Consortium for
Educational
Communication
Module on
ENERGY FLOW IN
AUTOTROPHIC AND DETRITUS
BASED ECOSYSTEMS
By
Zahoor Ahmad Itoo
M. Phil. Scholar
Department of Botany
Kashmir University
Srinagar
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TEXT
1.1. Physical laws govern energy flow in ecosystems
The behaviour of energy in an ecosystem follows laws of
thermodynamics. The first law of thermodynamics or the law of
conservation of energy, states that energy may be transformed
from one form into another but is neither created nor destroyed.
Light for example, is a form of energy, it can be transformed into
work, heat, or potential energy of food, depending on the situation,
but none of it is destroyed. The first law of thermodynamics
also states that energy cannot be created or destroyed but only
transferred or transformed. Thus, we can potentially account for
the transfer of energy through an ecosystem from its input as
solar radiation to its release as heat from organisms. Plants and
other photosynthetic organisms convert solar energy to chemical
energy, but the total amount of energy does not change: the total
amount of energy stored in organic molecules plus the amounts
reflected and dissipated as heat must equal the total solar energy
intercepted by the plant. The second law of thermodynamics may
be stated in several ways including the following: (a) No process
involving an energy transformation will spontaneously occur unless
there is a degradation of energy from a concentrated form into a
dispersed form, (b) because some energy is always dispersed into
unavailable heat energy, no spontaneous transformation of energy
(sunlight) into potential energy (protoplasm) is 100% efficient.
Every ecosystem possess certain thermodynamic characteristics:
can create and maintain a high state of internal order, or a condition
of low entropy (a low amount of disorder). In the ecosystems order,
a complex biomass structure is maintained by the total community
respiration, which continuously “pumps out disorder”. Accordingly
ecosystems are open, non-equilibrium thermodynamic systems that
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continuously exchange energy and matter with the environment
to decrease internal entropy but increase external entropy (thus
conforming to the laws of thermodynamics).
1.2 ENERGY FLOW
The behaviour of energy in an ecosystem is called energy flow.
Energy flow is a fundamental property of ecosystems that links
organisms with each other and to their environment. With respect
to energy flow the ecosystems are open systems i.e., they are
dependent on an external source of energy, which is the sun.
Except for the deep sea hydro-thermal ecosystem, sun is the
only source of energy for all ecosystems on earth. Of the incident
solar radiation less than 50 per cent of it is photosynthetically
active radiation (PAR). Only producers in an ecosystem have the
ability to convert light energy into chemical energy and thus act
as transducers or converters. Energy flows through an ecosystem
in one direction, which is called the food chain. The producers
contain the most energy; they are autotrophs and manufacture
their own food. In a terrestrial ecosystem, major producers are
herbaceous and woody plants. Likewise, primary producers in an
aquatic ecosystem are various species like phytoplankton, algae
and higher aquatic plants. All other organisms in an ecosystem
depend on producers to meet their energy requirements, hence
they are known as consumers. Consumers obtain energy by
eating the producers, they are also known as heterotrophs. There
may be several levels of consumers in an ecosystem, beginning
with herbivores and then to the carnivores and omnivores. If
they feed on the producers, (i. e., plants) they are called primary
consumers, and if the animals eat other animals which in turn eat
the plants they are called secondary consumers. Likewise, there
are tertiary consumers also. Obviously the primary consumers
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will be herbivores. Some common herbivores are insects, birds
and mammals in terrestrial ecosystem and molluscs in aquatic
ecosystem. The consumers that feed on these herbivores
are carnivores, or more correctly primary carnivores (though
secondary consumers). Those animals that depend on the primary
carnivores for food are labeled secondary carnivores. Finally, the
decomposers obtain energy from waste and dead organisms, e. g
bacteria and fungi. So there is unidirectional flow of energy from
the sun to producers and then to consumers, this is in accordance
with the first law of Thermodynamics. Further, ecosystems are
not exempt from the Second Law of thermodynamics. They need
a constant supply of energy to synthesize the molecules they
require, to counteract the universal tendency toward increasing
disorderliness. Starting from the plants (or producers) food
chains or rather webs are formed such that an animal feeds on
a plant or on another animal and in turn is food for another.
