• Energy is lost at each transfer in the food chain.

• Productivity must always decrease from primary producer (green plants) to secondary producers (grazers and predators).

• Productivity usually decreases by a factor of 10 at each trophic level.

• Gross production (P) consists of net production (NP) plus respiration (R).

• Net production (NP) is the amount of organic matter fixed and transferable to the next trophic level.

• In an ecosystem with plants only, net production by primary producers (NP) equals net ecosystem production (NEP).

• This net production (NP) is available for harvest and grazing by primary consumers (secondary producers).

• In an ecosystem with consumers (primary and secondary) present, NEP is the sum of net production not consumed plus the production (realized growth) of consumers (secondary production) (Fig. 2.1).

• With plants only, assuming R = 0.5 P for simplicity, the ratio of gross production to respiration (P:R) is 2.

• This represents energy surplus.

• With consumers present, the ratio P:R decreases.

• In mature, stable ecosystems, the ratio P:R approaches 1 from above (Say 1.01).

• In heterotrophic, or allochthonous-fed systems, P:R ratios are typically less than 1.

• This represents energy deficit.

2.3  EFFICIENCY OF ENERGY TRANSFER

• Photosynthesis is rather inefficient.

• For terrestrial systems, efficiency in light utilization is about 1%.

• In water it is much lower (0.1-0.4%); scattering efects by water on light results in lower efficiency.

• Utilization of primary productivity through secondary production is nominally 10%, but ranges from 5 to 20%.

• The 80% to 95% that is lost at each trophic level is through respiration.

• The steps involved in handling energy at the grazer level are shown in Fig. 2.2.

• Waste input disrupts structure by eliminating efficient energy utilizers.

• Large populations of unconsumed producers indicates a degradation of the ecosystem dynamics.

2.4  POPULATION GROWTH AND LIMITATION