An unprecedented downward spiral
Species decline from earliest human history onwards
Ever since humans have existed on Earth, we have been altering the ecosystems and biodiversity of the places we inhabit. Archaeological finds in Africa suggest that Homo erectus, an ancestor of the modern Homo sapiens, exterminated the first animal species after learning to cook game over a fire one to two million years ago. The first modern humans lived in Africa between 100,000 and at most 300,000 years ago. They also learnt to cook meat using fire. As socially organized omnivores, our ancestors hunted in groups. They mainly hunted large game together, taking selected plant species and some domesticated animals with them when they searched for new habitats. Both these behaviours altered the original ecosystems of newly colonized areas.
By the beginning of the Holocene around 12,000 years ago, modern humans had colonized three-quarters of the world’s terrestrial areas. By then, large game animals only persisted in sparsely populated regions. In what are now Israel, Egypt and southern Syria, for example, 10,500-year-old human remains demonstrate that people hunted much smaller animal species than their ancestors had many years earlier. Their hunting yield was just 1.7 per cent, a small fraction, of the animal biomass killed by the ancestors of modern humans in the same region 1.5 million years ago, as revealed by archaeological excavations in the region. The researchers also investigated possible reasons for this significant loss of biomass. They concluded that the decline could not be explained by climate fluctuations or sudden climate change. It is much more likely that humans and their ancestors in the Middle East decimated or even wiped out all large land creatures within 1.4 million years. People in the Mesolithic period (around 10,000 to 8000/6000 years before present) therefore had no choice but to prey on smaller wild animals and start arable farming a little later.
- 4.1 > Extinct giant bird: When humans first settled in New Zealand in the early 14th century, some 58,000 moas lived on the two islands. Just 120 years later, the settlers had wiped out these flightless birds, which could reach a height of up to 3.6 metres.
Population
The term “population” refers to a group of individuals of the same species that live together in the same location or area at the same time, interact with each other, and form a reproductive community. The development of a population is influenced by the various interactions between its members and their animate and inanimate environments.- Between the 7th and 14th centuries AD, the colonization of islands in the southern and western Pacific resulted in further demonstrable species extinctions caused by humans. In Polynesia, for example, it is estimated that some 1000 local bird species disappeared after humans arrived. Many of these extinct birds belonged to the rail family, which includes crane birds. These birds mostly only lived on the islands and, over the course of evolution, had lost the ability to fly. Unable to fly, these ground dwellers were easy prey for hungry humans and their “entourage” – dogs and pigs they had brought with them. These animals preyed on the birds, as did rats which arrived on the islands as stowaways.
Despite the consistently negative outcomes, humans have also always played a role in preserving species diversity and biodiversity, for instance by breeding new species of flora and fauna or preserving heritage species through targeted breeding programmes. However, a region’s species diversity can also benefit from small, targeted interventions in healthy ecosystems. For instance, when individual trees are removed from a dense forest, small clearings are created, providing a habitat with new conditions. A variety of species settle there more quickly than in the neighbouring forest, resulting in an overall increase in species diversity. Structurally rich gardens and parks have a similar effect.
All over the world, ethnic groups as well as traditional agricultural practices preserve certain habitats, and thus species, that would not exist without targeted intervention. Examples include heathlands grazed by sheep and high-altitude summer pastures in the Alps that are farmed extensively. These areas are home to many rare vascular plants, but only as long as the grasslands are regularly grazed or mown.
- 4.2 > The first modern humans mainly hunted big game. In doing so, they caused the extinction of many species. They also deprived themselves of an important food source, forcing them to adapt their way of life several times over the course of history in order to survive.
Initial cases of marine overexploitation
Although humans had already overfished coastal areas and decimated populations of marine mammals in ancient times, there was no mass extinction of marine species comparable to that of species on land. One decisive factor was that marine species generally inhabit larger areas than terrestrial animals and plants, and another factor was that humans were not yet able to reach every corner of the global ocean. This left marine life with sufficient refuges, particularly for species that were heavily fished or hunted. At the same time, humans for the longest time only fished and hunted as much as they needed to feed their clan or family.
