Energy Flow in Ecosystems
GISAT 112
Supplemental Reading for Chemistry in Context Chapter 41
1. Introduction
We have talked about energy and how we use it—to transport ourselves, to heat our
homes and schools, to power our appliances, etc. This energy involves the capture,
release, and transformation of energy usually stored in chemical bonds. For example, as
discussed in class, when we burn natural gas (CH4), the C-H and O=O covalent bonds in
our reactants absorb energy, while the H-O and C=O bonds in our products release
energy when they are formed. The energy released in this case is higher than the energy
absorbed—and it is released in the form of heat and light. A power plant might instead
choose to use this heat to generate steam, which it then uses to turn a turbine connected to
a generator. In that case, the thermal energy released by burning CH4 is converted to
mechanical energy (turning of the turbine) and then to electrical energy.
The question we now address is how ecosystems use energy. All plants and animals need
energy for growth, metabolic processes (like staying warm), and reproduction. Plants and
animals also get their energy by converting potential energy stored in chemical bonds.
For plants, energy is provided by the sun and triggers photosynthetic reactions that allow
plants to form simple carbohydrates (like glucose) from CO2 and water. These
carbohydrates store energy for future use. Animals that eat plants (herbivores) assimilate
the carbohydrates found in the plants and use those for energy. And carnivores eat other
animals and assimilate the energy that was stored in those critters. This supplemental
reading addresses energy in ecosystems, how that energy is stored, and how it is passed
along from plants to animals—and ultimately to us!
2. Energy in Ecosystems
2.1. Ecosystems
Before we delve too deeply into energy flow in ecosystems, we should understand what
ecosystems are. Ecosystems are defined as a community of different species interacting
with one another and with their nonliving environment of matter and energy. Ecology is
the study of the relationships between organisms and their environment. (Etymological
note: the word ecology comes from the Greek oikos meaning “house” + logos meaning
“study of”).
There is no correct size of an ecosystem—its size is somewhat arbitrary. Ecosystems are
often defined by the area we wish to study. Figure 1 shows how ecosystems fit into the
larger (and smaller) picture.
1
Much of the material in this supplemental reading is taken from G.T. Miller, Living in the Environment,
12th Edition, Thomson Learning, 2002; R.L. Smith and T.M. Smith, Ecology and Field Biology, 6th
Edition, Addison Wesley Longman, Inc., 2001. All figures are from Miller (2002) unless otherwise noted.
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Figure 1. Ecosystems relative to other special metrics
2.2. Produce-Consumer Chains
Since ecosystems are defined by the interaction of species among each other, another
important component of ecosystems are species relationships. One of the most critical
relationships is that of producer and consumer.
Producers (also called autrotrophs = "self–feeders") make their own food from
compounds obtained in the environment. Plants are producers. They capture sunlight to
make sugars and other organic compounds in the photosynthetic process. Chemosynthetic
bacteria are also producers, as they can convert simple compounds into complex nutrient
compounds to feed themselves without sunlight.
Consumers (also called heterotrophs = "other–feeders") get their energy and nutrients by
feeding on other organisms. Consumers include herbivores (who eat producers),
carnivores (who eat other consumers), omnivores (who eat both producers and
consumers), and detritivores (or decomposers, who eat dead matter). For most
consumers, complex carbohydrates are converted into energy via aerobic respiration that
occurs in the cells of these organisms. In respiration, organic molecules (e.g., glucose) are
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combined with oxygen to produce carbon dioxide, water, and energy in a process that is
almost the reverse of photosynthesis.
Glucose + oxygen
C6H12O6 + O2


