An analysis of over 70,000 fossils indicates that mollusc communities were incredibly resilient to major climate changes during the last ice age.
Scientists from the Florida Museum of Natural History and several European research institutes have tracked the history of Adriatic ecosystems through two warm periods that marked the most recent glacial expansion. Their results show that major changes in temperature, salinity and sea level have had much less impact on snail communities than the current environmental crisis caused by human activity in the region.
“It is sobering to consider that approximately 120,000 years of major climate change has not affected these ecosystems as much as human-induced changes over the past few centuries,” said lead author Micha? Kowalewski, Florida Museum Thompson chair of invertebrate paleontology.
Researchers have known for some time that modern Adriatic ecosystems are significantly altered from historical baselines.
“There are multiple human-induced stressors on these ecosystems, such as changes in land use that increase sedimentation rates,” said co-author Rafa? Nawrot, postdoctoral researcher at the University of Vienna, formerly at the Florida Museum. “This happened as early as the Roman Empire, when increased agriculture led to higher rates of erosion.”
While earlier civilizations along the Italian peninsula have left a noticeable signature on the ecosystems of the Adriatic, Nawrot explains that most of the changes have occurred over the past century. The influx of fertilizers into rivers and estuaries has triggered runaway reactions that deplete oxygen in marine and freshwater environments. Pollution from towns and cities creates a toxic mix for marine life, and cargo ships litter international shipping lanes with invasive species of stowaways that disrupt native ecosystems. Perhaps most detrimental to molluscs in the Adriatic, commercial fishing drags nets across the seabed, scouring the basin for bottom fish and bivalves.
“The Adriatic Sea is the most trawled area in the world,” Nawrot said, citing a large study published earlier this year.
What scientists have lacked, until now, are data on natural disturbances in the recent past of the Adriatic that they could use to assess the significance of current ecological changes.
“By looking at the fossil record, you can reconstruct a range of natural variability. If the current community falls outside of this range, it’s probably because of us,” said lead author Daniele Scarponi, associate professor at the University of Bologna.
To fill the gap in the area’s history, researchers sifted through long vertical sediment cores for marine mollusc fossils dating to the Late Pleistocene.
Ice Ages have been a turbulent time for global marine ecosystems. Sea levels rose and fell as water from the world’s oceans was cyclically released and trapped in massive continental glaciers. In total, there were about 17 ice ages during the Pleistocene, beginning about 2.5 million years ago and ending with the final retreat of the glaciers just 12,000 years before present.
The changes caused by the onset of the last ice age are particularly evident in the Adriatic Sea, which is mostly shallow in its northern extent. When sea levels dropped about 400 feet at the height of the Ice Age, the northern Adriatic nearly ceased to exist, its shores receding more than 150 miles south toward the Mediterranean.
“It would have been possible to walk from modern Italy to Croatia,” Scarponi said. “The entire northern part of the basin has been exposed and turned into a huge lowland plain.”
By studying fossils preserved before, during and after the last ice age, researchers were able to directly observe changes in mollusc communities. Their results show that as the oceans receded and cooled, molluscs that are today restricted to colder regions thrived in the Adriatic while heat-loving species declined.
The researchers note, however, that despite the sharp change in numbers, very few species have completely disappeared. “The main difference between snail communities across glacial/interglacial cycles is not extinction or the appearance of new species, but rather changes in relative abundance,” Nawrot said.
When the glaciers receded and the Adriatic reached its modern limits, mollusc species reverted to their earlier patterns of abundance. If scientists hadn’t known there was an ice age in between, it would have looked like nothing had happened at all based on the molluscs alone.
“Our fossil analyzes show that communities of mollusk species along the northern Adriatic coasts essentially reassembled into an almost identical picture of themselves when the seas returned,” Kowalewski said.
In itself, the discovery that molluscs are resilient to climate change is good news. Modern marine communities already face rising ocean temperatures associated with human-caused global warming.
“Temperatures during the last interglacial were actually a few degrees higher than they are now, and yet we see the same molluscan associations,” Scarponi said. “This means that near-shore snail communities are likely to be resilient to a slight increase in temperature in the future.”
But a warmer world brings with it a unique set of compounding problems, Nawrot warns. “Many stressors, such as anoxia and the effects of invasive species, will only intensify with warming, even if higher temperatures alone are not enough,” he said.
The future of marine ecosystems in the Adriatic and in the world’s oceans remains an open question, which the study authors say will require a variety of strategies to resolve. “We need international policies to tackle global climate change, but studies like these show we also need actions that mitigate local and regional threats,” Nawrot said.
The study was published in the journal Biology of global change.