This changed as the human population grew and markets for luxury items such as furs, feathers and ivory developed. People also began to earn a living by selling fish, mussels and other marine resources. Experts agree that the increasing destruction of marine habitats, and the subsequent collapse of marine populations and coastal ecosystems, has primarily been caused by the commercialization and subsequent industrialization of resource use and marine exploitation.
Just as on land, humans initially targeted the largest and most valuable species of marine mammals, fish and invertebrates. Once these populations had been depleted by intensive hunting and their slow reproductive cycles, the focus shifted to smaller, less valuable species. However, there had also always been smaller species that were fished. The hunt for the largest fish, seals, king crabs and other marine organisms continues to affect marine life today. First, large individuals disappeared from local biological communities, and shortly afterwards, entire populations of large organisms. The remaining populations quickly developed smaller body sizes and reached sexual maturity earlier. As a result, heavily fished marine organisms generally tend to be smaller today than they were in the past.
- 4.3 > Steller’s sea cow (Hydrodamalis gigas) could grow to 7.6 metres long and was hunted to extinction by humans just a few years after it was discovered. This species lived in the cold, shallow coastal waters of the North Pacific, primarily in the Bering Sea.
- The first evidence of a large-scale collapse in European fishing dates back to the High to Late Middle Ages (11th to 15th centuries). During this period, the Christian Church prohibited the consumption of meat. However, believers were permitted to eat fish and seafood on up to 130 days a year, which increased demand for seafood. At the same time, towns were growing, farmers cleared forests to create arable land, and rivers were dammed to power mills. All these developments led to significant changes in streams, rivers, lakes and estuaries. Water quality declined. The combination of increased fishing pressure and deteriorating water quality caused the populations of sturgeon and salmon to decline. Both species of fish disappeared from many European waters and coastal ecosystems. The European sea sturgeon (Acipenser sturio) vanished from Central and Eastern Europe in the 12th century. Atlantic salmon (Salmo salar) stopped returning to their spawning grounds from the late medieval period onward.
In response to declining fish stocks in inland waters and along the coast, fishermen in the North and Baltic Seas began to target nearshore fish. New shipping, fishing and preservation methods and technologies allowed them to venture further out to sea and catch larger quantities of fish. The main species caught were cod and herring.
The practice of catching fish and selling them at local markets, and later across regions, developed into a distinct business sector and became one of the driving forces behind the medieval trade revolution of the 11th and 12th centuries. The Hanseatic League, a powerful trading alliance of the time, amassed considerable wealth through the sale of herring. The price of salted herrings rose steadily during the Middle Ages. While 1000 of them cost 15 sous (or sol) in the coastal town of Calais in 1268, by 1300 the price had risen to 30 sous, reaching 75 sous just 42 years later. By that time, hundreds of millions of herrings were being caught in the North Sea and Baltic Sea every year, salted, stored in barrels, and shipped. However, the combination of intensive fishing, declining water quality and climate fluctuations gradually led to the collapse of important fish stocks in the late Middle Ages. By the late 13th century, herring shoals had disappeared from the southern Baltic coast. In the southern North Sea, herring stocks began to collapse from 1360 onwards.
- 4.4 > Until the 19th century, people used sailing boats to fish for oysters off the European coast. However, the subsequent introduction of motorized steamships led to the overfishing of oyster beds, causing them to disappear from an area of at least 17,000 square kilometres.
- Compared to Europeans, other cultures at that time were more careful with their marine resources. In Japan, for instance, fishing families developed various targeted methods to enhance the living conditions of marine organisms in coastal waters, thereby increasing their productivity and species diversity. They replanted destroyed seagrass meadows, for instance, and cultivated bivalves and macroalgae to improve water quality. This form of sustainable coastal and marine management is known as “Satoumi”. This approach, which deliberately rebuilds weakened coastal ecosystems for future sustainable use, is now internationally recognized as an important tool in the fight against marine species extinction, and is therefore more relevant today than ever before.
