In this blog we will begin the discourse on ecology at the ecosystem level for reasons already indicated;it is the level of greatest interest to everybody, regardless of whether or not the subject of ecology is to be pursued beyond beyond the introductory level. How, then, do we begin with something so formidable as an ecological system. We being just as we would begin the study of any level – by describing simplified versions, which encompass only the most important or basic properties and functions. Since, in science, simplified versions of the real world are called models, it would be appropriate at this point to talk a little about models.

A model is a simplified formulation that mimics real – world phenomena so that complex situations can be comprehended and predictions made. In simplest form, models may be verbal or graphic, that is, consist of concise statements or picture graphs. Although for the must part we shall restrict ourselves to such ‘’informal’’ models in this book, it is important that we consider at least the rationale of constructing more ‘’formal’’ models, because such formulations are going to play an increasing role in decision – making regarding man’s impact on his natural environment.

In its formal version a working model of an ecological situation would, in most cases,  have four components, as listed below (with certain technical terms, as used by systems analysts in parentheses).

                    Picture

A ayestem diagram showing four basic components of primary interest in modelling ecosystems. See text for explanation.

Figure 1-2 shows how these components can be linked together in a model diagram designed to mimic real – world situation. Shown are two properties p1 and p2 which interact at I to produce or affect a third property p3 when the system is driven by forcing E. five folow pathways are shown, with F1 representing the input and F5 the output for the system as a whole.

The diagram could serve as a model for photochemical smog production in the air over los angeles. In this case P1 could repre sent hydrocarbons and P2 nitrogen oxides, two products of automobile exhaust emission. Under the driving force of sunlight energy E, these  interact to produce photochemical among P3 is a more serious pollutant for man than is P1 and P2 acting alone.

Alternatively, figure 1-2 could represent a grassland ecosystem in which P1 are the green plants, which convert sun energy E to food. P2 might represent an herbivorous animal that eats plants, and P3 an omniverous animal that can eat either the herbiveres or the plants. In this case the interaction function I could represent several possibilities. It could be a ‘’no preference’’ switch if observations in the real world showed that the omnivore P3 eats either P1 or P2 according to availability. Or I could be specified to be a constant percentage value if it was found that the diet of P3 was composed of , say 80 percent plant and 20 persent animal matter irrespective of the state of P1 and P2. Or I could be a seasonal switch if P3 feeds on plants during one part of the year and animals during another season. Or it could e a threshold switch if P3 greatly prefers animal food and switches to plants only when P2 is reduced to a low level.