Warning Signs in Nature


To some, examining an old shell along the shoreline can be a highlight of a day’s outing at the beach. To others, a shell is much, much more. A shell can offer crucial pieces of ancient tales that describe where the environment has been and a warning sign about where it might be headed.

Jill Leonard-Pingel, a graduate student at Scripps Institution of Oceanography at UC San Diego, made four trips to Panama and Costa Rica between 2006 and 2011 to collect samples of Caribbean rocks and their embedded treasures of ancient oysters, clams, mussels, and scallops.

The shells of such invertebrates, known as bivalves, can divulge a wealth of information to biological researchers about their surrounding environment.

“For clams the shells echo what they are doing in the mud—the shell’s shape can tell us how the clam makes a living, where it lives, and what it eats,” said Leonard-Pingel.

Such archived information allowed Leonard-Pingel, Scripps Professor Emeritus Jeremy Jackson, and colleague Aaron O’Dea to travel back in time more than 11 million years to study the Caribbean’s past environment. Their results are published in the September issue of the journal Paleobiology and provide new information about how environmental changes, including those induced from climate, can sway the history of biological organisms.

“Disentangling the causes of extinction in the past is tricky but critical if we want to make accurate predictions about how life in the future will fare,” said study co-author O’Dea, a scientist with the Smithsonian Tropical Research Institute in Panama.

Focusing on one important chapter in the Caribbean’s history, Leonard-Pingel and her coauthors highlighted the transformations brought by the rise of the 3.5-million-year-old land bridge today known as the Isthmus of Panama, which led to a key separation of the Pacific Ocean to the west and the Atlantic Ocean’s Caribbean Sea to the east. While certain Caribbean species of bivalves, gastropods, and bryozoans went extinct as a repercussion of the land bridge’s formation, others emerged and thrived. Yet, such dramatic changes in species survival and extinction didn’t take hold until some two million years after the bridge’s formation, presenting a puzzling lag for scientists.

What took so long?

Leonard-Pingel was able to trace the answer through the stories told by bivalves and their shells. Studying bivalves in collections called “functional groups,” the shells revealed where the animals lived and what they ate.

“There were dramatic changes in the functional groups of bivalves through time, and they closely reflect the type of habitat the bivalves lived in,” said Leonard-Pingel.

Prior to the formation of the Isthmus of Panama, the bottom of the Caribbean Sea mostly consisted of soft mud and sand. With the rise of the land bridge, and cut off from the chilly temperatures of the Pacific Ocean, warmer conditions in the Caribbean nurtured the growth of coral reefs. Over the following millions of years, the newly changed habitat ultimately dictated which bivalves would survive and which would go extinct.

Leonard-Pingel said other ecological studies have shown that extinction can take place a long time after initial changes are set in motion. In certain cases it may be due to a phenomenon known as “extinction debt,” similar to repercussions in rain forests as they are broken into smaller parcels of land due to deforestation, hindering the area from supporting as diverse fauna as it previously did.

“This study provides us with more insight into how environment, particularly changes in climate, can influence biology in the long run,” said Leonard-Pingel. “This can provide a cautionary tale for humans. Although we may not be able to see changes in biology and species survival in the short term, it is important to remember that changes in environment can take a long time to have an effect on life.”

Leonard-Pingel believes changes that humans are causing in today’s environment, from pollution to ocean acidification, could have long-lasting effects that are difficult to predict.

“The example of the tumultuous history of the Caribbean is worrisome because it shows that the response of animals to change can occur long after the smoking gun,” said O’Dea. “It may be that the widespread degradation of marine life that has already taken place from the blitzkrieg of overfishing, pollution, and climate change may have consequences far into the future that are inescapable. Alternatively, it could be a chance of reprieve. If extinction takes so long we may have an opportunity to reverse the tide.”


-- Mario C. Aguilera

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