In the 18th century, the first marine species went extinct as a result of human exploitation. Steller’s sea cow (Hydrodamalis gigas), which inhabited the North Pacific, was hunted so intensively after its discovery by Europeans that it became extinct within 27 years. As the colonial powers advanced into North America and Australia, they began to overfish local stocks of fish and bivalves there as well. As a result, huge oyster beds in Europe and off the coast of North America disappeared after the 19th century development of machines that could mechanically detach and collect bivalves from the sea floor. The total area of oyster beds lost in European seas amounted to at least 1.7 million hectares – an area twice the size of the Mediterranean island of Crete.
- 4.5 > The three ecosystem categories of the Living Planet Index reveal a dramatic decrease in the number of wild animal populations. The index covers 1816 marine species, 2519 terrestrial species and 1472 freshwater species inhabiting lakes, streams, rivers and ponds.
The Great Acceleration
When fishermen began to replace their sailing ships and boats with steam-powered vessels in the 1880s, the range over which they could operate expanded once again. Motorized fishing boats could remain at sea for longer periods and use larger trawl nets in deeper waters. Further technological advances, such as larger and more powerful ships, new fishing gear, freezing and preservation techniques, and navigation and sonar equipment, led to a sharp increase in global fishing after the Second World War. Consequently, some of the world’s largest fish stocks collapsed. Moreover, between 1925 and 1975 whalers killed 1.5 million baleen whales in the Southern Ocean alone.
In hindsight, experts describe the period since the 1950s as the “Great Acceleration”. However, this term refers not only to the increase in global overfishing since the Second World War. It now represents the drastically increasing pressure that human activities are placing on all of the planet’s natural systems, including materials cycles, the climate, and species diversity on land and in the oceans.
The Age of Extinction
Today, more than 70 years after the start of the Great Acceleration, biodiversity on Earth is worse off than ever before. More than one million species of flora and fauna worldwide are at risk of extinction. The latest Living Planet Report by the World Wide Fund for Nature (WWF) and the Zoological Society of London found that the size of some 35,000 wildlife populations studied had declined by an average of 73 per cent over the past 50 years. This means that in an area that held 100 fish, amphibians, reptiles, birds and mammals in 1970, on average there are now only 27. The experts documented the highest population declines among freshwater species. Their populations have shrunk by 85 per cent on average. Terrestrial wildlife populations declined by 69 per cent, while marine species declined by an average of 56 per cent.
Experts attribute the somewhat reduced rate of decline in marine life to the composition of the populations studied. The main species observed here are fish whose populations are regulated by catch quotas and other management measures. While some of these regulated fish stocks have recovered in recent years, others have remained stable. WWF experts note that this trend is also evident in current statistics on marine organisms.
- 4.6 > In 2019, experts at the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) analyzed the extent to which direct factors were driving biodiversity loss on Earth. The oceans were found to be most affected by overfishing and other forms of resource overexploitation.
- However, the situation facing marine life is illustrated by the plight of hawksbill sea turtles (Eretmochelys imbricata), which nest on Milman Island in the northern part of the Great Barrier Reef in Australia. Although the island is located in a nature reserve with the highest protection standards, the number of female turtles nesting there has declined by more than half in just 28 years (1990 to 2018) – presumably because the turtles were illegally caught outside the protected area or killed as by-catch in fishing nets. Experts predict that this important sea turtle population could be wiped out completely by 2036. The hawksbill sea turtle would then be locally extinct.
WWF experts cite the destruction and loss of natural habitats caused by the expansion of food production for humans as the main cause of the dramatic decline in wildlife populations worldwide. This includes agriculture, as well as the fishing and hunting of wild animals. Other causes include overexploitation of natural biological communities, introduction of alien species, spread of disease, and the consequences of climate change and environmental pollution on land and at sea.
