Patterns of marine biodiversity
Key factors determining the distribution of marine life
The Coral Triangle in the Western Pacific is a marine region of superlatives. The island-studded waters around the Southeast Asian nations of Indonesia, Malaysia, Papua New Guinea, the Philippines, Timor-Leste and the Solomon Islands are recognized as the most biodiverse marine communities in several categories. The region hosts nearly 600 different species of coral and more shellfish species than can be found anywhere else in the world’s oceans. Experts have identified more than 2000 different species of reef fish, many of which are endemic. The Coral Triangle is also home to more species of sharks, more crustaceans and more species of anemones, macroalgae and sea squirts than any other region of the global ocean.
But why is this tropical marine region in Southeast Asia more species-rich than any other? What are the environmental factors that drive marine life and thus determine which single-celled organisms and species of flora and fauna occur where in the ocean and at what population densities? Science has some initial explanations and has already been able to confirm these for many marine organisms through experiments and measurements. However, it is not yet clear whether these mechanisms apply to all known groups of organisms.
- 2.1 > These various stony corals grow terrace-like in the Coral Triangle. Their horizontal, almost table-like growth form has the advantage that all the coral polyps receive sufficient sunlight and can use their tentacles to feed on zooplankton (small animals) floating in the water column.
- Best understood is the geographical distribution of marine animals. Which species can settle where in the ocean and find sufficient food is determined by the following key factors:
- the temperature and availability of oxygen,
- primary production and thus the total supply of food in a habitat, and
- the complexity of the marine habitat (this category includes parameters such as habitat structure, local currents, and also interactions between species).
- 2.2 > Generally, the more complex the structure of a marine organism, the lower the maximum temperature at which it can grow. For this reason, simple single-celled organisms such as bacteria or archaea can survive at or in hot springs, but more complex animals such as fish cannot.
Temperature and oxygen
Ambient temperature is the most important driver of biodiversity, because all living organisms on Earth can only live and function in the long term within very specific temperature ranges. This applies to viruses and bacteria as well as single-celled algae, macroalgae and plants, animals and humans. This range is termed the thermal tolerance window or thermal niche.
The width of this window or range varies from species to species. For oxygen-breathing marine organisms, if the ambient temperature is within their species-specific optimal range, they can provide enough energy to maintain all their bodily functions long-term by absorbing oxygen and food. However, if the temperature falls below the lower limit of the thermal tolerance window or rises above the upper limit, the body’s functional mechanisms fail and the organism’s life is at risk.
- 2.3 > Every living organism has a limited temperature range in which it can function and exist. This range is termed the thermal tolerance window. An organism exhibits the greatest performance when the ambient temperature is at the average value of its thermal tolerance window. As the organism gets warmer or colder, its performance declines.
- Moreover, we know of cold-blooded marine organisms such as fish that the width of the thermal tolerance window, and thus the individual’s heat tolerance, changes over the course of an animal’s development. Both parameters increase from the embryonic stage to young adulthood. However, as fish reach sexual maturity and increase in body size and mass, the width of the thermal tolerance window and hence the heat tolerance decrease again. Embryos and individuals ready to spawn therefore have the lowest tolerance to temperature fluctuations.
If we now compare all species and groups of organisms in the sea, we can see that the more complex organisms, such as animals and plants, not only have the narrowest thermal tolerance windows but they also reach their upper tolerance limit much sooner than other marine organisms. While unicellular bacteria and archaea can live in hot, oxygen-poor waters, marine organisms with a more complex structure reach their growth limit at a water temperature of 41 degrees Celsius at the latest. This upper temperature limit is evidently an insurmountable obstacle for their highly developed metabolic systems.
With the exception of birds and mammals, most marine animals are cold-blooded, meaning that their body temperature is determined by the ambient temperature. If a marine region heats up significantly in the summer and cools significantly in the winter, the organisms living there need a wide thermal tolerance window to be able to cope with both the warm summer temperatures and the cold winter. This is the case in the North Sea, for example. Long-term measurements off the North Sea island of Helgoland show that the average sea surface temperature there is below zero degrees Celsius during extremely cold winters, rising to 20 degrees Celsius in warm summers.
In tropical seas, however, seasonal temperature variations are much smaller. In the Philippine Sea, a marginal sea of the western Pacific Ocean and the most biodiverse region of the Coral Triangle, the sea surface temperature is around 20 degrees Celsius in winter. For the rest of the year, it averages between 24 and 28 degrees Celsius. As a result, the marine organisms living there only have to compensate for comparatively small temperature differences.
