Paleo detective work is at the core of an emerging field called conservation paleobiology. Its researchers hope to stem the loss of species and ecosystems in a rapidly changing world.

In the early 1990s, University of Arizona geoscientist Karl Flessa decided to scout out a new research site in the Colorado River Delta in Mexico. With few roads, it wasn’t easy: his four-wheel drive Chevy Suburban got stuck and he ended up hiring a fishing boat to travel to remote shores and islands in the Gulf of California.

Vast beaches of dead clams near the Gulf of California reveal the past impact of water diversions from the Colorado River. They also offer clues as to what it would take to bring back this once-productive ecosystem. Image courtesy of Karl Flessa (University of Arizona, Tucson, AZ) and reproduced from ref. 6.

But there was a big scientific payoff: the discovery of vast beaches made out of millions of clam shells. “For a paleontologist, that was a pretty cool thing,” Flessa says. And the discovery came with a mystery: Why had live clams almost completely disappeared?

That’s when Flessa had his “duh” moment, he says. The once-productive clam ecosystem must have been wiped out when dams and diversions on the Colorado River cut off the flow of water into the Gulf. “We realized that we could use the dead shells as a window into the past to measure the impact of those diversions, and to ask what it would take to bring the ecosystem back,” Flessa says.

From that insight—and similar ones—was born a new scientific discipline, which Flessa dubbed “conservation paleobiology” in 2002. The central idea is that the fossil record isn’t just a look backward; it also offers a crucial guide for strategies to conserve or restore species, communities, and ecosystems, today and in the future. “It’s putting the dead to work,” says Flessa.

The nascent field has brought new scientific support to restoring oysters in the Chesapeake Bay by using oyster sanctuaries. It has pinpointed dredging as the culprit in the collapse of scallop fisheries. It has documented dramatic changes in forest communities after big herbivores went extinct, and found that plants thought to be invasive in the Galapagos are actually returning natives. And Flessa’s own work shows that a relatively small pulse of water from the Colorado River to the Gulf of California at the right time of year could not only bring back clams, it could boost an important fish species, the totoaba. “It is exciting times for the field,” says Susan Kidwell, a geologist turned paleoecologist at the University of Chicago. Such insights are even starting to find a seat at the policy table, though it’s a tortuous path from research findings to concrete actions, and the extent of the policy impact remains an open question.

History Lessons Paleontology was being put to work in conservation even before the approach had a name. The rationale for reintroducing the California condor in the vicinity of the Grand Canyon in 1996, for example, hinged on fossil evidence in caves indicating that the condor naturally occurred in the area during the Pleistocene, according to Flessa. Since then, however, the approach has been made much more powerful by three technical revolutions, says Kidwell, which she described in a 2015 PNAS paper (1). One revolution is the ability to reconstruct key parameters, like growth rates, diet, and community composition from the fossil record. The second is the accumulation of much more (and better) data on past changes in the environment, making it possible to pinpoint the responses of species and communities to those changes. The third is technical refinements to dating techniques, such as radiocarbon and amino acid racemization (which measures the gradual change in the stereoisomeric forms of the amino acids), that have brought “stunning improvements in age determination,” Kidwell says, allowing the age of even an individual shell to be precisely known. “These revolutions have transformed our ability to reconstruct history,” she says. With these new tools—and a strong sense of mission—conservation paleobiology is now growing rapidly, with a 2011 National Science Foundation workshop (2) and review papers (3) that have defined the field and its challenges. “What’s new is a sense of community and a critical mass of people who call themselves conservation paleobiologists,” says Cornell University paleoecologist, Gregory Dietl. “Students are excited to go into it because they want to make a difference.” In a 2017 review in Science (4), more than 40 scientists argue that the knowledge gleaned from the fossil record “will be essential to sustain biodiversity and all of the facets of nature that humans need as we continue to rapidly change the world over the next few decades.”

