As a result of climate change, the oceans are also warming, which poses problems for many sea creatures to adapt. This is particularly dramatic in the eastern Mediterranean, as researchers have found. The water off the coast of Israel is already so heated that up to 95 percent of the native mollusc species there have become extinct. Introduced tropical species, however, multiply rapidly.
The rising temperatures due to climate change keep oceans and inland waters heating up. Organisms that are not adapted to these warmer water temperatures migrate to cooler water areas or die. Especially in very warm marine regions, such as the eastern Mediterranean, many marine species are already living at their tolerance limit with regard to the water temperature.
An international research team led by Paolo Albano from the University of Vienna has now investigated the effect of climate change on biodiversity in the eastern Mediterranean on the Israeli coast as an example. There the temperature on the water surface is between 17 and 30 degrees Celsius. This sea area is one of the warmest in the Mediterranean. For their study, the scientists first reconstructed the original occurrence of marine mollusks in this area, such as snails and mussels. To do this, they examined over 100 sediment samples on the empty shells of the species that used to live there. The researchers compared the biodiversity determined in the process with the current occurrence of molluscs.
95 percent of the former diversity of molluscs has disappeared
It turned out that the biodiversity of the mollusks indigenous to this marine area has decreased dramatically. “This is the largest climate-related loss of regional diversity in the oceans that has been documented to date,” the researchers explain. The shallow water habitats are hardest hit. The team found no living individuals of up to 95 percent of the more than 300 species that were previously found there. Most of the species have disappeared recently, possibly in the last few decades. In the deeper and cooler sea zones, Albano and his team documented around 50 percent of the previous biodiversity. According to observations, native species no longer grow sufficiently to reproduce – “a clear sign that the collapse of biodiversity will continue,” said Albano.
While the mollusc species originally native to the eastern Mediterranean are on the decline, tropical invaders have increased significantly, as the researchers found. They explain this with the fact that the species that migrate via the Suez Canal are well adapted to the warm water in the eastern Mediterranean and can therefore settle in large numbers. They now form large populations with fully reproductive individuals. “The most common native species are absent, while in contrast the tropical species are everywhere,” summarizes Albano. “The underwater scenario in Israel is unrecognizable.”
Further decline to be expected
According to the scientists, the future prospects for the Mediterranean are bad. The reason: Even if carbon dioxide emissions were stopped today, the sea would continue to warm for a long time, say Albano and his colleagues. This is ensured by the inertia of the ocean system – it only reacts to global warming with a long delay. The researchers therefore suspect that a similar collapse in biodiversity is already taking place in other, as yet unexplored areas of the eastern Mediterranean. In the future, the decline in native biodiversity is likely to spread and intensify further west.
Only in the deeper, cooler sea regions and in the tidal area, where organisms are better adapted to temperature extremes, the native species are likely to survive – at least for some time – according to the scientists. “But the future is bleak if we don’t act immediately to reduce our carbon emissions and protect marine habitats from other pressures that contribute to the loss of biodiversity,” warns Albano. “The changes that have already occurred in the warmest areas of the Mediterranean may not be reversible, but we could save large parts of the rest of the sea basin.”
Source: University of Vienna, Article: Proceedings of the Royal Society B – Biological Sciences, doi: 10.1098 / rspb.2020.2469