It's amazing if you think about it: In some parts of the ocean
(usually tropical), the plankton is so thick that some animals can
anchor themselves to a rock and spend their whole lives grabbing dinner
whenever they feel like it, never moving from one spot. Even
more amazing, quintillions of such creatures can colonize an area,
and live the good life over thousands, if not millions, of years!
Over time, the protective stony shells excreted by colonies of coral
polyps, hydrozoa, bryozoa, and sponges build up into massive formations of a limestone-like substance.
To geologists, it is an important process forming sedimentary
rock1. More importantly to oceanographers, marine biologists, and almost everyone else, the most beautiful
and diverse type of ecosystem in the world, not to mention the most wild
and woolly laboratory for evolution that ever existed, develops
around a coral reef.
A list of the wonders of a coral reef ecosystem would stretch forever:
Creatures from thousands of species congregate around the reef, competitors
for the rich broth of plankton, predators of the coral or their egg capsules,
predators to the predators, scavengers on the detritus from the ruin of
battle. Specialized ecological niches, parallel evolution, symbioses,
and opportunities for parasitism are everywhere. Fish are so thick that
sea anemones anchor themselves to the rocks in the same way as corals,
and pick off fish without having to budge. Other fish
use the sea anemones as a refuge from predators, occasionally bringing
in a meal for their hosts. Colors abound as grazing creatures announce
their toxicity to would-be predators.
The coral reef cycle was first articulated by Charles Darwin and the patriarch of geomorphology,
William Morris Davis:
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Coral polyps not only consume plankton, they are part of the plankton
before they settle down. Thus, larval coral polyps are spread throughout
the oceans. Eventually, the larvae find a place to anchor themselves.
Sometimes this is a new area without a reef, such as the shallows along
the coast of a volcanic island recently emerged from the ocean.
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Over millennia, the corals build fringing reefs hugging the coastline.
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Meanwhile, land erosion due to rainfall, as well as coastal erosion
due to wave action, erodes the island. Unlike the land, the reef
can renew itself. Inside the reef there is less plankton, and coral
doesn't grow as fast there. The reef becomes separated from the shoreline,
creating a barrier reef, with a lagoon separating the reef from the
shore. The largest coral reef in the world, the Great Barrier Reef, parallels the coast of Australia for 2000 kilometers (1200 miles).
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A barrier reef will protect the shoreline from the worst of the ocean's waves,
but erosion will still occur. Eventually an island with a barrier reef ringing
it is eroded completely away, leaving an atoll.
This description, although adequate for the time, has proven to be too simplistic; things are far more complex. Other processes complicate the development of coral reefs. The subsidence of the sea floor as it moves away from tectonic spreading centers drives coral reef growth. Sea level change is known to affect the cycle: As sea level rises, the land is drowned, but the reefs have room to grow. However, it is a race between the sea and the reef: If sea level rises too quickly, the zone where the all-important plankton grows can rise completely above the reef, drowning it. When sea level falls, reefs are left high and dry, and new areas become shallow enough for the whole cycle to begin again. This, and caldera-forming volcanic eruptions, most likely hasten an island's development towards an atoll.
Predation of the coral can serve to keep the reef in trim, but can also destroy it.
The most intriguing predator of coral, and the most important recycler
of coral reefs, is the parrot fish. Parrot fish use their hard
beaks to bite of chunks of coral, and grind it up to extract the yummy
coral polyps inside. They then excrete the sandy bits that are left;
most of the coral sand on the bottoms of lagoons, or on the beaches, has
passed through a parrot fish in this manner. A well-known recent
opportunist, the Crown-of-thorns starfish, has been wreaking destruction
of the coral reefs off the coast of California. Human predators also
exist: Methods of harvesting tropical fish in the Philippines are extremely
destructive.
Climate change and anything else affecting the location of the plankton
zones will also affect the reef. Over the past decade
or so, scientists have become increasingly worried as coral reefs die off
with no good explanation. Global warming and pollution have been
blamed but no clear link has been demonstrated. What is clear is
that something is unraveling the unimaginably complex fabric of ecological
dependencies that make coral reefs possible, rendering their bewildering
beauty into wasteland.
1Entire mountain ranges, such as the Guadeloupe Mountains of southwestern New Mexico, are made of coral.