Big Oysters At the heart of most of these approaches is a search for new ideas that help tackle complex environmental and ecological challenges. After arriving at the College of William & Mary in 2001, geologist Rowan Lockwood, now a full professor of geology there, realized the fossil record “could be leveraged to look at real world problems,” she says. One pressing problem: the precipitous decline in populations of Chesapeake Bay oysters. Before there were any human inhabitants, oysters filtered the water of the Bay about once every 3 days, scientists calculate. Now, that’s once every 300 days. Overall, oyster reefs around the world have declined by more than 85%. To compare the current oyster population to that of the past, Lockwood traveled up and down the tributaries of the Bay looking for fossil oysters poking out of the river cliffs. Some of those oyster reefs, laid down in warmer periods when sea levels were higher, are “spectacular,” Lockwood says. “The oysters are still oriented in the way they were when alive, a witness to an instant in time.” Lockwood collected more than 4,000 fossil oysters from 11 different sites, dating from 80,000 to 500,000 years ago. These ancient bivalves were enormous—three times the size of today’s biggest oysters at up to 30 centimeters—so Lockwood first suspected they must have grown faster back then. But her data showed something else: the ancient oysters lived to ripe old ages, up to 25 years old, compared to just 5 years old today. The surprise finding strengthens the case for creating sanctuaries where oysters are allowed to grow old and large, which leads to far greater production and increases the chances that more oysters will survive the two diseases that plague the population. Although closing areas to harvesting is politically difficult, researchers and fisheries managers have already shown that creating a few sanctuary areas has helped oyster numbers in the Bay climb some 12-fold from a near zero nadir in 2000. “There is still some frustration with the pace of restoration, but I’m much more encouraged about things than 20 years ago,” says Mark Luckenbach, professor of marine science at the Virginia Institute of Marine Science. Lockwood’s research “probably won’t change any fisheries managers’ ideas,” says Luckenbach, since they already understand the value of sanctuaries. But it does give them more ammunition in the fight, highlighting the importance of having large thick shells for oyster spat to settle on, he says. Current efforts to create new reefs involve dumping young shells, which last only a few years before breaking up or dissolving. “We knew about the need for big robust shells before [Lockwood’s] study, but she reinforced it,” says Luckenbach. In 2014, the pulse flow of Colorado River water (background) approached the tidal channels (foreground) of the Gulf of California; it was the first time the river had reached the sea since 1999. So much water is diverted from the river for drinking, agriculture, and other uses that water rarely makes it all the way to the sea. Image courtesy of Pete McBride (photographer).

Killer Cows In another tale of conservation paleobiology, researchers gained potentially useful restoration insights by leveraging not only biological samples and observations, but also archival data. The work was an ecological mystery story with a surprising twist. Kidwell, while studying the marine shelf off southern California, realized there was a treasure trove of information in the seabed samples that wastewater treatment plants were required to collect under the Clean Water Act, starting in 1972. “It was a wonderful unnatural experiment,” she says. The samples from the seafloor show that living communities of clams and other bottom dwellers have gotten healthier thanks to cleaner wastewater, as expected. But the samples also contain “death assemblages,” the shells that date back hundreds or even thousands of years, all the way back to when the continental shelf was first submerged 20,000 years ago. Kidwell and her collaborator, Adam Tomašových, were particularly intrigued by the brachiopod shells in the samples. Given that these filter-feeding creatures are extremely intolerant of mud, and that the current seafloor is very muddy, Kidwell figured they must have died out thousands of years ago. But when the researchers measured the ages of the brachiopod shells, they found that the populations had thrived until the 1820s before crashing completely by 1910. “We were completely surprised and nonplussed,” she says. “There had been absolutely no suspicion that such an ecosystem had existed, much less disappeared in modern times.” Kidwell ruled out warming trends or floods that might have brought more sediment to the seafloor. There had been no significant changes in temperatures or rainfall in the relevant time frame based on marine sediment and tree-ring data. Then she found the culprit: cows. Poring over the records of the Spanish missions and agricultural censuses after California became a state in 1850, Kidwell documented a huge expansion of livestock grazing that started in the late 1700s. “Los Angeles was the cow county of California, with a massive export industry of hides and tallow,” she says. The livestock trampled the ground, compacting the soil so that more rain ran off on the surface, picking up more sediment. Kidwell calculates that the amount of sediment flowing into the ocean jumped 10-fold, dooming the brachiopods. All that mud now makes it impossible to bring back the diverse and rich communities that once existed on the original seafloor of gravel and shells in pre-cow days. But if a “full natural” restoration isn’t possible, perhaps there is an “achievable natural” condition, Kidwell suggests. Along with wastewater quality improvements, interventions could include encouraging populations of deposit-feeding clams in the genus Nuculana that are associated with a diverse microbial community. “I’m very hopeful,” Kidwell says. “I think that in the next couple of decades, the muds will be getting back to where they used to be.” Piecing together the human impact on the coastal ocean off Los Angeles demonstrates the “huge role for conservation paleobiology,” says Kidwell. “Without this information, we would have had no idea how the continental shelf communities have changed.”

