The allocation of energy use by organisms.Source: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Debuke, IA). Every living organisms must use energy for six purposes; maintenance, growth, storage, reproduction, protection, and obtaining more energy. The natural world is a dazzling array of different strategies for accomplishing these tasks.

Endotherms such as mammals maintain a preferred body temperature with heat generated by their own metabolic processes, while ectotherms such as lizards obtain their body heat from the environment. Autotrophs (Greek for “self feeders”) such as green plants produce their own food via photosynthesis, while heterotrophs (Greek for “other feeders”) get their food by consuming other organisms. Humans store energy in the form of fat, while most trees store excess amounts of energy generated during the summer in their roots. Elephants produce one offspring and invest substantial energy in rearing it, while most fish produce huge numbers of offspring an invest little energy in parenting.

The desirability of various strategies for obtaining energy from the environment and allocating it among different uses is determined by natural selection. Strategies that work are rewarded while strategies that do not work are punished. If a strategy works, an individual will have sufficient supplies of energy and will allocate it in ways that allow the individual to produce many offspring that survive and prosper. If the strategy is inherited by the offspring, the number of individuals that follow this strategy will increase in the next generation. If a strategy does not work, the individual will produce few offspring and these offspring will have a small probability of survival. As a result, the number of individuals that follow the strategy will decrease in the following generation.

Aerobic organisms Aerobic organisms such as the cheetah (Acinonyx jubatus) are more abundant than anaerobic organisms because respiration with oxygen yields much more energy than the anaerobic pathway. If the relative success of a strategy persists for many generations, the strategy that generates the most energy and allocates it among maintenance, growth, storage, reproduction, and protection in a way that generates the greatest number of successful offspring will prevail. Other strategies will appear, but they will fail and disappear.

The role of natural selection in "choosing" strategies is illustrated by asking the question, why breathe oxygen? One pathway allows organisms to convert food to energy without oxygen, which is termed anaerobic respiration. Another pathway uses oxygen to convert food to energy, which is termed aerobic respiration. Most organisms on the planet use aerobic respiration. Why? An organism that respires a molecule of glucose anaerobically obtains 47 units of energy while the aerobic pathway generates 686 units of energy. Given the same amount of food, the aerobic organism has 15 times more energy available. All other things being equal, this difference allows the aerobic organism to expend greater efforts to maintain itself, to grow, to reproduce, to protect itself, and to obtain more food. Based on this advantage, organisms that depend mainly on aerobic respiration have been able to out compete organisms that are capable of anaerobic respiration only in environments where oxygen is present. As a result, aerobic organisms are the predominant form of life on the planet.

Notes

This article draws on material adapted from Chapter 5 of: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Debuke, IA).

Further reading

• Kay, James J., About...Thermodynamics and Ecology, University of Waterloo.
• Lotka, Alfred J. (1922) Contribution to the energetics of evolution [PDF]. Proc Natl Acad Sci, 8: pp. 147–51.