The ecosystem had thrived for thousands of years but collapsed less than two centuries ago.
The seabed off the coast of southern California is one of the most studied areas in the world, characterized by its high biodiversity and by its important roles in biogeochemical cycling and commercial fishing. Today, this seabed consists of soft sediments and is inhabited by mollusks, crustaceans, worms, and urchins that feed on organic matter. However, this was not always the case.
Paleontologists Susan Kidwell of the University of Chicago and Adam Tomašových of the Slovak Academy of Sciences recently discovered a lost ecosystem off the coast of Southern California that once stretched for nearly 250 miles from San Diego to Santa Barbara. Kidwell and Tomašových noticed an abundance of dead shells from scallops and marine organisms called brachiopods in the muddy California seabed and began examining the remains. Using geologic methods that Kidwell had developed since the 2000s, the researchers discovered that the now-muddy seabed was once decorated with shell-gravel habitats that housed these scallops and brachiopods for at least 4,000 years.
Kidwell and Tomašových analyzed 190 shells using a molecular dating technique known as amino acid racemization. They found that all of the shells were older than 100 years and that most were over 200 years old. No similar shells have been produced in the region within the past century, indicating that the sea creatures died off recently and relatively quickly.
Scallops and brachiopods prefer colder waters than those found off the coast of southern California, but Kidwell and Tomašových do not consider climate change to be a likely cause of the ecosystem’s collapse. Instead, they argue that the main culprit is siltation, the pollution of water by fine sediments such as silt.
In 1796, Spanish missionaries introduced livestock such as cattle and sheep to southern California. For the next century, the region’s economy was dominated by cattle production, subjecting the land to unmanaged, open-range grazing. The researchers believe that siltation resulting from this unmanaged grazing altered the ecosystem in the coastal seabed during the 1800s, leading to the decline and eventual collapse of scallop and brachiopod populations.
“This loss unfolded during the 19th century,” Kidwell explained, “Thus well before urbanization and climate warming. The disappearance of these abundant filter-feeding animals coincided with the rise of lifestock and cultivation in coastal lands, which increased silt deposition on the continental shelf, far beyond the lake and nearshore settings where we would expect this stress to have an impact.”
Kidwell and Tomašových published their findings online in the journal Royal Society Proceedings B, arguing that more research is needed to fully understand the ecological consequences of coastal land use and siltation.