- Furthermore, the performance of organisms also varies within their own thermal tolerance window. It rises towards the centre of the window and then drops again, resulting in a parabolic performance curve. This means that most organisms are only capable of long-term maximum performance when the ambient temperature is roughly in the middle of their own thermal niche and there is sufficient oxygen. If the air or water is warmer or colder, or if the organisms lack oxygen, they will not be able to perform optimally and will lose out in the competition between species. An example from Japan illustrates the form such a temperature-related defeat can take.
The waters off the east coast of Japan are home to both Japanese sardines (Sardinops sagax melanostictus) and anchovies (Engraulis japonicus). Both species form large shoals, but never occur in very large numbers at the same time. Instead, at times anchovies dominate, while a few years later Japanese sardines take over. This interplay can be explained by naturally occurring fluctuations in sea surface temperature in the western North Pacific and the different heat tolerances of the larvae of the two species.
Japanese sardine larvae grow best at a water temperature of 16.2 degrees Celsius. Anchovy larvae, in contrast, prefer 22 degrees Celsius. This gives the latter an advantage when the currents of the North Pacific bring warmer water to Japan’s east coast. But when the seawater cools down again, the sardine larvae find better growth conditions. The anchovies lose out during these cooler periods.
In addition to the temperature-dependent performance of marine organisms, there is a second way in which the water temperature of a marine region affects biodiversity. Warm temperatures, such as those found in the Coral Triangle, accelerate the metabolism of all organisms living there. This shortens the lifespan of individual generations. Offspring are produced at shorter intervals, thus increasing mutation rates. Scientists argue that this makes the emergence of new species more likely, and that the region’s biodiversity will increase in the long term as a result of evolutionary development.
- They cite the abundance of species in tropical seas as evidence for this theory. Unlike the polar oceans, their water temperatures have remained stable over the past 15 million years. Moreover, the Coral Triangle and other tropical marine areas were never iced over orglaciated – events which could have caused species extinctions. Spared from climatic extremes, the shallow waters interspersed with coral reefs instead became a source of biodiversity and, experts note, gave rise to many of the more advanced marine organisms we know today.
This raises two questions that science has yet to answer: Is there an upper limit to the species diversity of a marine area? Is there a saturation point? And secondly, do stable higher temperatures allow for greater species diversity? All considerations in this regard probably start with the question of how much food and oxygen are available to the organisms in a given marine area, and whether both quantities are sufficient to meet the marine organisms’ temperature-dependent basic needs.
Primary production as the key to species richness
Living organisms need food to grow and reproduce. Those that cannot find food must migrate or die. The food supply in a marine region therefore has a direct impact on biodiversity. In many places, algae, marine plants and cyanobacteria, also known as primary producers, provide the basis for a rich food supply. They absorb carbon dioxide and nutrients such as nitrogen, phosphorus, iron and silicon from the sea and use photosynthesis to create new carbon-rich biomass. For this reason, primary production is particularly high in marine areas where sufficient nutrients and sunlight are available, such as coastal waters. The greater availability of nutrients in such areas is also the reason why important primary producers such as unicellular calcareous algae or diatoms are much more likely to grow and form algal blooms around large land masses than far out in the open ocean.
- 2.4 > Brittle stars are not ascetics: They use their long, tubular arms to grab small organisms and animal or plant remains that are suspended or drifting in the water. They then sweep their prey to their mouth with its five toothed jaws on the underside of their central body disc.
- Primary producers in the Coral Triangle benefit from the region’s many islands and regular rain showers. The rain washes much-needed nutrients from the land into the sea. As these are immediately absorbed and used in the highly productive coastal waters, the primary production of the world’s oceans declines with increasing distance from land and increasing water depth. The same generally applies to species diversity.
Submarine mountains or continental slopes, where nutrient-rich water masses often rise from the depths, are an exception. These provide primary producers in the light-filled surface water with nutrients so continuously that they grow and die steadily and biomass sinks from the sea surface to the depths in sufficient quantities. Under such conditions, deep-sea biodiversity can be relatively high, even in temperate and high latitudes, as researchers have found in a study of the global distribution of 2100 species of brittle stars.
