The importance of transfer efficiencies in determining energy pathways

The proportions of net primary production that flow along each of the possible energy pathways depend on transfer efficiencies in the way energy is used and passed from one step to the next. A knowledge of the values ofjust three categories of transfer efficiency is all that is required to predict the pattern of energy flow. These are consumption efficiency (CE) assimilation efficiency (AE) and production efficiency (PE).

the relative importance of energy pathways depends on three transfer efficiencies:...

Benthic Microalgae

Figure 17.23 Relationship between the percentage of net primary production (NPP) consumed by herbivores and net primary productivity. o, phytoplankton; •, benthic microalgae; □, macroalgal beds; ♦, freshwater macrophyte meadows; ■, seagrass meadows; a, marshes; A, grasslands; o, mangroves; *, forests. (Data from a number of sources, compiled by Cebrian, 1999.)

... consumption efficiency,...

consumption efficiency,

Figure 17.23 Relationship between the percentage of net primary production (NPP) consumed by herbivores and net primary productivity. o, phytoplankton; •, benthic microalgae; □, macroalgal beds; ♦, freshwater macrophyte meadows; ■, seagrass meadows; a, marshes; A, grasslands; o, mangroves; *, forests. (Data from a number of sources, compiled by Cebrian, 1999.)

Repeated in words, CE is the percentage of total productivity available at one trophic level (Pn-1) that is actually consumed ('ingested') by a trophic compartment 'one level up' (In). For primary consumers in the grazer system, CE is the percentage of joules produced per unit time as NPP that finds its way into the guts of herbivores. In the case of secondary consumers, it is the percentage of herbivore productivity eaten by carnivores. The remainder dies without being eaten and enters the decomposer chain.

Various reported values for the consumption efficiencies of herbivores are shown in Figure 17.23. Most of the estimates are remarkably low, usually reflecting the unattractiveness of much plant material because of its high proportion of structural support tissue, but sometimes also as a consequence of generally low herbivore densities (because of the action of their natural enemies). The consumers of microscopic plants (microalgae growing on beds or free-living phytoplankton) can achieve greater densities, have less structural tissue to deal with and account for a greater percentage of primary production. Median values for consumption efficiency are less than 5% in forests, around 25% in grasslands and more than 50% in phytoplankton-dominated communities. We know much less about the consumption efficiencies of carnivores feeding on their prey, and any estimates are speculative. Vertebrate predators may consume 50-100% of production from

vertebrate prey but perhaps only 5% from invertebrate prey. Invertebrate predators consume perhaps 25% of available invertebrate prey production.

... assimilation efficiency...

assimilation efficiency,

Assimilation efficiency is the percentage of food energy taken into the guts of consumers in a trophic compartment (In) that is assimilated across the gut wall (A„) and becomes available for incorporation into growth or to do work. The remainder is lost as feces and enters the base of the decomposer system. An 'assimilation efficiency' is much less easily ascribed to microorganisms. Food does not enter an invagination of the outside world passing through the microorganism's body (like the gut of a higher organism) and feces are not produced. In the sense that bacteria and fungi typically assimilate effectively 100% of the dead organic matter they digest externally and absorb, they are often said to have an 'assimilation efficiency' of 100%.

Assimilation efficiencies are typically low for herbivores, detritivores and microbivores (20-50%) and high for carnivores (around 80%). In general, animals are poorly equipped to deal with dead organic matter (mainly plant material) and living vegetation, no doubt partly because of the very widespread occurrence of physical and chemical plant defenses, but mainly as a result of the high proportion of complex structural chemicals such as cellulose and lignin in their make-up. As Chapter 11 describes, however, many animals contain a symbiotic gut microflora that produces cellulase and aids in the assimilation of plant organic matter. In one sense, these animals have harnessed their own personal decomposer system. The way that plants allocate production to roots, wood, leaves, seeds and fruits influences their usefulness to herbivores. Seeds and fruits may be assimilated with efficiencies as high as 60-70%, and leaves with about 50% efficiency, while the assimilation efficiency for wood may be as low as 15%. The animal food of carnivores (and detritivores such as vultures that consume animal carcasses) poses less of a problem for digestion and assimilation.

