Since energy is an important common denominator in all ecosystems, whether designed by man, it provides a basis for that might be called a ‘’first – order’’ classification. Energy is always a major forcing function. The source a quantity of available energy determines to greater or lesser degree the kinds and numbers of organisms, and the pattern of functional and developmental processes – not to mention the life – style of man. Therefore, knowledge about the energetic of an ecosystem is always of key importance in understanding its properties.

An energy – based classification of ecosystems is outlined in table 2 -1 together with an order of magnitude estimate of the range of energy utilized in terms of kilo calories that flow through a square meter on an annual basis. Since different units (joules, calories, BTU’s, kilowatts and so on) are used by specialists in dealing with different forms of energy, and since the figures in table 2 -1 may not mean much to you in the absolute sense, this might be a good time to refer to appendix table 1 for explanations of units and for a list of convenient conversion factors. It is important to note that some energy units, such as the watt, have time built into the definition and are thus are thus energy time or power units. Other units, such as the calorie, represent potential energy (not time – specific): a unit of time must be added to convert these units to power rates. In this book we shall mostly use kilo calories (abbreviated: kcal) per day or per year to quantify energy flow. Thus, when we speak of ‘’power level’’ or use the term ‘’powered’’ we are referring to energy flow per unit of time. In order to compare various kinds of ecosystems we add a unit of area such as the square meter, acre, hectare, and so on.