When the scientists analyzed their data, they found that the greatest species richness of brittle stars was found in the deep sea far from the tropics, at the foot of continental slopes in the mid-latitudes of the northern and southern hemispheres. This observation may indicate that the geographical distribution and species richness of brittle stars at great depths is mainly dependent on food availability and only marginally on water temperature, which at very great depths is roughly the same everywhere – in polar seas as well as in the tropics. The situation is different for brittle stars living at depths of up to 2000 metres. Here, the highest numbers of species are found in the tropics and subtropics.
he role of structurally rich habitats and the coexistence of species
The Coral Triangle’s communities also owe their biodiversity to the many coral reefs, each of which is a habitat in its own right that, so to speak, forces its inhabitants to adapt to local conditions. Structurally rich habitats such as reefs, mussel beds, seagrass meadows and kelp forests provide many species with food, shelter and a wide choice of potential mates. They are therefore among the most species-rich marine ecosystems.
For organisms, it is always a risky business to colonize new habitats. If they leave their own coral reef, for example, they run the risk of being swept away by one of the ocean’s many currents or of being exposed to predators. And yet, throughout evolution, species have had to conquer new habitats. This is usually because competition for food in their original territory has become too great, local environmental conditions have changed, or animals have to find mates. Movements of the continental plates have also played a role in the history of the Coral Triangle. For example, where there are now large areas of shallow water supporting diverse marine life, about 20 million years ago there was a deep rift between the Southeast Asian plate and the Australian plate. As the two plates moved closer together, the shallow water areas were created that are so important today, allowing species to migrate and fill new niches.
2.5 > Off the coast of the Swedish island of Gotland in the Baltic Sea, wind and currents swirl large accumulations of shimmering green phytoplankton through the water. Microalgae are the first link in almost all oceanic food chains.- In order to minimize the risks associated with species migration, many marine species continue to follow one of three proven survival strategies:
- They stay relatively small and are carried to new habitats by local ocean currents. This saves energy.
- They grow to an impressive size to scare off predators, or they become master swimmers so that predators cannot follow them.
- They invest less energy into growing, leaving their safe shelters only at night to forage in the dark. This keeps them hidden from predators that hunt by sight.
In the long term, environmental factors such as the sea level also play an important role in the distribution of species and ecosystems. Sea levels determine which coastal areas dry up or are permanently flooded, and where sunlight-dependent organisms such as corals can colonize. Scientists have also developed a growing understanding of how the balanced coexistence of different species in an ecosystem can strengthen the community as a whole. A famous example is the interaction between sea otters (Enhydra lutris nereis), sea urchins (Strongylocentrotus purpuratus) and giant kelp (Macrocystis pyrifera) off the west coast of North America.
- 2.6 > The distribution and abundance of brittle stars changes with increasing water depth. From a depth of 2000 metres, the echinoderms live mainly at the foot of continental slopes in the mid-latitudes, as that is where they find the best food supply.
- Kelp forests are a biodiversity hotspot and provide a habitat for many marine organisms, including sea urchins, which normally hide on the seabed and feed on dead algae floating by (passive feeding). Under certain circumstances, however, they emerge from their burrows and switch to an active feeding mode, feasting on healthy algae. Researchers do not yet fully understand what makes them do this. However, it is known from observation that sea urchins in active feeding mode are capable of destroying large areas of kelp forest.
Healthy sea otter populations are the most effective defence against this. These marine mammals hunt the sea urchins and thus keep them from leaving their hiding places. In this way, the kelp forest remains largely intact – or so experience has shown. For almost a decade, however, scientists in Monterey Bay, California, have been observing that the interplay of the kelp forest species community is a lot more complex and that other species also play a role.
In 2014, the marine region was hit by a heatwave. This coincided with the outbreak of a starfish disease that caused mass mortality. Sunflower sea stars (Pycnopodia helianthoides) were among the victims. With an arm span of up to one metre they are among the world’s largest starfish – and they eat sea urchins. Their sudden disappearance and the heat stress caused the sea urchins to come out of hiding and actively attack the giant kelp, devouring the forests in many places except for a few remaining areas and reproducing extremely rapidly.
- 2.7 > Gray whales spend the summer building up fatreserves in the waters off the coast of Alaska, then migrate south in the autumn to give birth in the Gulf of California. Due to their enormous size only orcas hunt them.
