The world’s most devastating climate-driven loss of ocean life has been reported in the eastern Mediterranean, one of the fastest warming places on Earth.
Native mollusc populations along the coast of Israel have collapsed by about 90% in recent decades because they cannot tolerate the increasingly hot water, according to a new study, which raises concerns about the wider ecosystem and neighbouring regions.
Scientists said the sharp decline of native cockles, whelks and other shallow subtidal invertebrates is likely to have spread to waters off other countries in the region and would continue to progress westward to Greece and beyond as global temperatures increased.
The paper – published in the Proceedings of the Royal Society B journal – estimates native mollusc populations have fallen to 12% of their historical species richness in shallow subtidal sedimentary substrates, and to 5% on rocky substrates.
The authors of the paper expressed surprise at their findings. “The magnitude was totally unexpected,” said Paolo Albano, a marine biologist at the University of Vienna. “I expected a seascape that I was well accustomed to as a Mediterranean specialist but enriched with some interesting exotic species that had entered through the Suez canal. But what I found was a desert, totally devoid of even common Mediterranean species.”
The murex, for example, is a gastropod that has been used throughout the Mediterranean since Roman times for the Tyrian purple clothing dye. Albano said he found no members of this species on the 200km of coastline in the four-year study.
The research team took samples at multiple points, then compared living mollusc numbers with previous population sizes, which were estimated from empty shells found in sediment. The shortfall exceeded anything seen before. “This is the largest climate-driven regional-scale diversity loss in the oceans documented to date,” the paper says.
The change is visible at scuba-diving depths of between 5 and 40 metres. In deeper waters the temperatures are lower. In intertidal areas, species have evolved to adapt to wider temperature ranges. In between, native molluscs are disappearing.
As with the declines of pollinators and soil quality on land, this has wider consequences. Molluscs make up the largest marine phylum, accounting for 23% of all sea organisms. As well as providing meat for the seafood industry, they play an essential role in regulating the chemistry of the ocean by recycling nutrients and removing nitrogen and phosphorus. In part that role might be taken on by new invasive tropical species from the Red Sea, but preliminary results suggested they would not perform the same ecosystem role as the lost native ones.
“The ecosystem will be different and it will function in a different way. This is very clear. But the situation is so dynamic it is hard to predict the consequences,” Albano said.
The scientists believe the cause of the destruction is human-driven climate disruption. The Israeli coast – which is one of the hottest parts of the Mediterranean – experienced a temperature increase of 3C between 1980 and 2013. The average summer surface temperature is 32C. This is thought to have triggered the eradication of native mollusc populations – a phenomenon detected in previous studies elsewhere.
Pollution and the arrival of tropical species through the Suez canal were deemed less significant factors. Pollution tended to be localised around ports such as Haifa, and the canal has existed for over 150 years. Studies of the coastline in the 1970s and 80s showed healthy waters.
Albano said the collapse was likely to have occurred in the past 20 years and had affected the entire ecosystem: “The sea is completely changing from temperate Mediterranean to impoverished tropical. This turnover is rapid and in progress. It has not yet reached a stable state.”
The few native mollusc species that remained were struggling. They were widely scattered and 60% failed to grow to reproductive size.
Albano believed this was part of an irreversible trend as global warming made the relatively cool Mediterranean more like the tropical Red Sea. This echoes similar findings in the far north, where scientists say the cold Arctic Ocean increasingly resembles the warmer Atlantic in terms of biology, chemistry and temperature.
As well as stepping up protections of the still relatively pristine deep waters and tackling localised problems like pollution, Albano said the only way to address this shift was to tackle climate change by reducing emissions as soon as possible.
“In my opinion we have no choice. This should be first on the list of things to do when we consider how we are changing the planet. It is blind to think this should not be tackled immediately. Exactly like Covid, we need to treat this as an emergency.”