The drivers of marine species turnover
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) deals in even greater detail than the WWF with the factors and forces that drive human-induced species turnover. Scientists use the term “drivers”, but it is not fully elucidated what this encompasses. IPBES distinguishes between direct and indirect drivers of species turnover. The latter include the factors, values and behaviours that cause humans to make decisions which ultimately affect nature. In its 2019 assessment report, IPBES identified the following as the most important direct human influences on marine biodiversity: the direct extraction of raw materials and organisms; changes in the use of coastal areas and oceans; climate change and its consequences for marine life; coastal and marine pollution; and invasive species. The most significant indirect drivers are population growth and demographic changes, such as migration and urbanization, as well as socio-cultural changes, economic and technological developments, and changes in political, regulatory and administrative approaches (governance), as well as conflicts and epidemics.
- All of these factors have an impact, both individually and in combination with other influences. For example, the growth in the world’s population, alongside global economic growth and technological change, has led to changes in people’s lifestyles. Consequently, the demand for food, energy and natural resources such as rare earths, sand and metals has increased. At the same time, more greenhouse gases have been released into the atmosphere every year, accelerating climate change. Rising plastic production has caused more waste to be generated; in agriculture the use of fertilizers and pesticides has increased. Both these developments have contributed significantly to marine pollution in recent decades.
The direct removal of organisms
The main causes of species diversity loss in the oceans are overfishing and destructive fishing methods such as bottom trawling. Since the 1960s, the marine fishing industry has grown faster than any other marine industry. Seafood is one of the world’s most traded foods, with an estimated 61.8 million people currently employed in the global fishing industry. This growth, which has accelerated since the late 1990s, is mainly due to the increased farming of fish, mussels and other marine organisms in aquaculture. Wild catches officially peaked at around 82 million tonnes per year during this period. Since then, catches have stagnated at a high level, but fishing activities have expanded to greater depths and more remote regions.
- 4.7 > Researchers found 403 pieces of plastic in the digestive tract of a flesh-footed shearwater that was found dead on the Australian coast. When seabirds accidentally ingest plastic waste, the particles can cause cuts to the gastrointestinal tract, as well as leading to kidney and liver damage and dementia-like brain damage.
- According to the Food and Agriculture Organization of the United Nations (FAO), approximately 115 million tonnes of fish and seafood were landed in 2022. Of these marine organisms, 79.7 million tonnes were caught in the wild. Meanwhile, 35.3 million tonnes came from marine aquaculture – a new record high. However, the FAO emphasizes in its accompanying report that there are gaps in its catch statistics. For instance, catches from illegal fishing as well as by many artisanal fisheries and catches by recreational and sports fishers are not included. In order to accurately assess the extent to which fishing threatens marine biodiversity and the effectiveness of conservation measures, these removals must also be surveyed fully.
The FAO regularly monitors and documents the consequences of intensive oceanic fishing, using 445 fish populations as case studies. While 90 per cent of these populations were considered to be fished at “maximally sustainable levels” in 1974, this figure had fallen to 62.3 per cent by 2021. Conversely, the proportion of overfished stocks rose during this period from ten per cent in 1974 to 37.7 per cent in 2021. This means that according to FAO data more than one third of the stocks surveyed is now demonstrably overfished.
However, pressure on fish populations is growing not only because of the demand for fish meat, which is sold as food or used to produce fishmeal. In the coastal waters of Mexico, parts of Africa and South East Asia, fishermen have for some time now increasingly targeted species of fish with large swim bladders. These are removed, dried and sold to Asia as a luxury food, status symbol and alternative medicine for more than 15,000 US dollars per kilogram. The state of the affected fish stocks is only being investigated in a small number of locations.
4.8 > The hawksbill sea turtle (Eretmochelys imbricata) lives mainly in tropical coral reefs, where its diet consists of sea sponges, sea anemones and jellyfish. However, the number of these turtles has declined sharply due to illegal capture and destruction of their clutches of eggs. The species is now considered at risk of extinction.- Given the growing problem of overfishing, fish farming in marine aquaculture has gained increasing significance. Experts predict that this sector will continue to grow and occupy more ocean space in the future. Therefore, sustainable solutions are needed to address the environmental issues associated with aquaculture. These include the spread of diseases and parasites from farmed to wild fish stocks, increased water pollution from feed residues, faeces and pharmaceuticals, and the sourcing of feed (krill and fishmeal).
