University of Delaware
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The Messenger
Vol. 6, No. 1/1996
Evolution by Gluttons-When Productivity swamps an ecosystem

     For more than 200 years, Earth scientists have argued the
reasons behind the explosion and extinction of life-forms on the
planet. Paleontologist Ron Martin champions a theory that the
ocean's ecosystem collapsed under its own success.
     Martin's view was among three theories on the evolution of
life featured in Discover magazine earlier this year, as part of
the magazine's survey of the top 100 science stories. After
looking at fossil and isotope records, Martin, an associate professor of 
geology, says there is some evidence that large increases in nutrient 
productivity can "blow out the ecosystem, making way for the surviving 
organisms to try something different."
     Discover magazine called it evolution by "gluttony"-in which
plentiful food results in bursts of extinctions followed by a
surge of new, more efficient organisms.
     Life was slow on Earth for the first 3 billion years, Martin
says. But, some 500 million years ago, in the Cambrian Period, a
riotous array of marine animals in different shapes and
anatomical designs appeared. "We see very odd-looking creatures
in the Burgess Shale in British Columbia," he says. "Some look
like a shrimp crossed with a worm; others are crab-like
creatures; none exist today. Most all of the experimental forms
created during the Cambrian died out."
     According to Martin, there are two basic hypotheses for why
Earth had this burst of new life-forms in the Cambrian, followed
by the failure of most of the experimental types. Internal
control, or the genome hypothesis, suggests that genetic constraints are 
set in place in the organisms, keeping them from evolving into very 
different life-forms through the eons. The ecospace hypothesis suggests
external controls were more important, with ecological constraints 
ranging from predators to environmental changes.
     Some of the external controls that theorists suspect may
have affected these cyclic explosions of life include an increase
in ocean warmth from undersea volcanoes that sped up biological
activity; increased oxygen in the atmosphere diffusing into the
ocean, allowing more organisms to dwell in the sea bottom; and,
increased predation, which caused bottom dwellers to produce
shells to protect themselves.
     "Explosions and extinctions of life can be caused by a
variety of factors," Martin says. "Continents pull apart,
volcanism increases (which pumps more carbon dioxide into the
atmosphere) and temperatures rise. Continents come together and
destroy habitats. Sea levels rise and fall. But, one common
thread runs though it all: Nutrient levels go up and go down.
     "A modern example occurred in the Great Lakes. When sewage
and industrial wastes were dumped into the lakes, there were
algal blooms that swamped the lakes. Other aquatic creatures
died, and the whole ecosystem was upset."
     Plentiful food can knock over delicate ecosystems, Martin
says. "The organisms that reproduce the fastest will do so, and
they'll overcome the ones that reproduce more slowly. Ultimately,
they swamp the system and it collapses.
     "This appears to have happened in the Cambrian and many
other extinctions, as well."
     After the collapse and the consequent extinctions, Martin
says, the system begins to reorganize itself from the survivors.
As he said in the Discover article, "I think most people would
agree that somehow you have to break down the old ecosystem so
that organisms have a chance to try something that's different."
     Recognizing that other theorists have suggested that
extinction is associated with lower productivity and lower
nutrient levels, Martin says that this may be the case in some
extinctions. For example, the popular theory today is that
dinosaurs died out because dust from a meteor blocked sunlight
and rapidly cooled the Earth. "Decreased sunlight would have also
drastically decreased photosynthesis in the oceans, so marine
ecosystems would have collapsed, a situation called a
'Strangelove Ocean,' in reference to the movie of the same name,"
Martin says.
     "We have some chemical indicators of increased productivity
in the carbon isotopes," he says, "but we must keep in
mind that
    there are different ways to interpret what we see in the
fossil and isotope record. It's always hard to prove any
scientific theory, but, in Earth science, it can be especially
difficult.
     "What happens at one point in time is dependent on what
happened hundreds of millions of years earlier. It takes a
tremendous amount of time for things to happen, and it is very
difficult to measure the rates of processes that occur so slowly
that they appear to be constant over many, many human
generations. It's not like working in a controlled laboratory
setting. That's why historical sciences like geology and
paleontology are so important."
     Martin's version of what happened will appear in his book,
One Long Experiment: Scale and Process in Earth History, to be
published next year by Columbia University Press.
                                         -Cornelia Weil