We can view a forest, a stream, or an ocean as a system that absorbs, transforms, and stores energy. In this view, physical, chemical, and biological structures and processes are inseparable. When we look at natural systems in this way we view them as ecosystems. An ecosystem is a biological community plus all of the abiotic factors influencing that community. Primary production, the fixation of energy by autotrophs, is one of the most important ecosystem processes. The rate of primary production is the amount of energy fixed over some interval of time. Gross primary production is the total amount of energy fixed by all the autotrophs in the ecosystem. Net primary production is the amount of energy left over after autotrophs have met their own energetic needs. Terrestrial primary production is generally limited by temperature and moisture. The variables most highly correlated with variation in terrestrial primary production are temperature and moisture. Highest rates of terrestrial primary production occur under warm, moist conditions. Temperature and moisture conditions can be combined in a single measure called annual actual evapotranspiration, or AET, which is the total amount of water that evaporates and transpires off a landscape during the course of a year. Annual AET is positively correlated with net primary production in terrestrial ecosystems. However, significant variation in terrestrial primary production results from differences in soil fertility.Aquatic primary production is generally limited by nutrient availability. One of the best documented patterns in the biosphere is the positive relationship between nutrient availability and rate of primary production in aquatic ecosystems. Phosphorus concentration usually limits rates of primary production in freshwater ecosystems, while nitrogen concentration usually limits rates of marine primary production.Consumers can influence rates of primary production in aquatic and terrestrial ecosystems. Piscivorous fish can indirectly reduce rates of primary production in lakes by reducing the density of plankton-feeding fish. Reduced density of planktivorous fish can lead to increased density of herbivorous zooplankton, which can reduce the densities of phytoplankton and rates of primary production. Intense grazing by large mammalian herbivores on the Serengeti increases annual net primary production by inducing compensatory growth in grasses.Energy losses limit the number of trophic levels in ecosystems. Ecosystem ecologists have simplified the trophic structure of ecosystems by arranging species into trophic levels based upon the predominant source of their nutrition. A trophic level is determined by the number of transfers of energy from primary producers to that level. As energy is transferred from one trophic level to another, energy is lost due to limited assimilation, respiration by consumers, and heat production. As a result of these losses, the quantity of energy in an ecosystem decreases with each successive trophic level, forming a pyramid-shaped distribution of energy among trophic levels. As losses between trophic levels accumulate, eventually there is insufficient energy to support a viable population at a higher trophic level.
Stable isotope analysis can be used to trace the flow of energy through ecosystems. The ratios of different stable isotopes of important elements such as nitrogen and carbon are generally different in different parts of ecosystems. As a consequence, ecologists can use isotopic ratios to study the trophic structure and energy flow through ecosystems. Stable isotope analysis has helped quantify dietary composition of wild populations and the major sources of energy used by prehistoric human populations.