Antarctic krill (Euphausia superba) are mainly fished in the waters north of the Antarctic Peninsula and in areas further to the northeast. Since 1993, catches have quadrupled, causing particular problems where penguins, seals and an increasing number of baleen whales feed. Experts are therefore calling for stricter catch quotas and fishing bans in certain regions. Otherwise, they say, the increasing competition for these nutritious little crustaceans could jeopardize the recovery of baleen whale populations and threaten the survival of many other krill predators. Similar competition for food exists in the waters off South Africa, where the overfishing of anchovy and sardine shoals for fishmeal has led to a significant decline in the remaining African penguin colonies.
4.9 > “The World” is the name given to this string of artificial islands in the Persian Gulf. Built between 2003 and 2008, they are shaped like a world map and are located off the coast of Dubai. The islands are mainly made up of sand dredged from Dubai’s shallow coastal waters.Direct extraction of raw materials and its consequences
The ocean provides human societies with a wide range of raw materials, including oil, natural gas, minerals, metals, sand and gravel. The combination of rising global demand, technological progress and dwindling landbased sources has made the extraction of an increasing number of marine raw materials not only feasible but also economically viable. The industries involved have high expectations, as evidenced by the ongoing international negotiations on deep-sea mining, for example. However, the extraction of marine materials incurs significant initial costs and poses numerous environmental risks.
Researchers describe the dredging of sand and gravel as one of the most dangerous and harmful activities that can be carried out on a coast or in a river. This activity causes irreversible and irreparable damage to the land, water and biological communities of coastal and marine environments. It also often destroys entire ecosystems, such as beaches, dunes, spits and seagrass meadows. As a result, seabirds and sea turtles lose their habitats and nesting sites. As a consequence of the disappearance of seagrass meadows, vital refuges for the offspring of many marine organisms are also lost. According to the United Nations Environment Programme, sand extraction has tripled since the beginning of the 2000s, driven by population growth, urban expansion, economic growth and measures to protect against the effects of climate change. It is estimated that human societies currently use 40 to 50 gigatonnes of sand per year, and this figure is rising. This far exceeds the amount that the Earth system can replenish through the weathering of rocks.
- 4.10 > In Hong Kong, dried fish swim bladders are sold in sacks. Business is booming because swim bladders are considered a luxury food and a status symbol. Furthermore, many buyers believe that consuming fish bladders will slow down their ageing process.
- Dredging is not only carried out at sea for the purpose of extracting sand. Dredgers are also used when installing oil and gas platforms, wind turbines, pipelines and other offshore infrastructure. In all cases, the seabed must be levelled to stabilize and secure these structures. For offshore wind farms, dredgers often remove the upper layer of sediment from the ocean floor, which can be up to ten metres thick. When constructing oil or gas production platforms, a rock platform is often created instead. Before laying underwater pipelines and cables, the ocean floor must be cleaned and levelled, a trench dug or a rock bed constructed to reduce the load on the structure. This process completely destroys the biological communities on the ocean floor.
- 4.11 > Marine biologists on a research vessel approach a dead blue whale (Balaenoptera musculus) floating on the ocean surface in California’s Santa Barbara Channel. The giant whale had been struck by a ship and died from its injuries.
Changes in coastal and marine use
There are more than 1.6 million kilometres of coastline on Earth, spread across 123 countries. The coastal zone includes coastal waters, the associated ocean floor, adjacent land, estuaries, and other nearby bodies of water. It is estimated that between 23 and 37 per cent of the world’s population lives within 100 kilometres of the sea. This currently equates to between 1.86 and 2.99 billion people.
Due to population growth, increasing urbanization and economic development, coastal areas are being used more and more intensively. These areas serve as residential areas, transport hubs, industrial zones, economic centres and recreational spaces for an ever-growing population, and are being extensively redesigned to accommodate the requisite buildings and infrastructure. Expanding coastal metropolises such as Singapore, Hong Kong, Manila, Jakarta, Lagos and Mumbai are prime examples of this trend. At the same time, waste, sewage and pollutants, including those released further inland, are concentrated in coastal areas via rivers.
