Short notes on Ecosystem Control Mechanisms

If probability theory alone was responsible for directing the fortunes of the ecosystems to be found in our biosphere then ecological pattern and the forecasting of ecological events would be an impossible task.

Instead, ecosystems are ‘guided’ by external and internal control mechanisms more appropriately called the science of cybernetics (Beer, 1967).

Cybernetic control is widely used in industrial production lines. In its simplest state a cybernetic control is an on/off switch, for example a thermostat or a set of traffic lights which regulates the flow of traffic at an intersection. The cybernetic system can be refined, so that the traffic light example is extended to include a timing cycle, 30 seconds ‘go’ for main route A, 20 seconds for secondary route B and 10 seconds for pedestrian crossing. Or the system may have a traffic counter sensor or a photo-electric call to detect the presence of traffic to help optimize traffic regulation.

Even when taken to its ultimate development in which a central computer takes control of the entire traffic regulation via traffic lights, the system will remain a deterministic system because the programme of events will be controlled by the computer programme artificial intelligence (Al) into industrial robots but even in these examples the ‘intelligence’ is pre-programmed and is not ‘learnt’ in the organic sense.

Because natural variability is an inherent property of living creatures the organic system will never be deterministic in its behaviour. Lack (1946) provides a good example in his well known book about the behaviour of the common European robin (Eirthacus rubccula). The number of breeding pairs of robins fluctuated in a given area from year to year. This was due to a number of environmental changes. 1 .The robin population will increase under conditions of favourable (mild) winters and abundant food supply.

2. A succession of favourable breeding season produces an exponential increase in members of robins, and conversely, unfavourable conditions maintained for several years leads to a rapid depletion of numbers.

3. As the robin population increases then to individual territory size decreases leading to a small area from which food can be obtained. This is turn can lead to under nourishment and a reduction in the number of chicks reared per breeding cycle.

Fluctuations in population size of robins tend not to oscillate wildly between very large and very small numbers. Instead, the population cycles around a mean value which may show a 10 per cent annual variation? Considering all the possible causes which could create either a gain or a decrease in population size, the inter-year variation is surprisingly small.

Constancy in Ecosystems

The examples of the relative stability of population numbers as displayed by the robin illustrates a commonly observed feature of many ecosystems and species, that of constancy.

Constancy can be measured in a number of different ways, for example, by counting the number of individual organisms in an area, or by calculating the biomass, or by noting the dates at which significant stages of the growth history are reached. All these indices tend to confirm the feature of constancy, that is, individuals show only a small variation around the mean value.

Changes will take place, but will occur within boundaries imposed by feedback signals which the ecosystem and species will provide themselves.

The Feedback Mechanism

The success of an ecosystem is dependent upon the development of an accurate and speedy feedback mechanism. It can argued that an agressive, expanding ecosystem will replace and adjacent and less successful ecosystem because the former has a better developed feedback mechanim than the latter.

Any organism which responds quickly to a stimulus is better placed to survive than an organism with display several characteristics, all of which equip that ecosystem to outperform the system with poorer feedback responses. Good feedback channels permit the ecosystem to maintain input levels as near as possible to optimum levels.

To achieve this, the ecosystem will be capable of fineturing the input so that the amplitude of change rarely creates instability within the system.

Absolute stability is rarely found in an ecosystem although it is possible for an ecosystem to attain relative stability, in this condition, oscillation will occur between two extremes.

The system inputs can be considered to fluctuate as a series of independent wave-like curves and only when the majority of inputs come ‘in phase’ can stability be attained.

The condition of relative ecosystem stability is not commonly found in the biosphere at the present time. This is due to the process of simplification brought about by man, see figure. Removal of species by hunting or by habitat destruction has resulted in most ecosystems now existing in a state of permanent instability.

The ecosystem balance which has been achieved over tens, or even hundreds of thousands of years has been swept aside in just 10,000 years of time during which mankind has achieved biosphere dominance.

It is because of the magnitude and speed with which ecosystem simplification is now taking place that it has become necessary to actively promote the need for ecosystem and biosphere conservation. Had it been possible for mankind to be less destructive in his use of ecosystems then much of the present concern with conservation would have been unnecessary.