-
2.8 > A group of ravenous Pacific purple sea urchins (Strongylocentrotus purpuratus) head for a bull kelp forest (Nereocystis luetkeana) off the coast of Mendocino, California. After such an attack, all that is usually left is bare rock. - The increasing number of sea urchins meant more food for the sea otters, which benefited their young in particular. They survived in higher numbers, so that the otter population also grew. However, the sea otters only hunted sea urchins in the few remaining patches of kelp forests, effectively protecting them from overgrazing. The otters spurned the many sea urchins on the grazed residual areas, presumably because the echinoderms found less to eat on the bare areas and were therefore less nutritious than the well-fed sea urchins in the residual kelp forests.
Contrary to expectations, the sea otters did not contribute to any rapid depletion of sea urchins in the overgrazed areas, and the giant kelp was unable to recolonize these areas. This was a huge loss for the coastal ecosystem. It remains to be seen what will happen next.
This example demonstrates that the stability and functionality of marine ecosystems and their biodiversity also depend on the behaviour and coexistence of their inhabitants. When these relationships change, the communities reorganize, which can also mean that the species richness of a previously diverse marine region declines.
- 2.9 > The subtle interplay between sea otters, sea urchins, brown algae (kelp) and sunflower sea stars (starfish) off the coast of California shows how species in a habitat influence each other and thus stabilize their ecosystem.
Maps showing the distribution of marine life
Our knowledge of the abundance and distribution of marine life has increased dramatically over the past two decades. This is largely thanks to databases such as the Ocean Biogeographic Information System (OBIS), where researchers from all over the world contribute data sets on the occurrence of marine species. Looking at the ocean regions covered by the data sets, it is clear that experts in China, Australia and Europe have a particularly good overview of the species found in their waters. The waters of the tropical western Atlantic, the tropical eastern Pacific and the Canadian part of the Arctic Ocean are less well surveyed. In addition, there are marine ecosystems for which little or no information is available. These include deep-sea communities in particular, but also biotic communities in permanently ice-covered regions, as yet unstudied coral reefs, and chemosynthetic communities. The latter are lowoxygen deep-sea ecosystems in which bacteria use methane or sulphides as an energy source for primary production.
- 2.12 > Patterns of marine biodiversity: The number of known marine species increases from the polar regions towards the equator. It reaches a maximum in the transition zone between the tropics and subtropics and then slightly declines towards the equator.
- Not surprisingly, researchers know more about the abundance and distribution of commercially important marine species such as tuna, squid and mussels than they do about species that are not caught and sold. More research has also been done on larger marine organisms than on smaller species. In 2020, an international team of experts summarized that sufficient comprehensive data to track their global distribution are available for a mere 50,000 marine species. These well-researched marine organisms once again are mainly those of commercial interest, as well as marine organisms that are very popular with the public, such as whales and other marine mammals.
Using the OBIS datasets and other databases, several research teams have attempted in recent years to visualize marine biodiversity on a world map and to identify the most species-rich marine areas. This is not an easy task, as the first step was to check the comparability and applicability of a billion data sets. Some, for example, lacked precise location data, while in others, marine organisms had not been clearly identified or records had been entered twice. It is also well known that there is significantly more research data available from the oceans of the northern hemisphere than from southern or tropical regions. This creates a disruptive bias in the data, which also had to be corrected.
- 2.13 > Species richness is generally higher in coastal areas than in the open ocean. Researchers have documented the highest levels of biodiversity in the Indo-Pacific Coral Triangle, the central and western Indian Ocean, the Red Sea and around the islands of the Southwest Pacific.
- After several data processing steps a relatively clear picture of species richness in the global ocean emerges. The map presented here is the result of an analysis carried out in 2020. It captures taxonomic information and distribution data for 41,625 species across all groups of organisms, representing 17 per cent of known marine species at the time of publication.
The map shows that the marine regions with the greatest biodiversity are located at the edge of the tropics. This is particularly true of coastal waters. At the equator itself, there are significantly fewer species than 2000 kilometres further north or south. Scientists explain this significant dip in the diversity curve by the fact that fewer and fewer marine organisms are able to tolerate the increasing warmth at the equator. Thousands of species have migrated north or south since the 1950s to avoid dying of heat exhaustion in constant water temperatures of 28 degrees Celsius or more.
Major differences in species richness are also apparent when travelling around the globe along the equator. While the central marine regions of the Atlantic and Pacific Oceans can be described as species-poor, species richness increases as one approaches the Caribbean or the Coral Triangle in the Indo-Pacific. Other marine hotspots include the central and western Indian Ocean and the Red Sea. Researchers have also documented large numbers of species in parts of the Northeast Atlantic and the North Sea.
