Extinction in ocean’s mud presages key ecological changes

The loss of seemingly inconsequential animal species in the marine benthos – the top 6 inches or so of mud and sediment on the floors of the world’s oceans – is giving scientists a new look ahead at the consequences of the steady decline of the world’s biological diversity.

In new work published today (Nov. 12) in the journal Science, an international team of scientists describe work in which the ocean mud and the many animals that live there are used to forecast how the extinction of species alters important ecological processes that sustain life at the bottom of the ocean.

The team’s work was funded by the National Science Foundation.

“This is one of the first stabs at trying to see what will happen in ocean ecosystems as species go extinct,” says Bradley Cardinale, a University of Wisconsin-Madison postdoctoral fellow in zoology and a co-author of the paper. “What goes on in the sediment is important, not only because it affects life at the bottom of the ocean, but also because it has a big impact on the rest of the marine ecosystem.”

In the mud and sediments that have accumulated during many thousands of years at the bottom of the world’s oceans lives an astonishing array of animals – such as crabs, clams, sea urchins, brittlestars and marine worms. These animals play an essential role in churning up and filling the sediments with oxygen, making it possible for other forms of marine life to flourish.

The new study rests on a comprehensive survey of 139 marine invertebrates that inhabit the sediment of Galway Bay, Ireland, led by Martin Solan of the University of Aberdeen, Scotland. By looking at how extensively the sediments are mixed there, and matching that with data on each species’ size, abundance and movement through the mud, it is possible to construct mathematical models to predict the ecological consequences of losing species, according to Cardinale.

Running the models, the group found that the extinction of species is generally expected to reduce the amount of sediment mixing, and consequently diminish the oxygen concentrations that sustain bottom-dwelling life. The amount of change, according to the study, depends on the reasons species are going extinct and the order in which animals disappear. “We know certain types of species are at greater risk of extinction than others,” says Cardinale. “For example, large species often go extinct first, and that is important in the marine benthic environment because the bigger you are, the more sediment you are able to mix up.”

And while the creatures that inhabit the mud at the bottom of the ocean may seem remote and unimportant, Cardinale pointed out that oceans cover 70 percent of the Earth’s surface, and that the productivity of the sea is intricately linked to sediments that generate nutrients and food for other organisms such as fish. In places where human activities have disrupted marine sediments, such as the enormous “dead zone” in the Gulf of Mexico – where excess fertilizers are dumped by the Mississippi River – nearly all life has vanished. “One thing our study suggests,” Cardinale asserts, “is we need to know why species are going extinct in the first place. Even though extinction leads to less sediment mixing in our models, things can be far worse or not quite as bad, depending on the particular order in which species disappear.”

To maintain the functional necessities of an ecosystem, the study also suggests, that both the total number of species and the particular types of species going extinct matter: “There are certain really high-impact species that, if they survive, sediment mixing can be maintained for some time,” he says. “Even so, reducing the total number of species eventually leads to large changes. This suggests that conservation efforts should focus not just on the seemingly important species, but also on the total variety of life found in an ecosystem.”

Despite that, the discovery that the order in which animals go extinct is important and essential to knowing what the long-term environmental effects will be. “The finding is important because it argues that the particular cause of extinction ultimately governs the ecosystem-level consequences of biodiversity loss,” the researchers conclude.

In short, predicting how coastal environments will cope as animal species decline as a result of human activity will depend on a better grasp of why species are at risk and how that risk is amplified or minimized by an animal’s functional traits.

In addition to Cardinale and Solan, authors of the Science paper include Amy Downing of Ohio Wesleyan University, Katharina Engelhardt of the University of Maryland Center for Environmental Science, Jennifer Ruesink of the University of Washington and Diane Srivastava of the University of British Columbia.