Scientists reviewing 85 million years of fish fossil records say that the basic structure of the ocean ecosystem – the ratio of fish compared to elasmobranchs such as sharks, skates, and rays – has remained stable for periods of tens of millions of years, despite extreme environmental changes caused by climate shifts of the past. Distinct shifts between three stable states occurred rapidly, independent of climate shifts.

Scripps Institution of Oceanography at UC San Diego graduate student Elizabeth Sibert led an analysis of microscopic fossil fish teeth and mineralized shark scales known as denticles preserved in sediments on the seafloor for millions of years. The team found that there have been two major events in which the make-up of ocean life changed substantially.

Scripps Oceanography researchers identified three main periods of distinct marine life composition in the oceans: Image credit: Scripps Institution of Oceanography

One of those events was the Cretaceous/Palaeogene mass extinction 66 million years ago to which scientists attribute the disappearance of dinosaurs. Sibert and colleagues had earlier found that the abundance and diversity of fishes exploded once many of their main predators went extinct. Shark abundance, however, neither rose nor fell at the extinction.

For the next 45 million years, the ratio of sharks and fishes remained stable, while absolute abundances of both groups rose and fell in concert alongside changes in global climate, suggesting that the structure of the ecosystem was resilient to climate change.

Another transition 20 million years ago saw a sharp drop-off in the number of sharks in the world’s oceans, alongside a dramatic increase in variability of fish abundance, suggesting that sharks suddenly spent considerably less time in the open ocean ecosystem. The researchers conclude that what triggered changes in the community structure has to do primarily with how competition with other marine organisms – including plankton, invertebrates, seabirds, and marine mammals – influenced the balance of life in the oceans.

“It is really striking that the community structure is so stable during each of these long intervals,” said Sibert, whose research is supported through a National Science Foundation Graduate Research Fellowship. “It shows that it takes a major disaster or evolutionary regime shift to change fundamentally how the consumers in the ocean interact.”

The study, “Eighty-five million years of Pacific Ocean Gyre ecosystem structure: long-term stability marked by punctuated change,” appears in the May 18 edition of the journal Proceedings of the Royal Society B.

The researchers concluded that drastic swings in global climate, including periods of intense warming analogous to what Earth is experiencing at present, did little to alter the long-term structure of the marine vertebrate community. Sibert said those episodes from prehistory, though, do not serve as a guide for potential changes in marine ecosystem structure today, since the rate of today’s modern global climate change is much faster than anything experienced in the past, and the impact of human interactions with the oceans has no precedent.

Sibert’s co-author, Scripps paleobiologist Richard Norris, said that during the transition from “Cretaceous” oceans to “Paleogene” oceans 66 million years ago saw the disappearance of highly abundant invertebrate organisms called ammonites. Their mass extinction released fishes from predation and allowed them to explode in abundance in the warm greenhouse world of the Paleogene. Later, in the Modern ocean system, the evolutionary diversification of marine mammals, seabirds and large pelagic fish that compete with sharks, alongside the dramatic increase in variability of fish production, may have driven shark abundance down in the open ocean.

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