All forms of life are dependent on availability of energy at all levels, the creation, growth, and maintenance (defense, offense, and survival). The requirement and utilization of energy are mainly in two forms; the most important are nutrient and environmental energy in the form of heat and light.

It is easy to observe that extremely cold or hot regions are not favor­able for the growth of living things. Likewise, the absence of light limits the propagation and proliferation of photosynthetic biotic species.

The sun, of course, radiates energy into space of which only an insignif­icant part is shared by this planet of ours called Earth. Because of its spin and its orbital rotation, a seasonal variation occurs in the total insolation on the earth’s surface, which averages approximately 20 kcal/(m2 • yr). The incident radiation comprises 2000-8000 A, 50% of which is in the visible range (3700-7700 A); only a small part of the incident energy is utilized by living systems.

Solar constants are given as 1.968 cal/(cm2 • min) = 3.86 X 1033 erg/s = 1.373 kW/m2. There are variations in the figures, depending on the source of information. However, the energy received on the earth’s sur­face is mostly thermal and wasted. Biological fixation is restricted to pho­tophosphorylation.

Let us look at the components of ecosystems that are capable of uti­lizing incident energy and some interrelationships between them.

Autotrophs (meaning self-surviving), also known as producers, mainly the photosynthetic systems, are the largest users of sunlight. Theoretically, anywhere there is light they should grow, provided other inputs are favorable. In arid land, the lack of nutrients; in deserts, the lack of water; and at higher-altitude, low temperatures, low CO2 tension and other adverse conditions will prevent the proliferation of autotrophs, leaving otherwise sufficient insolation unutilized (energy fixation by photosynthetic pathway is treated elsewhere). Producers growing on detritus (dead organic materials) are not well described in the literature, but these could be autotrophs.

Heterotrophs (mixed surviving or unlike surviving), on the other hand, survive partly depending on the nutrient sources made avail­able by other living systems. Most animals are heterotrophic. Therefore, animals are also called consumers.

If animals survive mainly on autotrophic materials, they are called primary consumers, commonly known as herbivores. If animals largely survive on other animals as their source of food, they are called secondary consumers, popularly known as carnivores. Predators are animals that hunt their animate food, known as prey. The prey-predator relationship plays an important role in nature and con­tributes to the ecologic balance.