... and production efficiency...

Production efficiency is the percentage of assimilated energy (An) that is incorporated into new biomass (Pn). The remainder is entirely lost to the community as respiratory heat. (Energy-rich secretory and excretory products, which have taken part in metabolic processes, may be viewed as production, Pn, and become available, like dead bodies, to the decomposers.)

Production efficiency varies mainly according to the taxo-nomic class of the organisms concerned. Invertebrates in general have high efficiencies (30-40%), losing relatively little energy in respiratory heat and converting more assimilate to production.

Aquatic Trophic Levels
Figure 17.24 Frequency distribution of trophic-level transfer efficiencies in 48 trophic studies of aquatic communities. There is considerable variation among studies and among trophic levels. The mean is 10.13 % (SE = 0.49). (After Pauly & Christensen, 1995.)

Amongst the vertebrates, ectotherms (whose body temperature varies according to environmental temperature) have intermediate values for PE (around 10%), whilst endotherms, with their high energy expenditure associated with maintaining a constant temperature, convert only 1-2% of assimilated energy into production. The small-bodied endotherms have the lowest efficiencies, with the tiny insectivores (e.g. wrens and shrews) having the lowest production efficiencies of all. On the other hand, microorganisms, including protozoa, tend to have very high production efficiencies. They have short lives, small size and rapid population turnover. Unfortunately, available methods are not sensitive enough to detect population changes on scales of time and space relevant to microorganisms, especially in the soil. In general, efficiency of production increases with size in endotherms and decreases very markedly in ectotherms.

trophic level transfer efficiency, ... which combine to

TLTE = Pn/Pn-1 X 100. give trophic level transfer efficiency

The overall trophic transfer efficiency from one trophic level to the next is simply CE X AE X PE. In the period after Lindemann's (1942) pioneering work, it was generally assumed that trophic transfer efficiencies were around 10%; indeed some ecologists referred to a 10% 'law'. However, there is certainly no law of nature that results in precisely one-tenth of the energy that enters a trophic level transferring to the next. For example, a compilation of trophic studies from a wide range of freshwater and marine environments revealed that trophic level transfer efficiencies varied between about 2 and 24%, although the mean was 10.13% (Figure 17.24).

(a) Forest

Respiration \

Grazer system

Respiration

Decomposer system

Decomposer system

(b) Grassland

Respiration

Respiration

Respiration

Respiration

(c) Plankton community

Respiration

Grazer system

Grazer system

Decomposer system

Respiration

Decomposer system

Respiration

(d) Stream community

Respiration

From terrestrial catchment

Figure 17.25 General patterns of energy flow for: (a) a forest, (b) a grassland, (c) a marine plankton community, and (d) the community of a stream or small pond. The relative sizes of the boxes and arrows are proportional to the relative magnitudes of compartments and flows. DOM, dead organic matter; NPP, net primary production.

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Responses

  • ELANOR GARDNER
    Is assimilation efficiency greater for invertibrates?
    6 years ago
  • mulu
    Why assimilation are typically low for herbivores, detritivores and high for carnivores?
    3 years ago
  • maire
    How to calculate assilimilate efficiency in community?
    2 years ago
  • girma awate
    Why carnivores get less energy but have high efficiency?
    2 years ago
  • Augusto Sagese
    Why assimilation efficiency of herbivores is lower than carnivores?
    2 years ago
  • dehab
    Why consumption efficiency low in herbivorous?
    1 year ago
  • Niklas
    What is the importance of trophic level transfer efficiency?
    11 months ago
  • bruna
    Why do herbivores have low growth efficiencies?
    4 months ago

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