Using shells from recent fossils, scientists have discovered that the interactions between predators and prey in the northern Adriatic Sea have sharply declined due to human influences.
If observations of marine life in the Adriatic Sea had ceased in the mid-20th century, one might have viewed the situation positively. During the late 1800s and early 1900s, there was a noticeable increase in the populations of snails and the clams they prey on, indicating a thriving ecosystem.
However, a significant change occurred. Both predator and prey populations saw a drastic drop, with some species vanishing entirely. These were replaced by the common corbulid clam (Varicorbula gibba), which can slow down its metabolism under unfavorable circumstances. The presence of this species in fossil records often signals a harsh environment unsuitable for many other organisms.
“This species has increased in number and size compared to before, primarily due to reduced predation and competition,” explained Martin Zuschin, a paleontology professor at the University of Vienna. Together with colleagues from Slovakia, New Zealand, Austria, Italy, and the United States, they have released a new study highlighting the reduced predator/prey dynamics in the Adriatic Sea.
The results contribute to a growing evidence base showing that human activities have dangerously disrupted marine ecosystems in the region. Increased fishing practices, bottom trawling, runoff of nutrients, introduction of non-native species, and rising water temperatures due to climate change have profoundly altered marine life along certain areas of the Italian coastline.
“Our research in the northern Adriatic Sea indicates that the species variety in these habitats is much less complex now than in the past. Nowadays, many locations lack predators, grazers, and organisms that dwell on the sediment, while other species, like deposit feeders and sediment-dwelling animals, are more prevalent,” continues Zuschin.
To draw a parallel with terrestrial environments, the northern Adriatic can now be likened to a golf course, characterized by low biodiversity and nutrient overload. Zuschin and his team have been assessing the Adriatic’s decline for several years, comparing present-day organisms with fossils from the period before human influence was felt in the area.
This research falls under conservation paleobiology, which helps scientists assess biodiversity losses and propose methods to restore natural habitats.
The authors of this study had a unique opportunity to delve deeper. Beyond just examining declines in numbers and species, they investigated whether the interactions between species were also affected. This investigation is often challenging with most fossils. While physical evidence like bite marks can indicate ancient predator-prey interactions, such fossils are seldom found and are difficult to link to specific animal types when they are.
However, seafloor environments are one of the notable exceptions. Marine invertebrates with external shells have coexisted alongside predators capable of drilling into those shells for eons. Marine snails, worms, and even octopuses possess adaptations to grind and break these shells.
“Certain snails have specialized organs that produce acid to weaken the calcium carbonate in shells, facilitating the drilling process,” noted co-author Michal Kowalewski, the Thompson chair of Invertebrate Paleontology at the Florida Museum of Natural History.
The circular perforations left behind serve as indicators, allowing scientists to quantify predation levels.
The researchers collected samples from two areas: one in the northwest Adriatic at the Po River’s mouth and the other in the northeast Gulf of Trieste. At both sites, they extracted sediment cores using long, cylindrical tubes. The top layers contained younger sediment, more recently settled than that found in the deeper layers.
Both sites exhibited a similar pattern. The populations of predators and prey and the frequency of drill holes remained stable until the mid-19th century when all three showed a marked increase. Zuschin attributes this brief period of heightened activity to the early stages of industrial development in Italy.
“A moderate increase in nutrient levels can benefit the ecosystem,” he stated.
Unfortunately, this period of improvement was short-lived. An overabundance of nutrients in the Adriatic led to excessive algae growth, which settled on the seafloor after dying. Bacteria that broke down this organic matter depleted much of the available dissolved oxygen, suffocating nearby marine life. “The situation became unmanageable, and the entire system collapsed,” Zuschin remarked.
Interestingly, these oxygen-depleted episodes, known as eutrophication, did not negatively impact everything. They potentially benefited the common corbulid clam, Kowalewski pointed out. “Compared to their competitors, these clams are better equipped to survive low oxygen levels and can multiply quickly.”
Corbulid clams also seem less appealing as a food option for drilling predators. Their shells exhibit drilling holes but at a lower rate than other species. With their primary limitation being their feeding capacity, corbulid clams have thrived in the nutrient-deficient waters of the northern Adriatic.
However, an even graver issue looms—climate change is warming the Adriatic Sea, leading to increased stratification of the water. This phenomenon occurs when warmer surface water mixes less with the cooler water below, hindering oxygen distribution to deeper depths. In regions already plagued by eutrophication, these problems may worsen.
Despite these challenges, Zuschin believes there is a glimmer of hope. Initiatives are being implemented to reduce pollution entering Italy’s rivers, and findings from one area in the Po River Delta indicate a slight increase in drill-hole frequency. However, Zuschin cautions that restoration efforts will be complex and will only become more challenging the longer they are delayed.
“Environmental degradation comes with heavy costs. The impacts of what has been lost on quality of life are immeasurable and cannot be easily quantified in monetary terms.”
