The Transformation of Energy

The transformations of energy in an ecosystem begin first with the input of energy from the sun. Energy from the sun is captured by the process of photosynthesis. Carbon dioxide is combined with hydrogen to produce carbohydrates (C₆H₁₂O₆). Energy is stored in the high energy bonds of adenosine triphosphate or ATP.

 The prophet Isaah said "all flesh is grass", earning him the title of first ecologist, because virtually all energy available to organisms originates in plants. Because it is the first step in the production of energy for living things, it is called primary production. Herbivores obtain their energy by consuming plants or plant products, carnivores eat herbivores, and detritivores consume the droppings and carcasses of us all.

     Figure 1 portrays a simple food chain (The sequence of organisms, each of which is a source of food for the next, is called a food chain), in which energy from the sun, captured by plant photosynthesis, flows from one trophic level to the next trophic level (Each step in the flow of energy through an ecosystem is known as a trophic level; Organism’s feeding status), via the food chain.

(Although we have been talking about food chains, in reality the organization of biological systems is much more complicated than can be represented by a simple "chain". There are many food links and chains in an ecosystem, and we refer to all of these linkages as a food web. Food webs can be very complicated, where it appears that "everything is connected to everything else".)

Each step in the flow of energy through an ecosystem is known as a trophic level (Organism’s  feeding status). Producers (green plants) : first trophic level. Herbivores (called primary consumers) :  second trophic level. Carnivores (secondary consumers): third trophic level. Carnivores (tertiary consumer) : fourth trophic level and so on. Omnivores, parasites, and scavengers occupy different trophic levels, depending on what they happen to be eating at the time

If we eat a piece of steak, we are at the third trophic level; if we eat celery, we are at the second trophic level

As energy is transferred from one trophic level to another, energy is lost due to limited assimilation, respiration by consumers, and heat production (heat is dissipated to the surroundings and warms the air, water, or soil). As losses between trophic levels accumulate, eventually there is insufficient energy to support a viable population at a higher trophic level.

 Approximately 90% of the useful energy is lost with each transfer to the next highest trophic level. So in any ecosystem, the amount of energy contained in the herbivore trophic level is only 10% of the energy contained in the producer trophic level. The amount of energy at the third trophic level is approximately 1% of that found in the first trophic level. Ecologists often use biomass (weight of living material) to approximate the relationship between the amounts of energy at each trophic level.

A general rule of thumb is that only about 10% of the energy in one consumer level is represented in the next higher level. For example, it generally takes about 100 kg of clover to make 10 kg of rabbit and 10 kg of rabbit to make 1 kg of fox.

The more trophic levels or steps in a food chain or web, the greater the cumulative loss of usable energy as energy flows through the various trophic levels.

Energy flow pyramids explain why the Earth can support more people if they eat at lower trophic levels by consuming grains, vegetables, and fruits directly (for example, grain – human) rather than passing such crops through another trophic level and eating grain eaters (grain – steer – human).

The large loss in energy between successive trophic levels also explains why food chains and webs rarely have more than four or five trophic levels. In most cases, too little energy is left after four or five transfers to support organisms feeding at these high trophic levels. This explains why there are so few top carnivores such as eagles, hawks, tigers, and white sharks. It also explains why such species usually are the first to suffer when the ecosystems that support them are disrupted and why they are so vulnerable to extinction.