It must be said, however, that these general patterns do not apply to all groups of organisms. Seals, sea lions and walruses, for example, are mammals that produce their own body heat and have evolved to develop a thick, insulating layer of fat. Especially in cold regions this gives them a distinct advantage. This is why they are most abundant in polar and subpolar waters. In contrast, seagrasses prefer the water temperatures of temperate latitudes and are most abundant in their coastal waters.
Far out in the open ocean, researchers have documented the greatest biodiversity in marine areas between 30 and 40 degrees north and south latitude. These regions are home to most species of zooplankton, such as copepods, foraminifera and krill, as well as large numbers of whales and deep-sea fish such as tuna and sharks. Comprehensive data on the distribution of single-celled algae (phytoplankton) and marine bacteria were not yet available in 2020.
The diversity of habitats and biogeographical areas
The researchers also looked at where the twelve best-known marine ecosystems occur in the global ocean, and whether there are regions in the global ocean where different ecosystems coexist in close proximity. The ecosystems analyzed included the biotic communities of river estuaries, mangrove forests, tidal marshes, seagrass meadows, coral reefs, continental-shelf incised valleys and deep trenches, cold-water coral reefs, seamounts, deep-sea trenches, hot springs and submarine mountain ranges. The results of the analyses confirmed the findings on the species diversity of the oceans: the number of ecosystems, and therefore the diversity of different biotic communities, was found to be highest in the Coral Triangle, the Caribbean and off the east coast of Australia.
Three years earlier, in 2017, another team of researchers had published a map dividing the world’s oceans into 30 different zones, known as biogeographical regions. For this classification, the scientists had analyzed OBIS data on the distribution of 65,000 marine species and used statistical methods to examine how similar marine communities were in terms of their species composition. Where there were clear differences between neighbouring communities, the scientists drew a boundary.
Using this zonation, they were then able to answer a number of questions about the distribution of marine life. They found that of the 100 most widespread marine species, 72 were plankton and only 27 were what are known as marine megafauna. These are large inhabitants of the seas such as whales, sea turtles, seabirds and sharks, some of which are known to migrate over long distances. According to the study, the foraminifera Globigerinita glutinata has the largest habitat. This tiny single-celled organism was found in 28 per cent of the marine area surveyed.
- The analysis also showed that marine organisms swimming in the water column are generally more widespread than those living on the ocean floor. The marine area with the highest proportion of endemic species is the Black Sea: 84 per cent of the species living there were not found in any of the other zones. The researchers explain this leading position with the inland sea’s low salinity. Due to the high level of freshwater inflow, freshwater and brackish water species can also live in the Black Sea.
The study also provided new insights into the depth distribution of species. Based on their data, the researchers were unable to detect any depth zones other than those already known. These include the surface layer flooded with sunlight (euphotic zone), where algae and their predators thrive. It extends from the ocean surface to depths of around 200 metres.
Below this is the twilight zone (mesopelagic zone), where plants no longer thrive. It extends from 200 to 1000 metres and provides a daytime refuge for organisms that only come to the surface at night to feed. Below the twilight zone is the deep sea (bathypelagic and aphotic zones), where communities of organisms have to cope with low food availability, constant cold and permanent darkness.
- 2.16 > The foraminifer Globigerinita glutinata lives in the water column and is one of the most widespread marine organisms in the world. The shell of adult animals consists of three distinctive chambers. It is less than half a millimetre in diameter.
- The scientists hope to use this knowledge of the distribution of marine life to help protect it – for example, by using the biogeographic zones to determine the best possible location, size and connectivity of marine protected areas.
The distribution map and associated database will also be used as a benchmark for future observations of marine biodiversity.
Reports of species migrations and ecosystem shifts as a result of climate change are now coming in from all regions of the global ocean. This is no surprise, as the Earth’s climate directly regulates the key factors that determine the distribution of marine life. These primarily include water temperature, the oxygen content of the water masses, their degree of acidification (pH), terrestrial nutrient inputs as a result of precipitation, the strength of ocean currents, sea levels and, in the case of the flora and fauna of the polar seas, the extent, thickness and lifetime of the sea ice that protects and sustains them.
- 2.17 > The biogeography of marine life: After analyzing data on the distribution of 65,000 species of flora and fauna, researchers were able to divide the global ocean into 30 different biogeographical zones. Each of these zones is home to a marine species community that differs in its composition from the communities in the neighbouring zones.
- 2.17 > Table 1
- 2.17 > Table 2
