Today, mud-dwelling clams occupy a stretch of sea bed some 250 miles off southern California. However, this was not always the case in the past.
Paleontologists discovered that the site had hosted thriving communities of scallops and shelled marine organisms (called brachiopods) for thousands of years in the past.
Then, during the 19th century, these communities started dying off, leaving behind their shells, and the site was progressively taken over by clams.
"This loss unfolded during the 19th century, thus well before urbanization and climate warming," said Susan Kidwell, the William Rainey Harper Professor in Geophysical Sciences, from the University of Chicago. With climate change ruled out as a cause of the communities’ die-off, the paleontologists turned their attention to human activities instead.
Both scallops and brachiopods have a low tolerance for high levels of suspended sediment, so they were in trouble when Los Angeles and Orange counties engaged in unmanaged, open-range grazing: "The disappearance of these abundant filter-feeding animals coincided with the rise of livestock 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," said Professor Kidwell.
She and fellow researcher Adam Tomašových of the Slovak Academy of Sciences concluded that siltation associated with the prolonged period of unmanaged land use probably led to the collapse of the brachiopod-scallop populations.
A paper in the 7 June issue of the Royal Society Proceedings B journal describes their discovery.