Image of a lionfish in the sea, surrounded by smaller fish.

Alien species and climate warming are irreversibly altering the Mediterranean

One of the world’s leading biodiversity hotspots is under siege. Marine biologists warn that prevention and adaptation are the only ways to save endemic species and keep the Mediterranean alive.

A perfect storm is brewing in the Mediterranean Sea, threatening its unique biodiversity and the people who depend on it.

Despite covering just 0.18% of the global surface area, it is one of the world’s foremost biodiversity hotspots. It is estimated that approximately 7% of all known marine species live in the Mediterranean and somewhere between 3,400 and 5,100 are endemic, which is the highest degree of endemism globally.

However, this immense biodiversity and all the economic and ecological benefits that come with it are under threat. Experts are concerned that rising ocean temperatures and the onslaught of alien species through the Suez Canal have pushed the Mediterranean to a tipping point.

Mediterranean warming faster than anywhere on Earth

According to MedECC, a climate research network that advises policymakers, temperatures have risen steadily since the 1980s, with the trend accelerating in the 1990s.

MedECC estimates that average sea temperatures have increased by about 0.29 ºC to 0.44 °C per decade since 1980 compared with the preindustrial average. This phenomenon is particularly pronounced in the Eastern Mediterranean, which is surrounded by desert and is generally shallower than the centre and west of the sea.

Stelios Katsanevakis, a researcher and marine ecologist at the University of the Aegean, says that maximum summer temperatures now exceed 32 ºC, well above the temperature thresholds of native and endemic species, which are adapted to much cooler water.

“Based on the projections of how the temperature will be in the following decades, it seems that this situation will expand westwards and northwards in the Mediterranean,” says Katsanevakis. “We are going to see many extinctions.”

As average temperatures rise across the Mediterranean, so too do the occurrence of deadly marine heatwaves.

“Thirty years ago, the percentage of the Mediterranean impacted every year by marine heatwaves was quite low, less than five per cent. From 2015 to 2019, this figure exceeded 90 per cent,” says Katsanevakis.

“So almost the entire Mediterranean is affected by at least one marine heatwave every year,” he adds. “And this causes massive mortalities for many native species.”

A steady stream of invaders from the south

The completion of the Suez Canal in 1869 changed the world. Lloyd’s List, a shipping data journal, estimates that USD 9.6 billion of cargo passes through the canal daily. The continued dredging of the canal to allow ever-larger ships to navigate it has also opened the way for the tropical species of the Red Sea and the Indo-Pacific to enter the Mediterranean.

“We know that about 800 species have come through the canal and colonised the Mediterranean,” says Jason Hall-Spencer, professor of marine biology at the University of Plymouth.

“There are some that fit in quite well and don’t cause any noticeable problems to the economy, human health or the ecology,” he adds. “But there are others that cause havoc.”

Many of these invasive species thrive in the Eastern Mediterranean as they are well-adapted to warmer waters. They occupy existing niches and prey on native species or other aliens.

Furthermore, many of the species have no natural predators, allowing their numbers to grow rapidly in a boom-and-bust cycle. Rising populations during the boom period, which can last decades, permanently damage ecosystems adding pressure to native and endemic species.

The only thing you can do is prevent alien species from coming in, and this is why treating ballast water is good because it’ll help prevent the problem.

Paolo Albano, Marine biologist

Prevention and preservation are the only solutions

Paolo Albano, a marine biologist at the Stazione Zoologica Anton Dohrn in Naples, says eradicating alien species in an environmentally friendly manner is impossible as they are already too widespread.

As a result, the emphasis must be placed on preventing the continued spread of invasive species and native habitat preservation.

Along with attaching themselves to the hulls of container ships, a process known as biofouling, half of the invasive species in the Mediterranean travel in the ballast water, which is used to provide stability to vessels and offset the weight of the cargo.

Albano and Katsanevakis believe the International Convention for the management of ballast water that entered into force globally in 2017, requiring cargo ship operators to treat their ballast water, is a positive step.

“The only thing you can do is prevent alien species from coming in, and this is why treating ballast water is good because it’ll help prevent the problem,” says Albano.

Hall-Spencer believes the problem of biofouling and species swimming through the canal can be prevented by recreating the Great Bitter Lake, an area of high salinity in the Suez Canal that served as a natural barrier to invasive species until a recent enlargement.

He adds that this can be done by pumping the brine extracted from water in Egypt’s desalinisation plants into a narrow section of the canal to recreate the barrier. However, a lack of political will in Egypt means this solution is unlikely to be implemented soon.

Jan Steger, a postdoc at the University of Vienna who studies the Eastern Mediterranean, says the immediate focus should be on native habitat preservation.

“What always helps is to protect the habitats from point source anthropogenic interferences,” he says. “You can prevent or mitigate some of the negative effects of the invasive species if you don’t add other stresses.”

This means protecting the natural environments of endemic species from human-created interferences, such as pollution, overfishing and destructive activities such as bottom trawling.

Marine Protected Areas (MPAs) have a critical role to play in this respect. They allow the preservation of native species, such as groupers, which are natural predators of some alien and invasive species.

However, Albano warns that establishing new MPAs takes a long time, and there may be opposition from stakeholders.

We are starting to have many local extinctions, and the problem is that if native biodiversity declines and species are lost continuously, this will translate into losing ecological functions and services.

Stelios Katsanevakis, Marine ecologist

Red alert in the Mediterranean

As a result, some degree of adaptation will have to take place for human communities on the Mediterranean coast.

Still, experts warn that only a tiny percentage of invasive and alien species present new economic opportunities. The most prominent example is the blue crab in Tunisia, which has become a multi-million-dollar fishing industry for locals.

Commercial and sport fishing of the lionfish – considered by the World Wildlife Fund to be the single most damaging invasive species – for food and jewellery is another example in Cyprus. Additionally, many alien and invasive fish are caught by commercial fishers.

But the new opportunities these species create pale in comparison to the damage they are doing to endemic and native species in the Mediterranean.

Due to environmental pressures caused by climate change, the presence of alien species in the Eastern Mediterranean has some benefits. Although marine biologists emphasise that this is because global warming otherwise destroys the ecosystem.

“We are starting to have many local extinctions, and the problem is that if native biodiversity declines and species are lost continuously, this will translate into losing ecological functions and services,” says Katsanevakis.

“So, in one way, these alien species that arrive and replace native species have a positive impact as they secure these functions that will have been lost,” he adds.

However, marine biologists are generally pessimistic about this development, and its consequences are not yet understood.

“It’s really a new combination,” says Steger. “It’s not the Indo-Pacific. It’s not the Red Sea but also no longer the Mediterranean as we know it.”

Albano says the devastation caused by alien species and climate change in the Eastern Mediterranean should be a stark warning to the rest of the basin and the world.

“What you see there now may be seen in the central and western Mediterranean in 20 to 50 years,” he warns.

Written by
Daniel Dawson
Contributing Writer at Spoon Agency