Download Article 11

Article 13
Oceans
Divided
The Isthmus of Panama is a land bridge—and also a dam that divides Atlantic
from Pacific. What happened to marine life when the dam was built?
BY KATHY A. SVITIL
W
HEN THE ISTHMUS of Panama
emerged from the sea 3.5 million years
ago, the history of life on Earth was forever
and dramatically changed. North American animals migrated across the thin land
bridge into South America and did well
enough to supplant their southern counterparts; southern animals moved northward
and, except for the opossum and a few others, died out. In the sea the situation was
just the reverse: organisms that once swam
between the Atlantic and Pacific oceans
became permanently confined to one or the
other side of the isthmus, isolated from one
another. How did this affect them?
There has been vague talk over the
years of a mass extinction in the Caribbean, and a vague assumption that the biological isolation of Atlantic from Pacific
happened more or less suddenly. Researchers at the Smithsonian Tropical Research Institute in Panama are now
replacing the vagueness with hard data.
There was no mass extinction in the Caribbean, they say—or rather, none that can be
laid directly at the feet of the Panamanian
isthmus. And the rise of Panama began
separating Atlantic species from Pacific
ones well before the land bridge itself was
completed.
The story of Panama begins more than
60 million years ago, when a giant tectonic
plate carrying the floor of the Pacific
Ocean began diving underneath the Carib-
bean plate to the east. As the plate sank, it
melted, and that molten rock began to
force its way up to the surface, through the
thin crust of the Caribbean plate, emerging
as a chain of spectacular volcanoes. The
chain stretched from Mexico in the north,
down through what is now Nicaragua, and
curved out into the Atlantic. At that time a
large seaway still separated the chain from
the continent of South America; a strong
current flowed west from the Atlantic into
the Pacific, and marine life moved freely
between the two oceans.
While the volcanic arc steadily grew,
South America was slowly drifting toward
North America. “Then, 10 million years
ago, South America began to collide with
the volcanic arc,” says geologist Anthony
Coates of the Tropical Research Institute.
“When that happened, the arc and parts of
the continental shelf of South America began to buckle.” The ridge that would become an isthmus started to rise out of the
sea.
By 6 million years ago, the geologic
record shows, the ridge was still a few hundred feet below the sea surface, and the Atlantic and Pacific were still connected.
Certain animals, however, had already felt
the ridge’s influence. Nancy Knowlton, an
evolutionary biologist at the Tropical Research Institute, has studied the genetic
makeup of closely related species of snapping shrimp—shrimps with a claw that
1
makes a finger-snapping sound—from either side of the isthmus. She has found that
many Atlantic and Pacific shrimps had already become isolated from one another
and had started to evolve into distinct species, millions of years before the seafloor
ridge became an isthmus. Those species,
Knowlton says, normally live in deeper
water or clear water, so their path between
the seas was blocked by the shallow and
murky water above the ridge long before
the ridge rose above the waves. The barrier
that is obvious to us was to these
shrimps—and probably to other marine
species as well—a mere aftermath.
By 3 million years ago, the isthmus that
is now Panama and Costa Rica had fully
emerged, with global repercussions. The
strong equatorial current that had flowed
from Africa to Asia through the Panamanian seaway now veered northward,
strengthening the Gulf Stream; the warm
water now flowed into the North Atlantic
instead of the Pacific. Since the trade
winds continued to push Pacific surface
water to the west, that surface water now
had to be replaced by cold water welling
up from the depths along the west coast of
South America. The cold water was and is
rich in nutrients—which is why the Pacific
off South America now supports huge populations of plankton and rich fisheries. The
Caribbean is warm and relatively nutrientpoor—which is why its waters are clear
Article 13. Oceans Divided
(there are few plankton), full of corals
(they thrive in nutrient-poor environments), and also full of scuba divers (they
like clear water and corals).
Another difference between the Pacific
and the Caribbean today is that the Pacific
is still home to a group of conspicuous
snails, known as the paciphiles, that went
extinct in the Caribbean sometime after the
rise of the isthmus. Based on this observation and little else, paleontologists had
constructed a picture of a mass extinction
that had supposedly wiped out a large percentage of Caribbean mollusks. “Basically,”
says Jeremy Jackson, a paleontologist at
the Tropical Research Institute, “that picture was based on crummy data.”
Jackson has compiled better data. He
has counted more than 800 different types
of fossil snails and clams of the Caribbean,
dating from the past 8 million years. He
has found that rather than killing mollusks,
the emergence of the isthmus triggered a
40 percent increase in the number of mollusk types in the Caribbean. The diversity
increase continued until 1.5 million years
ago. At that point, a spate of extinctions
did wipe out one-third of the types. “But
those losses were replaced by new species,” Jackson says, “so that overall, diversity did not decrease.” In any case, he says,
the extinctions could not have been caused
directly by the emergence of Panama, because they happened 2 million years later.
Yet the isthmus may well have led to
the extinctions in some sort of complicated
and indirect way that researchers haven’t
figured out yet—just as they hadn’t recognized, until Knowlton’s study, that the
evolutionary impact of the isthmus began
millions of years before it became land.
Therein lies, as Jackson sees it, the lesson
of his and Knowlton’s work, a lesson he
says may be relevant to our current efforts
to foresee the effects of global environmental change.
“We shouldn’t expect to see a simple,
direct correspondence between changes in
the physical world and changes in the biological world,” says Jackson. “Everybody
is wondering about global warming: it’s
going to get a little warmer—what’s going
to happen? What this research suggests is
that maybe nothing is going to happen, until, all of a sudden, something big will happen. You can’t go out and measure a
change in temperature and automatically
expect to see a change in biology. It’s going to be much more complicated than
that.”
From Discover, November 1993. With permission of the author, Kathy A. Svitil. © 1993 by Discover magazine.
2