When millions of people live in confined spaces, the remaining coastal marine ecosystems are exposed to intense, chronic stress. In particular, biological communities suffer from the overloading of coastal waters with waste, sewage, nutrients and pollutants. This often results in low-oxygen zones where few marine organisms can survive. In Jakarta Bay (Indonesia), for example, the number of fish declined by 80 per cent as the city grew. In 2015, corals covered only one tenth of their original range. Off the coast of Hong Kong, scientists have shown that the degree of water pollution determines which organisms survive on the ocean floor. Areas with heavily polluted water are primarily settled by mussels that are unaffected by the poor water quality and large quantities of particles stirred up by construction work. In contrast, in regions with less polluted water, macroalgae, bryozoans and other molluscs, which are sensitive to sinking particles and sediments, also survived.
Our increasing reliance on the sea is placing new stresses on marine organisms, including noise pollution from construction work for wind farms and other infrastructure, as well as from the growing volume of shipping traffic. For example, on the west coast of the Canadian province of British Columbia, 300-metre-long natural gas tankers will soon be sailing through the narrow Kitimat fjord system to load liquefied natural gas at a new shipping port and transport it to Asia. This archipelago has long been considered a safe haven for fin whales and other baleen whales. Local whale researchers fear that it could soon become a death trap for these marine mammals.
The consequences of climate change for marine species diversity
Climate change is altering the living conditions of marine organisms in many unprecedented ways. Rising water temperatures force cold-blooded organisms to invest more energy and oxygen in their metabolism, thus requiring them to expend more energy to maintain their bodily functions. This is possible as long as there is sufficient food and the ambient temperature remains within the species-specific comfort range, enabling individuals to acclimatize. However, if the temperature rises above this range, the organisms are at risk of circulatory collapse. Warmer seawater also stores less oxygen than cold water. This means that heat-stressed organisms find it more difficult to mitigate that stress by increasing their respiratory rate. In addition, heat increases the number and size of low-oxygen zones at mid-water depths and shifts the biological calendar of many species. This can occur in such a manner that the calendars of different species drift apart: in the western Baltic Sea, for instance, due to warming phytoplankton blooms only occur after cod and herring larvae have hatched. As a result, the juveniles are at risk of starvation.
Climate change is also altering ocean chemistry because atmospheric carbon dioxide dissolves in the surface waters. This acidifies the ocean, making life difficult for many organisms, especially calcifying species such as bivalves, true conchs and calcifying marine algae. These organisms must expend far more energy building their protective shells and skeletons, as well as repairing acidification-induced damage.
Last but not least, climate change is causing sea levels to rise and the area of sea ice to decline. These developments have far-reaching consequences: breeding and nesting areas in low-lying coastal regions are flooding, and mangrove forests are becoming permanently inundated, resulting in the trees dying off. The disappearance of sea ice not only means the loss of a key habitat in the polar seas. It also means the loss of the most important food source, as nutritious polar diatom communities form the foundation of marine food webs in the Arctic and Antarctic.
- 4.12 > Due to ocean warming, many marine species are moving to cooler waters, albeit to differing extents and at different speeds. This has been proven to be the case for species such as the American lobster, the Humboldt squid and the Tasmanian dwarf cushion star.
4.14 > Storm damage: A year on from Hurricane Ian sweeping across the US state of Florida, this fishing boat remains stranded in a mangrove forest ravaged by wind and waves. As hurricanes become stronger, coastal ecosystems such as mangroves, tidal marshes and seagrass meadows are increasingly at risk.- The impacts of climate-induced warming, acidification and increasing oxygen depletion on marine life are mutually reinforcing and having an increasingly disruptive effect: mobile species are leaving their traditional territories, migrating towards the poles or to deeper waters. Among the many species fleeing the increasing heat is cod. The cod population in the North Sea is shrinking, while the heat-loving European hake (Merluccius merluccius) is migrating northwards from the south into the North Sea. Hake are more cunning hunters than cod. Experts therefore assume that the newcomers to the North Sea will put much greater hunting pressure on native species and could permanently alter the ecosystem.
