Researchers use historic remnants like antlers, shells, teeth and pollen to learn how natural communities once worked. The clues serve as guides for restoration.
Conservationists seeking to restore shark populations on the Atlantic coast of Panama were facing a problem all too familiar to biologists: No records existed to document what pristine shark communities looked like before overfishing decimated the animals over the past few decades. Without that information, how could the restoration workers know what they should be aiming for?
Erin Dillon, a paleoecologist at the Smithsonian Tropical Research Institute in Panama, thought she had the solution. By sampling microfossils — dermal denticles, the “little teeth on the shark’s skin,” as she describes them — deposited on the ocean floor, Dillon was able to reconstruct a picture of shark communities in the region before human impact. Shark abundance on the Caribbean reefs has declined by over 70 percent, she found, with fast-swimming, open-water sharks hit the hardest.
Dillon is one of the rising stars in the burgeoning new field of conservation paleobiology, which uses the fossil record to inform and assist present-day conservation efforts. “We often need some sense of the way things used to be before there was extensive human impact,” says Karl Flessa, a paleobiologist at the University of Arizona who coined the term “conservation paleobiology” two decades ago and coauthored an early look at the field in the 2015 Annual Review of Earth and Planetary Sciences.
Conservation paleobiologists are using the past to establish pre-disturbance baselines, as Dillon has done. They are also documenting long-term patterns of habitat use and revealing previously unsuspected changes in ecosystems as a result of human activity. By uncovering how species have responded as past climates changed, they are helping to understand how the same species may respond to climate change today. And their results are guiding management plans for some of the world’s most endangered ecosystems.
Tracing caribou migrations of the past
Often, paleontological data offer the only practical way to understand the long-term ecological patterns that are so critical to conservation decisions. That’s the case for caribou herds on the Arctic coastal plain of Alaska, which have proved difficult to study in real time. The animals migrate extensively, and they use different parts of their home range each year, so ecologists have a hard time knowing which areas are crucial to maintaining caribou populations.
“There’s so much year-to-year variability,” says Joshua Miller, a paleoecologist at the University of Cincinnati. “It can be challenging to make conservation decisions when you don’t know the long-term value of a place.”
So Miller turned to the paleontological record — specifically, accumulations of the antlers the animals shed each year. Unusually for members of the deer family, females as well as males have antlers, which they shed shortly after calving. In the Arctic climate, these antlers remain intact for hundreds or thousands of years, providing a long-term record of where calving occurs. “You really can walk on the landscape today and get some essence of what caribou were doing thousands of years ago,” says Miller.
By counting and radiocarbon-dating hundreds of antlers, Miller was able to document that caribou have relied for thousands of years on the same calving grounds along the Arctic coast that a well-known major herd, the Porcupine herd, still uses today — including a period 3,100 years ago when summer temperatures were even warmer than today. “That gives us some confidence that the patterns we see today should be maintained over the next period of climatic change,” says Miller.
And that’s not all the information to be gleaned from shed antlers. Miller also measured the ratio of two stable isotopes of the element strontium, which gets deposited in the animals’ antlers each summer because it’s chemically similar to the calcium that builds antler bone. Different habitats contain different ratios of the two strontium isotopes, so the ratio provides a way to track the animals’ summer range.
As with the calving grounds, the summer range of the Porcupine herd has remained stable over time, Miller found. But that’s not the case for the Central Arctic herd, which lives farther to the west. Before there was much human activity, the strontium isotope ratio shows that the caribou spent much of their summer along the coast. But beginning about 1980 — roughly when oil development began along there — they began avoiding the coast and summering farther inland. While that is not conclusive proof that oil development caused the shift, Miller notes, it does point to the coastal region’s importance for the caribou — a key consideration for conservation.
Cattle grazing in historic Los Angeles
Occasionally, the fossil record completely changes the way conservationists think about an ecosystem. For example, ecologists had assumed that the muddy seafloor off the coast of Los Angeles had always been that way. But when sedimentary geologist and paleoecologist Susan Kidwell of the University of Chicago and her colleague Adam Tomašových of the Slovak Academy of Sciences in Bratislava began studying seafloor samples as part of a wastewater monitoring program, they were surprised to find remains of shelly creatures called brachiopods. These don’t live on muddy seafloors but on hard, sandy or gravelly bottoms.
Chemical dating of the shells revealed that the youngest remains dated from the late 19th century — about the time when the Los Angeles area was heavily grazed by cattle. Runoff from overgrazed, eroding soil, Kidwell and her colleagues concluded, must have smothered the hard surfaces the brachiopods needed, resulting in the local extinction of an entire ecosystem. “Despite 50 years of close monitoring on one of the best-known continental shelves in the world, it was utterly unsuspected,” Kidwell says.
