Humans can only consume, not produce -- Trophic Categories
Doug Bashford
� Area: Sci.Environm
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Msg#: 60 Date: 03-12-94 17:06
From: Alan Mcgowen Read: Yes Replied:
No
To: All Mark:
Subj: Ecocentral 16
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From: Alan McGowen <al...@igc.apc.org>
ECO CENTRAL 16
Energy Basics
Trophic Categories
Organisms can be categorized by how they obtain their energy. All
organisms ultimately store energy in the form of ATP (adenosine
triphosphate) for use in enzymatic reactions, but a variety of
different *metabolic pathways* are used to produce ATP from ADP
(adenosine diphosphate). Intermediate storage forms such as
sugars, starches and fats are also used.
*Autotrophs* ("self-feeding") produce their own food requirements
without using energy stored in other organisms. To do this they
need some source of abiotic energy. Autotrophs are also called
*producers*, and when they form the base of a food chain,
*primary producers*.
*Chemoautotrophs* produce their energy by oxidizing inorganic
chemicals; there are a number of different reactions employed;
e.g. oxidizing sulfide ions to free sulfur, sulfur to sulfate
ions, ammonium ions to free nitrogen gas (N2) or to nitrate ions,
or nitrite ions to nitrate ions: in each case the oxidation
releases energy.
*Photoautotrophs* use light to form energy-capturing chemical
bonds. The most important of these is photosynthesis employing
chlorophyll which releases oxygen from H2O, but some anaerobic
bacteria use a photochemical reaction which releases sulfur from
H2S. Photosynthesis employing chlorophyll is the only form of
autotrophy found in eukaryotes; all the other types of autotrophy
are found in bacteria, which are metabolically very diverse. Some
of these bacterial metabolic reactions are extremely important in
nutrient cycling.
*Heterotrophs* ("other-feeding") are organisms which depend upon
energy produced by other organisms. They oxidize high-energy
molecules such as fats, carbohydrates, and proteins which are
made by autotrophs or other heterotrophs. Heterotrophs are also
called *consumers*.
*Herbivores* or *primary consumers* consume primary producers,
e.g. green plants.
*Carnivores* consume herbivores (secondary consumers) or other
carnivores (tertiary and higher-order consumers). Primary
carnivores are secondary consumers, secondary carnivores tertiary
consumers, etc. Carnivores may feed at multiple trophic levels.
*Omnivores* utilize energy from both plants and animals.
Generally omnivores can subsist on either plants or animals
but do best on a mixed diet. Humans are omnivores.
*Saprotrophs* (decomposers) utilize energy in dead animal or
plant organic matter. Intestinal organisms also fall into this
category.
These trophic categories are not absolute -- a carnivore may eat
plants, or an herbivore flesh, when starving. Some algae absorb
complex substances which they cannot synthesize themselves but
get from other organisms, making them simultaneously autotrophs
and heterotrophs. Some plants trap and digest insects. Some
phytoplankton in far northern lakes are autotrophic in summer and
heterotrophic in winter. Some protists have chloroplasts but are
also capable of mobility and ingestion.
Trophic Chains and Webs
Energy is transferred through ecosystems in characteristic
sequences known as trophic chains or food chains. A sequence
autotroph -> heterotroph -> carnivore is known as a *grazing
trophic chain*. Chains commencing with dead organic matter and
involving saprotrophs are *detritus (or decomposer) trophic
chains*. The successive steps along a trophic chain are called
*trophic levels*.
An early study of a grazing trophic chain examined a
vegetation -> mouse -> weasel
chain in an abandoned agricultural field in Michigan [Ehrlich,
1987, pp 522-23]. Of 94.2 E8 kcal of solar energy falling on each
hectare of the study plot, about half, 47.1 E8 kcal lies in the
part of the spectrum which is used in photosynthesis (0.4 - 0.7
microns). A little over 1% is actually captured to produce the
plants' *gross primary production* (GPP) of 58.3 E6 kcal. [Part
of this falls on bare ground. The part which actually falls on
photosynthetic organs is utilized with an efficiency that is
higher than this estimate of 1%.] About 15% of this, 8.76 E6
kcal, is used for the plants' own *respiration* (production and
use of ATP) leaving a *net primary production* (NPP) of 49.5 E6
kcal. This is the energy which is stored in plant tissues and
which forms the base of the trophic chain.
Of the NPP, about 30% (15.8 E6 kcal) is in forms which mice can
consume. However, mice consume only 1.5% of this, about 250 E3
kcal. The mice use 170 E6 kcal for respiration and about 2% of
this, 5170 kcal, becomes incorporated into mouse tissues as
(secondary) production.
The weasels consume 5824 kcal -- more than the mice produce per
hectare of the study plot, the difference arising from
immigrating mice and other prey species. Again only about 2%, 130
kcal, becomes production of weasel tissue, and some 5434 kcal is
used by the weasels for respiration. Of the solar energy which
was incorporated into the plants, only about 0.00026% ends up as
more weasel.
Ecosystems contain many such trophic chains, impinging on each
other at many points (as was mentioned above, weasels eat other
prey than mice, so other trophic chains pass through weasels.)
These multiple intersecting chains form *trophic webs*, often of
great complexity.
Ecological Efficiencies
The transfer of energy between trophic levels can be quantified
by the use of *efficiency* factors. At trophic level n, define
*ingestion* In (the total quantity of energy taken in; in the case of
the primary producers this is the light energy falling per unit
area; in the case of herbivores it is the total intake of food).
