According to Odum, the structure of an ecosystem needs to be considered from various angles in order to understand the relationship between the structure and function in an ecosystem.
1. Trophic Structure:
The producer-consumer arrangement is one kind of structure called trophic structure (trophic = food), and each food level is known as the trophic level.
The amount of living material in the different trophic levels or in a component population is known as the standing crop. This term is applied equally well to plants or animals.
The standing crop can be expressed in terms of the number per unit area or in terms of biomass i.e., organism mass. Biomass can be measured as living weight, dry weight, ash-free dry weight, carbon weight, calories, or any other unit that may be useful for comparative purposes.
The standing crop not only represents the potential energy; but also may be important as a buffer against physical oscillations and as a habitat, or living space for organisms. Thus, trees in a forest not only represent energy that provides food or fuel; but they modify climate and provide shelter to birds and men.
2. Biochemical Structure:
The amount and distribution both of inorganic chemicals and organic materials present either in the biomass or in the environment are important factors in any ecosystem. We may consider this as the biochemical structure of the ecosystem. For example, the quantity of chlorophyll per unit area of land or water surface and the quantity of dissolved organic matter in water are two items of great ecological interest.
3. Species Structure:
Still another aspect of great importance is the species structure of an ecosystem. Species structure includes not only the number and kinds of species present but also the diversity of species— that is, relationship between species and numbers of individuals or biomass and the dispersion of individuals of each species that are present in the community. As mentioned above, the amount of energy gained by the system and the mode and quantities of it transferred, affect the density and diversity of species.
4. Productivity Concept:
A very important factor in the ecosystem study is the rate of energy-trapping by green plants, which in turn governs the rate of production of organic material from simple inorganic substances (in a given area over a given period of time). This rate of energy conversion or increase in organic biomass produced is called primary productivity. This total production as a result of photo- or chemosynthesis is called gross primary productivity (GPP). Naturally some of the primary produce is all the time lost by way of the breakdown of organic matter in respiratory metabolism of plants and animals.
Thus, net primary productivity (NPP) is the rate of production when loss due to respiration is deducted out of the gross production rate. Primary productivity refers to total increase in weight in all parts: fruits, leaves, stems, and roots, as against the agricultural productivity which refers to useful parts as grains or fodder parts.
The efficiency of any ecosystem greatly depends upon the production rates of its primary producers. Oceans form the largest ecosystem and its productivity varies in different climatic regions. On the shores, the productivity may be 2 to 3.5 grams per square metre per day, and in deep seas only 0.5 gram. In highly productive lakes, the productivity value may be 5 to 10 grams/ m2/day and even upto 50 grams in exceptionally favourable conditions.
The net productivity of crop plants (whole plant) ranges from 0.25 kg to 1 kg or a little more for rice and wheat crops per square metre per year. Sugarcane is one of the very efficient converters of solar energy and its net primary productivity value ranges from 2 to 4 kg/ m2/year or even more. All productivity values are expressed on dry weight basis of organic matter or on the basis of energy contents in unit time and space.