Novel Forests Sometimes, putting the fossil record to work not only suggests ways to bring back individual species, such as clams or oysters, it also offers lessons from the past that apply to multiple species or even to entire ecosystems, something closer to “full natural” restoration. At the University of Maine, for example, Jacquelyn Gill has been doing some clever sleuthing to figure out why novel forest communities flourished after the end of the last Ice Age. Today, spruce–fir forests dominate the northern United States and Canada, “We were completely surprised and nonplussed. There had been absolutely no suspicion that such an ecosystem had existed, much less disappeared in modern times.” —Susan Kidwell while hardwood forests reign further south. But about 14,000 years ago, the ash tree was king, living alongside spruce trees across vast areas of the planet in forests that were more open than today’s forests. “These novel tree communities lasted for 1,000 years and have no modern analog,” says Gill. What caused these ash forests to suddenly appear? Gill suspected it might be linked in part to the extinctions of mammoths, North American camels, and horses at the end of the Pleistocene, when “we lost half of the animals on the continent larger than a German shepherd,” she says. Proving that was challenging, however. Whereas there is an extensive fossil record of plant communities in the form of pollen in sediment cores, fossil animal bone finds are spotty and difficult to correlate with changes in the plant community. So Gill turned to a surrogate for the presence of big grazing animals, a fungus that lives only in animal dung. “The spore of this fungus is preserved in the sediment record,” she explains. “When you lose the spores, it shows that the dung—and the animals—are gone.” The combined spore and pollen fossil record makes it possible to reconstruct the past with remarkable precision. Gill showed that within 20 to 30 years after the spores disappeared some 13,500 years ago, the open ash–spruce forest emerged. It lasted for more than 1,000 years, until the warming planet created conditions that favored a forest with more oak trees. Gill’s findings provide support for a controversial idea, called “rewilding,” in which big grazing animals would be reintroduced to mimic the conditions of the Pleistocene (5). Opponents argue that such reintroductions could wreck ecosystems and drive currently endangered species to the brink. But proponents believe that the result could be communities that are more diverse and biologically productive, and more resilient in the face of climate change than today’s ecosystems. In fact, conservation paleobiology has revealed at least one natural experiment that provides evidence for such an improvement. Wild horse populations on the coasts of North Carolina and Virginia, descendants of horses brought by Spanish explorers starting in the 1500s, seem to be “assuming the role of now-extinct natives,” Dietl and his coauthors report in their review paper (3). And as those feral horses graze on salt marshes, they are increasing the diversity of birds and crabs. These successful applications of conservation paleobiology are aimed at understanding the impact of humans on the natural world and then using that past record to better shape conservation efforts. But the value of the field is larger than that, researchers say. Over millions of years, the planet has changed dramatically, with temperatures and atmospheric carbon dioxide levels periodically rising and falling. Even before the term “conservational paleobiology” was coined, the paleontological record was revealing how species and communities responded to those changes, providing a glimpse of what we might expect now in a changing climate. “We have all these natural experiments that we can learn from,” says Gill. The fossil record shows, for example, that most species responded to the extraordinary planetary warming leading up to the Paleocene–Eocene Thermal Maximum 55 million years ago in a similar way: they got smaller. That’s an experiment “that’s hard to do in the [laboratory], but which gives us a scenario that we can plan for,” says Dietl. The past record also shows that ecosystems take tens of thousands of years to recover from big increases in CO 2 in the atmosphere, a sobering reminder of what lies ahead if countries fail to curb their greenhouse gas emissions fast enough.