A portion of the energy ingested is not used; in the case of
consumers this is what is defecated; in the case of plants it is
energy which is not absorbed by photosynthesis. The portion of the
energy which is *assimilated* is An. Of the assimilated energy, a
portion is used for respiration and the remainder becomes *net
production* NPn. Of this net production, a portion is unused by
the next trophic level n+1 -- this part goes into a *decomposer
chain* when the organism dies or drops leaves. The rest becomes
ingested at the next trophic level, In+1.
The *assimilation efficiency* is An/In -- the proportion of what
is ingested that is assimilated.
*Growth efficiency* is NPn/An -- the proportion of what is
assimilated that is fixed in tissues at trophic level n.
*Exploitation efficiency* is In+1/NPn -- the proportion of the
net production of level n that is ingested by level n+1.
*Ecological efficiency* is In+1/In -- the proportion of energy
ingested at level n that is ingested at level n+1.
In the plant --> mice --> weasel chain, the assimilation
efficiencies increase as one goes up the chain: the assimilation
efficiency of the plants was small:
A1/I1 = 58.3 E6/ 47.1 E8 = 1.2%
and the assimilation efficiencies increase as one goes up the
chain. In general, assimilation efficiencies are higher in
carnivores than in herbivores. Some assimilation efficiencies
are: millipede feeding on decaying wood, 15%. Elephant, 30%.
Kangaroo rat, 80%. Weasel, 96%. [Ehrlich, 1987]
Growth efficiency, on the other hand is highest at the producer
level. In the chain discussed above:
NP1/A1 = 49.5 E6/58.3 E6 = 93.5%
In animals, growth efficiencies are lower in endotherms than in
ectotherms, since a larger proportion of energy must be expended
in respiration to maintain body temperature. Insects have growth
efficiencies of about 33%, mammals and birds, 1-5%. Young animals
have higher growth efficiencies than old animals.
Above the producer level, the ecological efficiency is as a
(crude) rule of thumb about 10%. Applying this rule of thumb to a
population of humans who eat exclusively corn as opposed to one
who eat exclusively beef that eats the same quantity of corn, the
corn-eating human population can be about 10 times the size of
the beef-eating one.
Trophic Pyramids
The effect of these efficiencies -- which, by the second law of
thermodynamics, must all be less than 100% -- is to cause the
energy available at each trophic level to be smaller than that at
the level below it, forming an *energy pyramid* Pyramids may also
be constructed for the *number* of organisms at each trophic
level, or for the *biomass* at each trophic level.
Pyramids of numbers or biomass may be inverted; for example the
pyramid of numbers of leaf-eating insects on large trees will be
inverted, though the pyramid of biomass will not be. In marine
ecosystems, pyramids of biomass can be inverted: the total
biomass of consumers of plankton can be greater than the biomass
of the plankton, because the *turnover* or *productivity*
(production per unit area per unit time) of the plankton is
extremely high, higher than the *rate* of consumption. Energy
pyramids cannot ever be inverted as this would violate
conservation of energy.
Some Production Figures
The Net Primary Production of the entire biosphere is estimated
at 11.2 E17 kcal/year. This figure may be in error by 50% or
more.
In the following table, production figures are in dry weight of
*phytomass*, plant biomass. (Data from [Ehrlich, 1987])
Ecosystem type NPP (g/m2 yr) total total
production living phytomass
dry phytomass (billion tons)(billion tons)
Forests total 48.68 950.5
tropical humid 2300 23 420
tropical seasonal 1600 7.2 112.5
temperate conifer 1500 4.5 90
temp. deciduous 1300 3.9 84
Savanna 39.35 145.7
low tree/shrub 2100 12.6 45
grass dominated 2300 13.8 13.2
Desert 3 16.5
scrub 200 1.8 9.9
degraded lands 100 1.2 6.6
Extreme desert 0.13 0.78
Perpetual ice 0 0
Swamps and Marsh 7.25 26.25
temperate 2500 1.25 3.75
tropical 4000 6 22.5
Cultivated land 15.05 6.64
temperate annuals 1200 7.2 0.6
temp. perennials 1500 0.75 2.5
tropical annuals 700 6.3 0.54
trop. perennials 1600 0.8 3
Human area 500 0.4 3.2
Terrestrial total 897 (average) 132.2 1243.9
Lakes and streams 400 0.8 0.04
Marine 254 91.6 3.9
Aquatic total 255 (average) 92.4 3.94
Grand Total 440 (average) 224.6 1247.8
The total "ingestion" of human civilization is over 30% of
terrestrial NPP -- this is the part which is diverted from use in
all other (nonhuman) food chains. The amount which humans and
domestic animals actually consume as food is about 4% of total
terrestrial NPP and 2% of total marine [Vitousek, 1986], i.e. the
assimilation efficiency of human civilization of terrestrial NPP
is about 10% -- significantly lower than that of millipedes.
Refs.
Ehrlich, Paul R. and Jonathan Roughgarden. 1987. _The Science of
Ecology_, MacMillan, New York
Kimmins, J. P. 1987. _Forest Ecology_, MacMillan, New York.
Vitousek, P.M., P. R. Ehrlich, A. H. Ehrlich, and P. A. Matson,
1986. "Human Appropriation of the Products of Photosynthesis,"
BioScience, vol. 36, pp. 368-73.
-!!!!!!!!!!-
Alan McGowen
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- If you scratch a cynic,
- you'll find a defeated idealist.