Depending on whether ocean currents carry their eggs, larvae or juveniles to new, suitable habitats, organisms attached to the ocean floor, such as warm-water corals, will either die out locally or become extinct as the oceans warm and acidify. The remaining habitat of cold-adapted organisms such as penguins, Arctic cod and Antarctic krill in the polar seas is declining in area due to the reduction in seasonal sea ice cover and absence of permanently low temperatures.
Extreme events such as marine heatwaves and severe cyclones pose a particular threat. Due to their scale and destructive force they quickly exceed the stress limits of organisms. Therefore, extreme events can cause immense damage to marine ecosystems within a short period of time, especially if they occur at increasingly shorter intervals, preventing ecosystems from recovering from previous shocks.
- 4.15 > Antarctic krill swim underneath ice floes to feed on polar diatoms growing in the cracks and channels of the sea ice. Krill are a keystone species in the Southern Ocean food web. They are a vital food source for penguins, fish, whales, seals and many other animals.
- The structure of local biological communities, and with it their ability to perform certain ecosystem services, changes when species disappear or new species arrive. Off the coast of Japan, for instance, the migration of algae-consuming reef fish from tropical waters due to rising temperatures has led to a decline in native kelp forests. This has had negative consequences for the many species that previously relied on the macroalgae forests for their survival. However, at the same time, the fish have allowed for the establishment of corals that prior to the ocean water warming up had only been found in coastal areas further south.
Such dramatic changes to marine ecosystems have a direct impact on the people who depend on them for their livelihoods. Fishing grounds are shifting geographically and becoming inaccessible, especially for artisanal fisheries. This is particularly true of tropical and subtropical waters, where significantly fewer species will be able to survive in the now warmer waters.
In Alaska, fishing for opilio snow crabs (Chionoecetes opilio) was banned in 2022 after a prolonged marine heatwave caused stocks in the Bering Sea to collapse by more than 90 per cent. Prior to this extreme event, fishers landed annual catches worth 200 million Euros. It is uncertain whether this level can ever be reached again. The probability of the water temperature in the Bering Sea exceeding the snow crab’s arctic cold comfort zone is now 200 times higher than in the 1850s. Researchers also found that the snow crabs died during the heatwave due to a lack of food. In the warm water, they required far more food than the Bering Sea ecosystem could provide.
This example underscores that climate change has enormous consequences for marine biological communities, making it more difficult for us to sustainably utilize and preserve them in the long term.
Coastal and marine pollution
Human-induced pollution of the oceans and coastal areas is increasingly contributing to the decline in marine biodiversity. Today, oceanic pollution encompasses not only the deposition of chemicals, silt and contaminated wastewater, but also the contamination of marine ecosystems by artificial light, pharmaceuticals, anthropogenic noise and plastic waste in all its forms. The effects on ecosystems are correspondingly diverse. For example, pharmaceuticals and environmental toxins can cause reproductive issues and increased mortality rates. They may also reduce the food supply for certain species. Artificial light can irritate nocturnal marine organisms. For instance, if lamps are lit in the immediate vicinity of sea turtles’ beach nests, the fresh hatchlings can’t find the most direct path to the ocean. Underwater noise caused by humans can drive marine organisms away from their habitats and make it difficult for them to hunt, find mates and communicate. In extreme cases, it can injure or kill them.
The millions of tonnes of plastic waste in the sea are destroying marine habitats, with plastic debris accumulating on a massive scale. Carelessly discarded fishing nets pose a risk to marine life, as animals can become entangled in them and die. Marine organisms may also mistake plastic waste for food and ingest it. These animals then virtually starve to death with full stomachs. Microplastics particles are a particular danger. Researchers have now shown that more than 1300 different terrestrial and marine animal species ingest microplastics particles, including invertebrates, insects, fish, birds and mammals. The adverse impacts of this pollution are felt at all levels of biological organization, from the cellular level to the food web and entire ecosystems.
