Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 154

such as sea urchins and starfish. The purple sea star is a classic key-
stone animal. It lives in the shallow tidal zone off the coast of north-
west America. It is not the top predator in its ecosystem, and indeed
it is relished by sea otters. It in turn consumes the mussels of the
intertidal zone. The presence of just a few sea stars can keep mussel
populations down to levels in which they form only part of a diverse
intertidal community. Remove the sea stars, and the mussels rapidly
multiply to become dominant, crowding other species out.
In ecosystems where there are many species, vulnerability to the
loss of a few keystone species tends to be less. In Cambrian ecosys-
tems, with perhaps a few tens of species, disruption of one or more of
the keystone species may have collapsed entire ecologies. Examina-
tion of the fossil record of the Cambrian shows a number of major
extinction events, perhaps as many as six within its roughly 50 mil-
lion year duration, and many more than at any other comparable time
in the Phanerozoic Eon. These extinctions affected familiar groups
such as trilobites, and less familiar groups such as the reef-building
but sponge-like archaeocyathids, which flourished briefly in the trop-
ical oceans worldwide, before most went extinct before 510 million
years ago. Environmental factors may have played a major role in
these extinctions, as the anoxic waters of the deep oceans periodically
spilled on to the continental shelf, suffocating much of the life there.
It is a signal of the increasing sophistication of life in the Cambrian
seas that such mass suffocation events may have spurred the rise of
other groups of organisms. The biological pump is the engine that
delivers carbon from the surface waters to the deep oceans, and it is
thus an integral part of the ocean food supply, and of the carbon cycle.
Seabed anoxia, in the Cambrian and elsewhere, is implicated in forcing
many organisms into the water column, especially those that had
the right pre-adaptations, such as a life cycle with planktonic lar-
vae, strong swimming abilities, or a penchant for buoyancy. Once