The chain or web is formed because of this interdependency. No
energy that is trapped into an organism remains in it for ever.
The energy trapped by the producer, hence, is either passed
on to a consumer or to a decomposer when the organism dies.
Energy transfer during these consumption events is not perfectly
efficient. As no energy transfer occurs in an ecosystem unless
there is loss of energy as heat. So in a food chain producers have
maximum energy followed by primary consumers and secondary
consumers and so on. Death of organism is the beginning of the
detritus food chain. On average about 10 percent of net energy
production at one trophic level is passed on to the next level.
Processes that reduce the energy transferred between trophic
levels include respiration, growth and reproduction, defecation,
and non predatory death (organisms that die but are not eaten by
consumers). The nutritional quality of material that is consumed
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also influences how efficiently energy is transferred, because
consumers can convert high-quality food sources into new living
tissue more efficiently than low-quality food sources.
The low rate of energy transfer between trophic levels makes
decomposers generally more important than producers in terms
of energy flow. Decomposers process large amounts of organic
material and return nutrients to the ecosystem in inorganic forms,
which are then taken up again by primary producers. Energy is
not recycled during decomposition, but rather is released, mostly
as heat.
1 Flow of energy in an ecosystem
Fig.
1.2.1 Energy flow in an autotroph based ecosystems
These ecosystems are characterized by a dependence on energy
capture by photosynthetic autotrophs and secondarily by movement
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of that captured energy through the system via herbivory and
carnivory. Autotrophic ecosystems are directly dependant on
an influx of solar radiation. The sun is the ultimate source of
energy for these ecosystems. Living organisms can use energy
in basically two forms: radiant or fixed. Radiant energy exists in
the form of electromagnetic energy, such as light. Fixed energy is
the potential chemical energy found in organic substances. This
energy can be released through respiration. Producers utilize the
radiant energy of sun which is transformed to chemical form, ATP
during photosynthesis. These ecosystems depend on autotrophic
energy capture and the movement of this captured energy to
herbivores. Organisms that can take energy from inorganic sources
and fix it into energy rich organic molecules are called autotrophs.
Organisms that require fixed energy found in organic molecules
for their survival are called heterotrophs. Heterotrophs who
obtain their energy from living organisms are called consumers.
Decomposers or detritivores are heterotrophs that obtain their
energy either from dead organisms or from organic compounds
dispersed in the environment.
Energy flow pathway in Cedar Bog Lake
The energy flow in this ecosystem was studied by Lindeman
(1942) who reported that the total solar input was 118872 gcal/
cm2/year of which 118761 gcal/cm2/year remained unutilized.
The autotrophs showed a gross production of 118872-118761=
111 gcal/cm2/year (i, e., 0.10%). From this energy 23gcal/cm2/
year (21%) was consumed in respiration, 3.0 gcal/cm2/year in
decomposition and about 70 gcal/cm2/year remained unutilized.
The net primary production was therefore, 111 – (23+ 0) = 88
gcal/cm2/year. Thus, the autotrophs transferred 17% of their net
primary production to herbivores and accumulated about 79.5%
of food energy. Out of 15 gcal/cm2/year the herbivores used 4.5
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gcal/cm2/year (30%) in metabolic activities, 0.5 gcal/cm2/year
in decomposition and 7% gcal/cm2/year remained unutilized.
Only 3.0 gcal/cm2/year (28.6% of net production) was passed
onto carnivores. Thus, carnivores used 60% (1.8 gcal/cm2/year)
of energy in metabolic activities and 40% (1.2 gcal/cm2/year)
remained unutilized. Thus, according to Lindman (1942) from
gross primary production of 111 gcal/cm2/year by autotrophs,
a total of 29.3 gcal/cm2/year was used in respiration, 3.5 gcal/
cm2/year in decomposition and 78.2 gcal/cm2/year remained
unutilized. It may be noted that there was a progressive decrease
in energy at each tropic level.
Fig. 2 Energy flow diagram for Cedar Bog Lake, Minnesota (Energy
in gcal/cm2/year) R. Lindeman 1942.