The discovery gives local conservationists a new target for their restoration efforts, though it could take centuries for the mud to wash away. In the meantime, Kidwell notes, it becomes more important to protect gravelly or sandy seafloors that still remain farther offshore, near the Channel Islands.
Fossils aren’t only useful for learning about the past, however. They can also suggest how plants and animals might respond to future events — most pressingly, climate change. For example, Jenny McGuire, a conservation paleobiologist at the Georgia Institute of Technology, and her colleagues studied fossilized pollen grains to see how 16 important plant taxa from North America responded to climate change over the past 18,000 years. Did the plants shift their ranges to follow their preferred climate, the researchers wondered, or did they stay put and make the best of things as climate changed around them?
Twelve of the 16 taxa changed their geographic distribution to maintain similar climate niches, the researchers found — even in periods when the climate was changing rapidly. But such shifts may not be as easy today due to loss and fragmentation of their habitats. The lesson, McGuire says, is that plants that shifted instead of adapting locally could be at the greatest risk today and require extra conservation aid. “It tells you which plant taxa you have to worry about,” she says.
Conservation paleobiology is new enough that its insights are only starting to percolate through to the government agencies that make conservation decisions on the ground. That’s largely because institutional change takes time. “Any of us who actually work with agencies — as well as people who work for agencies — can tell you just how slowly and carefully and thoughtfully agencies change anything about what they do,” says Kidwell.
It is happening in a few places, though, most notably in the Florida Everglades, where decades of water diversions and drainage have significantly altered the natural flows of fresh water that maintain the ecosystem. Federal, state and local governments are working to return the region’s water regimen closer to its natural state — but no records exist of what flow rates were before drainage began.
So Lynn Wingard, a paleoecologist with the US Geological Survey, turned to the fossil record. Wingard knew that each species of mollusk living in the Everglades has its own preferred level of salinity. By making a census of the relative abundance of shells of 68 kinds of mollusks in sediment cores and comparing it to data from living communities, she could estimate the average salinity at each point in time in the past.
Then one day she found herself in a meeting room with a hydrologist who knew how to predict salinity from water flow rates — and they and others in the room realized that they could turn his equations around and use salinity to figure out historic flow rates. “We all had this massive brainstorm: Yes, we can do this, and it would allow us to calculate flow before there was any flow monitoring,” says Wingard. Wingard’s salinity numbers are now the official targets for Florida Bay restoration.
Conservation paleobiology has limits
In theory, paleobiologists could apply their techniques to explore ecosystems millions, or tens of millions, of years in the past. By doing so, they could treat the history of life as a vast experiment — examining, for example, repeated known periods of rapid climate change to see what characteristics put species at greatest risk of extinction.
But looking into deep time this way brings risks, experts say. Ecosystems do change, so ones indicated by fossil assemblages may differ from modern ones in important ways. “The farther back you go in time, the more difficult it is to predict things directly, because the species are different, the ecosystems function differently,” says Michal Kowalewski, a conservation paleobiologist at the University of Florida who heads a research network of practitioners in the field. “So the last few hundred years give us the most information.”
A further limitation of fossil data is that historic time periods get somewhat blurred. “However carefully you take a sample, it’s going to be a mixture of organisms that lived at different times,” says Kowalewski. That can make it difficult to use the fossil record to track changes that were rapid, especially as you go deeper into the past where the blurring is often greater.
And practitioners note one more concern: Even if we can correctly identify the way ecosystems were in the past, it may be impractical to try to restore them to that state today. “It’s not as easy as ‘This is what it used to be, we should bring it back to that,’” says Jonathan Cybulski, a historical ecologist at the Smithsonian Tropical Research Institute and the University of Rhode Island. Sometimes — as is the case for the ocean floor off Los Angeles — conditions have changed so much that restoration is impractical. But even so, he notes, paleoecological data can help conservationists refine their targets.
Other times, restoration may even be undesirable. Grizzly bears, for example, used to thrive in coastal California, now among the most heavily settled parts of the state. Few would endorse returning grizzlies there.
Despite these concerns, conservation paleobiologists see a bright future in digging into the past to guide the future, because so many plants and animals leave fossils of some sort: pollen, teeth, shells or other traces, especially from relatively recent times. “These archives are pretty much everywhere, both in terrestrial habitats and marine habitats. We can pretty much go to any region of the world and look at the young fossil record,” says Kowalewski. “In many ways, it’s even easier to do this than to inventory living biodiversity.”
10.1146/knowable-080323-1
This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.
