The food chain concept

The constituent parts of the marine system do not stand alone and must not be considered in isolation; they are interrelated through a complex of relationships not always easily understood even by those specialists who have worked for years on the marine environment.

The food chain concept provides a reasonably satisfactory model as a first approach to the functional aspects of the marine environment.

Some indispensable terminology, definitions and descriptions. The photosynthetic organisms, also known as primary producers, use solar energy, nutrients and CO₂, to produce vegetal material and oxygen .

Herbivores of various types and sizes, also known as secondary producers or primary consumers, make use of the production of the photosynthetic organisms to cover their energy needs.

In exactly the same way, first order carnivores, or tertiary producers, feed on the biomass of herbivores until they in their turn are eaten by higher carnivores, called quaternary producers.
Biomass, that is to say, the weight of live body tissues from each of these links in the food chain at any given time, is called standing crop; while the production to time ratio (i.e. the biomass produced in a time interval divided by the length of this time interval) is defined as productivity.

Productivity is not the same in each link of the food chain and is very closely related to the general characteristics of the environment (nutrients concentration, temperature, solar radiation etc.), as well as on food availability and size. Since small organisms usually have high productivity, this should be seen as an important characteristic of the marine environment because the latter is based on the microscopic phytoplankton.

General rules in operation

They utilise only part of the available standing crop of the previous trophic link; in other words, not all the vegetal production is consumed by herbivores, nor are all herbivores consumed by carnivores etc..

Some of the energy consumed is used for body development, some for respiration and some for reproduction. Thus the higher an organism is located in the food chain, the higher the energy losses they have sustained and consequently there is less overall disposable energy.

During their lifecycles, all organisms produce excretions, some of which can be and are utilised by other organisms as food, and all organisms produce carbon dioxide which can be utilised by plants for photosynthesis.

The excretions as well as the dead bodies of the marine organisms, will sooner or later end in bacterial activity which through mineralization produces the raw/ primary material for photosynthetic organisms.

Complexity of Concept

All the above general rules have been simplified to some extent, without straying far from the truth. One of the problems of the food chain paradigm is that it can give the impression there is a one-to-one relation between successive links while in reality the relations are much more complex: many organisms adopt alternative feeding methods which correspond to food availability, and it is quite common for adults to belong to a different trophic link than the juvenile individuals of the same species. The contemporary concept is that of the food web, an example of which is given in Figure. 20 Food chains or nets can be of different length or complexity. An example for a short food chain may be found in antarctic waters from primary producers (diatoms) over krill (herbivorous crustaceans) to baleen whales that filter the krill out of the water. Much longer food chains may be found in tropical waters where 5-7 steps lead to the top predator.



Competition

The complexity of these trophic relationships is linked to the concepts of competition and stability There is competition between two species when both have the same preferences for a single limited natural resource. This concept has immense repercussions because it obviously covers all the life parameters of any given species.

As far as food is concerned, however, it is easily understandable. In Figure 19 it can be seen that herring juveniles and sand eel have to a large extent overlapping feeding preferences. Some decades after this Figure was produced, the herring population of the North sea fell drastically, and almost reached extinction levels, because of overfishing. One of the consequences of this extreme drop in population was a correspondingly drastic increase in the sand eel population in the area (normally preyed on by herring).[see Assessment of the Shetland Sandeel Fishery - 1966]

Stability of the ecosystems

The stability of ecosystems has attracted the attention of many marine ecologists. In Figure 21 (global - multistable equil) the two main concepts related to this aspect are presented.




The global stability concept

According to this, the disturbance of an ecosystem (caused by a pollution event or incident, or an ecological catastrophe) may alter its faunal and floral composition, but sooner or later it will recover and come back to its initial regime, once the activity of the disturbing factor has ceased.

The multiple stable states equilibrium concept

According to this, the restoration to the initial state will occur only if the disturbance is relatively small, otherwise various alternative states may occur, and some of these may be quite different from the original.
The latter concept has gained ground over the last few years, because human activities now have in their turn the potential to cause large scale and high intensity disturbance. Of course anthropogenically induced intense disturbance is directly related to pollution and, in particular, is related to the depletion and loss of natural resources. For instance, when a natural population dies out in a certain area because of overfishing the species composition is altered. New species combinations are established and these may inhibit the re-establishment of the local population which has died out. Serious problems will still arise even if the population has not totally disappeared, but is only severely depleted.

This is the concept of the critical population size below which reproduction fails, while the random (stochastic) variation of mortality can wipe out an entire population. Critical population size is a theoretical concept which has a practical meaning mainly in the case of K-strategy species i.e. relatively large size species with few offspring .These species are much more susceptible to environment disturbance and the decrease of their population. Perhaps this is the reason why conservation movements usually organise campaigns to save large size animals (seals, dolphins, whales) which give birth to only one youngster. No efforts are ever made to save polychaetes, worms or small crustaceans which lay thousands of eggs, although it could be argued that both categories have a similar right to survive. In addition, it cannot be asserted that either category is of lesser importance to an ecosystem. One step towards this goal is the formulation of red lists of endangered species or habitats.