An illustration of the Barents Sea ecosystem, where the Institute of Marine Research is carrying out a wide range of research and monitoring activities.
An ecosystem can be defined as “a dynamic complex of plants, animals and micro-organisms that interact with the non-living environment to form a functional unit”.
The basic conditions that define an ecosystem are set by the physical environment, which includes the depth and type of seabed as well as the characteristics of the sea in terms of temperature, salinity and currents. Geographical location is another decisive factor in determining the degree of seasonal variation, for example in the amount of light.
Interactions and energy flows
The interaction between the organisms in an ecosystem – eating and being eaten – is the basis of the energy flow in the food chain. In the sea, this interaction results in a pyramid with a huge quantity of plankton at the base and diminishing amounts of biomass as we move up the food chain. Between each level there is a loss of energy, because not all the energy consumed is taken up by the organism at the next level, and because all organisms use energy for respiration, movement and reproduction. The energy transfer efficiency of each link in the food chain is around 10 percent. Marine ecosystems may thus contain ten times as much zooplankton biomass as plankton-eating fish. However, this is not always the case; phytoplankton, for example, are capable of supplying a greater biomass at the next level of the food chain because they reproduce as fast as they are eaten.
In order to understand how ecosystems work, it is important both to be aware of the interactions between the organisms involved and the resulting food pyramid, and to bear in mind that organisms live out their lives on the basis of what pays in evolutionary terms. The following figure illustrates precisely these dimensions of the ecosystem, with interactions, biomass and the underlying biological driving forces. Similarly, the arrows demonstrate that these factors influence each other and that ecosystems are not static.
Central elements in the structure and function of marine ecosystems. Interactions between organisisms (top), the food pyramid (lower right) and evolutionary motivation (lower left).
Just as on land, production in the sea is based on plant photosynthesis. Plants use the energy of light to transform water, carbon dioxide and nutrient salts into energy-rich organic compounds and oxygen. In the sea, photosynthesis is largely carried out by phytoplankton, (plant plankton) since seaweeds have low productivity in comparison with phytoplankton. All other organisms are dependent on plants to survive, grow and reproduce. Phytoplankton are grazed by zooplankton (animal plankton) such as copepods and krill, which are eaten in turn by certain types of fish and baleen whales. Still higher in the food chain we find fish-eating predators such as cod, seals and whales, and right at the top, polar bears and killer whales.
The previous paragraph summarises our everyday concept of the organisms that comprise an ecosystem, but it also contains a rich fauna of micro-organisms. In what is known as the “microbial loop”, organic debris, for example from phytoplankton, is taken up by bacteria which in turn are eaten by protozoa (single-celled animals) and parasitised by viruses. The protozoa are eaten by tiny zooplankton such as copepod nauplii (larvae). Some of the energy that is metabolised in the microbial loop thus benefits the rest of the food chain, in spite of the microbial detour taken relative to the energy that goes directly from phytoplankton to zooplankton.
Succession, evolution and learning
Ecosystems are formed in the course of a gradual process known as succession, with colonisation, growth and adaptation, in which random aspects of the succession of species during colonisation often have consequences for subsequent colonisation and thus for the species composition and function of the ecosystem. Successions normally move from being ecosystems with a small number of species and low biomass towards a climax community with high species diversity and high biomass of plants and animals. Some species have specialised in rapidly exploiting spare “space” in the ecosystem, while others exploit colonising species and may outcompete them in turn. In fjords on Svalbard that are exposed to ice scouring, for example, recolonisation takes place regularly because the ice scours away attached plants and animals. Another example is kelp forests, which are home to an incredibly wide range of other plants and animals. Areas in which kelp is harvested by trawling thus pass regularly through a succession after trawling, with the formation of new kelp forest being followed by colonisation by other plants and animals that depend on the kelp forest for survival.
The organisms in an ecosystem adapt to the physical environment and to each other through the processes of genetic evolution and learning. Evolution is a process in which change tends to take place very slowly, in the course of thousands or millions of years, as a consequence of repeated cycles of reproduction, mutation and natural selection. Learning , on the other hand, takes place within the life cycle of the individual, and is particularly important for long-lived fish and marine mammals, which may experience relatively large climatic changes in the course of their lives. For example, there is good reason to believe that changes in herring migration patterns are driven by “cultural” changes. Patterns of migration are maintained by young herring as they learn from the older fish in the stock. If the stock collapses and there are few older fish left to learn from, the culture will be lost. The collapse of the stock of Norwegian spring-spawning herring at the end of the 60s thus led to major changes in the pattern of migration of this species.
Our marine ecosystems are made up of many different species. Some of these are particularly important in an ecosystem simply because they are so numerous. These species include the copepods. These little hoppers feed on phytoplankton and small zooplankton, and they exist in huge numbers in the Norwegian Sea. Copepods spend the winter in “hibernation” at great depths and are at little risk of being eaten by fish or other predators. Towards the end of winter they migrate vertically and spawn both before and during the spring bloom, so that their nauplii, i.e. the smallest copepod stage, have access to plenty of food and can grow rapidly. Copepods are the basis of our large stocks of herring and mackerel, which feed on them throughout their lives. They are also food for the larvae of a number of other species such as cod and saithe. Altogether, most of our fish species “plan” their lives around the copepod life-cycle, spawning in the spring so that their larvae will encounter as many copepod nauplii as possible. Herring are perhaps the species that is most highly dependent on copepods. Like them, herring stocks overwinter, which means that they do not eat for more than four months!
Bottom-up and top-down
We often speak of the dynamics of an ecosystem or a species being driven by “bottom-up” or “top-down” processes. This has to do with the fact that population growth may be limited either by food intake or by predation. When bottom-up processes are assumed to be important, this suggests that the biomass attained by a fish species is limited by the amount of food it can obtain. If top-down processes are important, the biomass is limited by predation. Both bottom-up and top-down control can be important ways of regulating the size of a population under different circumstances. For example, capelin are exposed to predation for much of their life. However, predation pressure varies from one year to another, and the capelin stock may often become extremely numerous. A large capelin stock leads to the zooplankton on which it feeds being grazed down, which means that the capelin grow slowly. Thus, the capelin stock can be regulated by both bottom-up and top-down processes.
Ecosystems are in a permanent state of change. At our latitudes, variations in the physical environment – wind, currents and temperatures – are central drivers of change in the ecosystem. The physical environment is thus a supplier of premises for the rest of the ecosystem. There are wide variations in climate on both annual and decadal scales, and we are currently in a period of rapid global warming. This may result in long-term changes in our ecosystems, since warming is likely to alter competitive consitions in the sea. We can already see that a number of species that used to be infrequent guests in our waters are now in the process of establishing themselves here, while our traditional species are moving northwards. Changes of this sort have taken place before; for example, it is known that cod move their spawning grounds to the north and east during warm periods.
However, changes in the ecosystem are not only climatic. Human activity, for example, also leads to pollution, overfishing and the introduction of new species, all of which may bring about important changes in the structure and function of ecosystems. Ecosystem-based management involves having an overarching plan for managing the ecosystem as a whole, where we try not merely to maintain a high level of commercial resource harvesting in the long term, but also prevent human activity from having negative effects on the rest of the ecosystem. The present generation must not despoil the environment at the expense of future generations. This means that, to a greater extent than until now, we need to understand the structure and function of the ecosystem in order to be able to predict the consequences of human activity. For this reason, we will need to invest more in ecosystem research in the future.