Historically, studies of multispecies assemblages have often focused on 'energy'. There were three general reasons for this:

1. Units of energy relate to all biological activity.

2. There was, for many years, the hope that a focus on energy would somehow bring ecology under the purview of thermodynamics.

3. Finally, once it had become customary to use energy units, there was no strong reason to change.

Concern with energy began as part of an attempt to extend the formal rigor of thermodynamics to biology. Lotka developed a diagram of an abstract ecological system at steady state. At the suggestion of Hutchinson, Lindeman used the notation of Lotka to describe the passage of energy through a lake.

In this trophic dynamic scheme, ecosystems are represented as a set of trophic levels. All the photosynthesizers (green plants, algae, and many kinds of colored bacteria) can be labeled as trophic level 0, the herbivores as level 1, carnivores feeding on herbivores as level 2, carnivores feeding on them as level 3, etc.

Ecological efficiency was rigorously defined by Lindeman as the fraction of the energy that is consumed by organisms on one trophic level that serves as nourishment for organisms on the next higher trophic level. It is a dimensionless ratio, both the numerator and denominator having units of energy/time. Estimates of the empirical value and the utility of ecological efficiency have been studied ever since Lotka and Lindeman and are still of concern.

Ecological efficiency can be measured for either plants or animals. Any definable portion of a community has an ecological efficiency if, and only if, it is possible to measure the energy per unit time taken from it by predators and the energy that it consumed from its prey or food organisms or, in the case ofphotosynthetic organisms, from solar radiation.

The trophic dynamic diagram of Lindeman was not perfect. There are omnivores that consume both flesh and vegetation, thereby feeding on several trophic levels. There are multiple tropic levels among the decomposers. There are also cannibals and organisms that change their trophic level as they mature. These considerations complicate assignment of particular species to particular trophic levels.

With the exception of some deep sea and hot spring bacteria, that use chemical energy in the absence of light, photosynthesis directly, or indirectly, provides the energy for all organisms. Photosynthetic organisms gain radiant energy from sunlight and transform it into potential or chemical energy. Some of the chemical energy in plants is passed on to herbivores whence energy may be passed on either to higher-order carnivores or parasites or detrito-vores. The chemical energy consumed by herbivores is dissipated as heat, stored, or passed further up the food chain. In addition, all trophic levels contribute energy in the form of dead organisms and excreta to decomposer organisms, like bacteria, and molds.

The energy income to plants has been variously defined as the energy of the sunlight impinging on the plants or as the energy actually fixed by photosynthesis. This makes an enormous difference in the estimated value of ecological efficiency for the plants but does not matter for the ecological efficiency estimates of higher trophic levels.

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