4.16 > The population of African penguins has plummeted over the past 35 years. While there were still around one million penguins living on the coasts of South Africa in the early 1990s, by 2023 this figure had dropped to fewer than 10,000. This decline is due to the overfishing of sardines and anchovies, as well as an increase in floods and heatwaves during the breeding season causing the penguins to abandon their nests.Greater damage occurs when many stressors combine
Human activities and interventions in the ocean affect marine organisms in different ways. The intensity of individual stress factors varies between marine regions and depending on the water depth at which organisms live. For example, the inhabitants of shallow coastal seas feel the effects of rising water temperatures, increasing ocean acidification and the direct consequences of marine pollution much more acutely than deep-sea biological communities far out at sea. Nevertheless, the following applies to the entire ocean habitat: if stress factors occur simultaneously, they may interact and thus increase their destructive power. Stressors reinforce each other, creating a cascade effect.
This intensifying effect is particularly evident in tropical coral reefs. Those that are already weakened by water pollution and intensive fishing bleach more quickly and extensively during periods of extreme warmth than those that are not exposed to additional stresses. The latter also recover much faster. Additionally, reefs in over-fertilized coastal waters are more susceptible to destruction by voracious crown-of-thorns starfish. The nutrients deposited from land-based sources promote the growth of microalgae. These, in turn, serve as food for the starfish larvae during the first six to twelve months of their lives. If microalgae are available in large quantities, the starfish grow in large numbers and, once sexually mature, prey on the polyps of stony corals. Each spiny starfish can destroy up to ten square metres of coral per year.
- 4.17 > Scientists use the term “tipping point” to describe the point in time at which the cumulative effect of many small changes or pressures causes a system to exceed its stress limit. The system then changes in a fundamental way and assumes a new state. A return to the original state is usually impossible.
Critical tipping points are nearing
The Living Planet Index and other indicators of global species diversity reveal the alarming rate at which the Earth is losing its wild animal and plant species, either through extinction or drastic population decline. This decline is particularly evident in the ocean, where there has been a widespread loss of seagrass meadows, tidal marshes, kelp forests and coral reefs, as well as a dramatic collapse in shark and ray populations. Hunted and fished by humans for centuries, populations of sharks and rays have declined by a further 71 per cent since 1970. According to the Red List, more than one third of the approximately 1200 shark and ray species are threatened with extinction – a disaster of inconceivable proportions for the marine environment.
Although human activities often cause small-scale changes in species diversity, the cumulative impact of these interventions can be highly significant. The pressure on the affected ecosystem, or even the entire ocean, can increase to such an extent that the change originally driven by humans suddenly continues of its own accord. This results in further, very abrupt changes that may be irreversible. Moreover, they usually pose enormous additional risks to humans and nature. In such instances, experts speak of reaching a tipping point. If crossing this tipping point jeopardizes the survival of large numbers of people and species of flora and fauna, it is referred to as a global tipping point.
The most prominent global tipping points include the melting of the Greenland and Antarctic ice shields, the collapse of the Atlantic meridional overturning circulation in the North Atlantic, and the thawing of permafrost worldwide, which could result in the release of huge quantities of carbon dioxide and methane. In terms of the oceans, the worldwide extinction of tropical coral reefs would constitute a global tipping point. Their complete disappearance would lead to the collapse of important fisheries. Moreover, many tropical islands and coastlines would lose their natural breakwaters, not to mention the impact on marine species diversity. It is estimated that tropical coral reefs harbour a quarter of all known marine species.
Although it is difficult to prove that global tipping points have been exceeded, there are now many signs of local or regional tipping points. In 2023, for instance, many of the remaining coral reefs in the Caribbean bleached during an exceptionally intense and prolonged marine heatwave. Researchers witnessed, powerless to respond, the affected stony corals dying on a scale never before seen by coral experts. The consequences of this massive die-off for species diversity in the region are thought to be far-reaching but scientific proof will take time. This is partly because stony corals that survive a bleaching event can take up to a year to recover from the heat shock and resume growth and reproduction.