From the energy flow diagram shown in (Fig. 2) two things become
clear. Firstly, there is one way direction in which energy moves
(unidirectional flow of energy). The energy that is captured by
the autotrophs does not revert back to sun; that which passes to
the herbivores does not pass back to the autotrophs. As it moves
progressively through the various trophic levels it is no longer
available to the previous level. Thus, due to one way flow of energy
the system would collapse if the primary source, the sun, was cut
off. Secondly, there occurs a progressive decrease in energy level
at each trophic level. This is accounted largely by the energy
dissipated as heat in metabolic activities and measured here as
respiration coupled with unutilized energy. In Fig. 2 the boxes
represent the trophic levels and the arrows depict the energy
flow in and out at each level. Energy inflows balance outflows
as is required by the first law of thermodynamics, and energy
transfer is accompanied by dispersion of energy into unavailable
heat (respiration) as required by the second law. Fig. 2 presents
a very simplified energy flow model of three trophic levels, from
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which it becomes evident that the energy flow is greatly reduced
at each successive trophic level from producers to herbivores and
then to carnivores. Thus, at each transfer of energy from one
level to another, major part of energy is lost as heat or other
form. There is a successive reduction in energy flow whether
we consider it in terms of total flow (i, e., total energy input
and total assimilation) or secondary production and respiration
components. Thus, shorter the food chain greater would be the
available food energy as with an increase in the length of food
chain, there is a corresponding more loss of energy.
1.2.2 Energy flow in detritus-based ecosystems
These ecosystems depend less on direct solar energy and more
on the flux of dead organic material or detritus produced in this
or other ecosystems. Indeed, some ecosystems, such as caves,
are completely independent of direct solar energy and are instead
completely energy dependant on the influx of detritus. Such
ecosystems can be regarded as detritus based ecosystems. In
other instances, sub-components of an ecosystem derive their
energy entirely from that systems detritus through decomposition.
Decomposition of organic material occurs in a variety of ways,
among them leaching and fragmentation, but primarily by the
activity of organisms that may, in turn, facilitate both leaching
and fragmentation. The primary agents of the final stages of
decomposition are microbes that act through the process of
metabolism. Detritus can be broadly defined as any form of nonliving organic matter, including different types of plant tissue
(e.g. leaf litter, dead wood, aquatic macrophytes, algae), animal
tissue (carrion), dead microbes, faeces (manure, dung, faecal
pellets, guano, frass), as well as products secreted, excreted or
exuded from organisms (e.g. extra-cellular polymers, nectar, root
exudates and leachates, dissolved organic matter, extra-cellular
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matrix, mucilage). The relative importance of these forms of
detritus, in terms of origin, size and chemical composition, varies
across ecosystems (Moore et al. 2004).
Detritus is a source of energy and nutrients to living organisms
in most food webs. The amount of energy flowing through the
detrital pathway can equal or exceed that of the grazing pathway
(Heymans et al. 2002; Mulholland et al. 2002). Some food webs,
such as those in caves, small streams in forested watersheds, and
below-ground are based almost entirely on detritus pathway. For
other food webs the detritus pathway can have strong influences
on the structure and dynamics of the grazer pathway by providing
energy that can sustain higher densities of consumers than would
otherwise not be maintained if these consumers fed exclusively
on energy derived from the grazer pathway (Moore et al. 2003).
Energy flow in a temperate deciduous forest
Gene Likens and F. Herbert Boreman have carried out extensive
and long-term studies on the Hubbard Brook Experiment forest,
a sugar maple, beech, and yellow birch forest in New Hampshire
and the data obtained by them is as:
Fig. 3 Fate of energy in c/m2/yr in the Hubbard Book Experimental
forest (data from Gosz et al. 1978. Scientific American 238: 93102)
From above study it is clear that a substantial portion of energy
(75%) from net primary production passes through detritus food
chain and only 1% through grazing food chain. The sources of all
the energy flowing through the detritus pathway is as follows:
Leaves
83%
Root death
12%
Nonleaf litter fall
2%
Organic matter via precipitation 2%
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Fecal matter
0.9%
Exuded by roots
0.1%
Animal death
traces
Total
100%
In this forest about 150 C (4%) of this detritus material was not
consumed by detritus feeders (bacteria, fungi, many invertebrates)
or transferred to carnivores (beetles, centipedes) or omnivores
(salamanders, rodents, birds) and thus it accumulated annually
on the forest floor.