Energetics in Ecosystems

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Biomes represent the big picture -- a grand scheme -- to categorize the living world. Within each biome any number of smaller biological communities coexist and at the boundaries between biomes special interactions may also occur.

The populations of organisms that interact, compete and form stable associations in habitats such as caves, streams, ponds, north facing slopes or other unique physical places are called ecological communities.

Two views of communities:

  1. The following ideas are part of H. A. Gleason's individualistic concept of communities.
    • Communities are chance collections of species that are in the same area because of similar environmental requirements.
    • There should be no distinct boundaries between communities.
    • Species are distributed independently along environmental gradients according to a tolerance range.


  2. E. E. Clements advanced the "interactive" concept of a community functioning as an integrated unit. When the consistent composition of a community is based on interactions between plants, animals, etc. the community can then function as an integrated unit. Thus plant species are often clustered into discrete communities.

Both views can be tested and both occur. Observations by and large show that plants are more loosely associated and tend to change on a continuum (Gleason) but animals with narrow niches tend to remain within a given community (Clements).

Before we can understand a biome we must look first at the communities and populations of which it is composed.

An overview of ecological communities and ecosystems.

Investigating the flow of energy when it interacts with both the living (biotic) components and nonliving (abiotic) factors within a community allows us to define units of the biome called ecosystems.

 To understand ecology (our house - our world) the following aspects of ecosystems must be considered.


Energetics in Ecosystems


Energy is not recyclable. Once degraded to its lowest level -- generally referred to as heat -- it is no longer available to do work. We therefore speak of the flow "downhill" from high to low energy.

About 0.1% of incoming solar radiation is captured and turned into biomass by photosynthesis, yet this seemingly small amount of energy becomes 150 to 200 billion tons of dry organic matter annually.

Energy accumulates in an ecosystem as autotrophs such as plants and algae convert sunlight into chemical energy (bonds between organic molecules). As the chemicals are passed from one organism to another (consumers) much of the energy is wasted or lost, and the energy flow diminishes -- each trophic level retains approximately 10% or less of the energy of the previous level. (See Diagram below.)

Figure 1. Adapted from Wallace (Page 1166) - ENERGY FLOW IN AN ECOSYSTEM. Solar energy is converted into sugars by photosynthesis- primary production. Sugar -- chemical bond energy -- is used by each level of consumers (and by producers) to make all the rest of organic matter. Eventually, some of the energy is passed on to decomposers but most is lost as heat. At each trophic level less and less energy is available to do work since the transfer is only about 10% efficient.

A simple food chain can be used to show the depletion of usable energy.


Energy flow is described in terms of trophic levels.

  • Producers - photoautotrophs and chemoautotrophs (deep sea thermal vents) form the wide base of chemical energy on which all other organisms depend.
  • Consumers - heterotrophs utilize organic matter produced by other living things. They may be organized into the following trophic levels.


  • Primary consumers or herbivores - organisms which form the second trophic level, feed on plants or algae (or some other producer). Many insects are at this level as are some fungi. The large hoofed mammals are classical examples of herbivores.
  • Secondary consumers or carnivores comprise the third trophic level or higher. In the web of life animals eating other animals (snakes eating birds eating insects) may produce several additional trophic levels. There is a limit however. Usually by the 4th or 5th trophic level so much energy has been wasted or lost by previous levels that not enough resources are available to support another level. "The tremendous reduction in available energy is what ultimately limits he number of trophic levels."
  • Decomposers or saprobes are an essential part of all ecosystems for without them no amount of energy would be sufficient because most raw materials necessary for life would be locked up in the dead bodies of previous generations. Fungi, bacteria, and some protists ensure that essential molecules cycle back to producers. These important chemicals include ammonia, sulfates, nitrates, nitrites, and phosphate.


Trophic levels are often represented as pyramids. These Eltonian pyramids -- first used by Charles Elton -- describe changes in some factor from one trophic level to another. Although there are pyramids of numbers and pyramids of biomass the pyramids of energy are most commonly used.

An example of an energy pyramid where humans are second-level consumers is shown below.

Figure 2

As is obvious from such radically stepped diagram an enormous amount of energy is lost between levels. If the earth is to support more humans in the future more of us must become vegetarians (or dramatically reduce meat intake).

Energy and Productivity

The rate of energy storage is called primary productivity (See Figure 1). Net community productivity, NCP, is the total rate of energy created by producers minus the rate of energy utilized by all the organisms living within the ecosystem.

This important measure, NCP, can indicate the health and state of a community. The NCP can determine whether a community is declining, growing or most importantly in a nonchanging or climax state.

Modified Nov. 8, 2005