps (Cellana radians),
and a host of tiny snails. Notice the various forms of crustose calcareous
algae (Lithothamnion spp.). Mimiwhangata.
f049611: detail of some little actors on the urchin barrens.
Notice the rough terrain created by some crustose calcareous algae possibly
officinalis. Cavalli Islands.
Examples Enjoy the examples of storm barrens and how varied they can be, and
also how beautiful.
f022424: for demoiselles to lay their nests of eggs, they
need barren rock as shown here. The blue males guard the eggs while the
green females feed in the currents. Demoiselles keep the rock face free
from small snails and they even prod sea urchins along. Poor Knights.
f048302: demoiselles have cleared their rocks from grazers,
but are thereby allowing the sea lettuce to invade. Poor Knights.
f017317: as the kelp boundary moved up due to increasing
murkiness of the water, it left behind a barren area where sponges fight
deposition by mud. Martins Bay.
f017316: sponges in a degrading environment enjoying the
cleaning services of sea cucumbers (Stichopus mollis). Martins Bay.
f036001: a colourful storm barren with isolated shrubs of
tangleweed (Carpophyllum flexuosum). The tangle weed has been trimmed
by storms and it is too tough for sea urchins to eat. Notice the many white
sea anemones. Mimiwhangata.
f045827: urchins in a diverse storm barren with red carpet
sponges, large grey sponges, white anemones, tangle weed and pink coralline
algae. Apparently the carpets can repair themselves rapidly. Notice how
all urchins sit on pink paint patches. Mimiwhangata.
f051809: a storm barren covered in encrusting sponges (Crella
incrustans) that survive the gnawing from sea urchins as the only grazers.
f020605: short red seaweeds (Champia spp) and
a flat tough green seaweed (Gigartina sp) surviving in a storm barren
where urchins can't. Poor Knights.
f051822: turfing calcareous algae dominating in a very exposed
storm barren. Cuvier Island.
f020915: detail of turfing calcareous algae in a storm barren.
f036520: detail of turfing calcareous algae in a storm barren.
f034100: detail of one of the toughest calcareous algae in
a storm barren. Mayor Island.
f036523: detail of a most varied storm barren coralline garden
- standing room only. These plants made from bits of stone hinged together,
are able to survive in the most exposed conditions. Cape Brett.
f049016: a storm baren covered in jewel anemones and encrusting
sponges. No unoccupied space left. Truelove Reef Cavalli Islands.
f028104: a sheer storm barren cannot be populated by urchins
for once they fall down, they cannot make it back up again. Cape Brett
f051912: a very rich storm barren at a very exposed pinnacle.
In the distance nothing but storm barrens. Never Fail Rock, Mercury Islands.
f051900: detail of the storm barrens around Never Fail Rock.
Huge diversity. Standing room only. No urchins.
f031419: storm barrens on a basalt bommy at Raoul Island,
Kermadecs. Two kinds of urchin and a large snail in the foreground. Note
that there are no large seaweeds (kelps) at the Kermadecs, possibly because
the storm barrens extend over the whole photic (light-) zone.
f031804: storm barrens populated by large urchins (Centrostephanus
rodgersi) a far as the eye can see. McDonald's Rock, Kermadecs.
f047302: Because Niue island does not have a continental
shelf, hurricanes can arrive with exceptional strength, creating storm
barrens from the surface down to 80m deep, where no branching corals
can survive. See Niue ecology
f047110: At night millions of sea urchins appear from their
dens and tunnels in the soft rock, to graze the film of algae. Notice their
scrape marks. Molluscs like seashells also join in.
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