carbon dioxide + water + energy
6 CO2
+ 6 H2O + energy
These producers, consumers, and decomposers are continuously cycling matter and
energy through their ecosystem. Figures 2 and 3 depict these producer-consumer
relationships for an aquatic and terrestrial ecosystem, respectively.
Figure 2. Producer-consumer relationships for an aquatic ecosystem.
Figure 3. Producer-consumer relationships for a terrestrial ecosystem.
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As shown in Figures 2 and 3, we can also categorize consumers based on the plants or
animals they eat. Primary consumers are herbivores those that feed directly on
producers. Secondary consumers are carnivores that feed on primary consumers.
Tertiary consumers feed only on carnivores. Note that some animals (like humans) can
be primary (think salad), secondary (think rabbit stew), or tertiary consumers (think gator
bites)!
We can also divide the decomposers into two groups: decomposers that consume remains
and wastes of other organisms and complete the breakdown and recycling of organic
materials; and detritivores which feed on detritus (partially decomposed organic matter,
such as leaf litter and animal dung). Figure 4 identifies examples of decomposers and
detritivores. For this reading we will refer to these groups as one.
Figure 4. Distinction between decomposers and detritivores.
3. Primary Production
In discussing ecosystems, it is often useful to talk in terms of that ecosystem’s ability to
produce energy. Gross primary production (GPP) is related to the ability of producers in
an ecosystem to capture solar energy and produce the simple organic carbon compounds
that they will use for growth and reproduction. That is, GPP is a measure of energy that is
captured by an ecosystem’s producers. GPP is also represented as a rate, and so the units
of measurement are energy per area per time (usually kcal/m2/year).
However, not all of the energy captured by plants is stored as plant tissue. Producers also
need energy for metabolic processes. This is done through the oxidation of their stored
carbon compounds through respiration. Another term, net primary production (NPP),
represents the amount of energy that remains after accounting for energy used for
respiration. Thus, we have
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NPP = GPP – Energy used in respiration
NPP is available for consumers to feed upon. Different ecosystems have more or less
NPP capacity, based on biotic and abiotic conditions of the ecosystem (such as sunlight,
temperature, precipitation, etc.). Figure 5 shows the NPP of various ecosystems around
the world.
Figure 5. Average NPP for different types of ecosystems.
4. Secondary production
The NPP of an ecosystem is available to herbivores, which can capture this energy by
eating producers. However, not all of the energy stored in plants is consumed by
herbivores—some just can’t be accessed (for instance, plants that are inaccessible to the
herbivores) or might be removed by humans (e.g., through agriculture). The amount of
NPP available to herbivores varies by time and location.
If an herbivore does capture some of the NPP energy through a nice vegan meal, not all
of that energy is assimilated into the herbivore’s body. A large amount of the plant
material may pass through the animal undigested, depending on the type of herbivore.
For example, grasshoppers assimilate only about 30% of the grass they consumes,
leaving the other 70% for decomposers, while mice assimilate about 85-90% of what they
consume.
The energy (i.e., food matter) that is consumed is used for maintenance (heat production,
metabolism and respiration), growth (stored as animal tissue) and reproduction (stored in
a new organism), or passed as waste. (Note that the waste matter really isn’t “wasted”, as
it represents a gourmet meal for the detritivores, who receive energy by eating it).
Usually the maintenance energy is relatively higher for small, warm-blooded animals.
To help keep track of these energy flows, we can set up an energy budget for a given
consumer. The energy budget might look something like this:
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I=A+W
where,
I = ingested energy (what is taken in)
A = assimilated energy (what is used in the body)
W = lost through waste (what is passed as feces and urine)
In addition, we have:
A=P+R
where,
P = secondary production (the formation of fat and other animal tissue)
R = respiration (used for metabolic processes)
Combining these equations would give us:
I=P+R+W
To make this a bit more concrete, let’s apply some numbers to these equations for a
typical white tail deer. For a deer that takes in 100 units of energy (I), about 35 units
leave as feces, urine, and methane from digestion (W), 48 units leave as heat production
(R), and 12 units are stored in body tissue (P).
Based on these concepts, we can think in terms of the efficiency of a certain consumer to
convert energy from its diet. Three efficiency metrics often used in ecological studies are:

A/I = assimilation efficiency; this is a measure of how efficient the consumer is at
extracting energy from the food it consumes.

P/A = production efficiency; this is a measure of how well the consumer can
incorporate the assimilated energy into new tissue growth, or secondary
production.

P/I = production to ingestion ratio; this is a measure of how well the consumer
converts the energy ingested into new tissue. Since this tissue is available for
another higher-level consumer to eat (e.g., the deer might be eaten by a wildcat),
then the P/I value is quite important to carnivores.
Table 1 shows some approximate efficiency values for four different types of animals.
Table 1. Approximate efficiency values for four consumer types.
Efficiency
Homeotherms
(constant body temp)
Grazing arthropods
(insects or arachnids)
A/I
70-80%
35-40%
P/A
2-3%
42-47%
P/I
1.5-2.5%
15-18%
Source: Data from Smith and Smith (2001).
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Lepidoptera (e.g.
butterfly, moth)
42-52%
45-55%
21-24%
Poikilotherms (e.g.,
cold-blooded, like
frog)
39-45%
42-48%
17-19%
5. Food chains and energy flow
As we have seen, energy is stored by producers and passed through to consumers by
means of a food chain. That is, solar energy is captured by plants, which assimilate that
energy and store it as plant tissue. This plant tissue is eaten by herbivores, which
assimilate the energy into their own animal tissue. That herbivore’s tissue is then eaten by
a carnivore who assimilates that energy and stores it as carnivore animal tissue.
Ultimately, the energy stored in the carnivore is returned to the ecosystem when that
animal dies and decomposes.
Each level in a food chain is called a trophic level. All organisms that obtain their energy
in the same number of steps from the sun are in the same tropic level. The first trophic
level represents the primary producers. The second trophic level represents the herbivores
(or primary consumers). The third trophic level represents the carnivores (secondary
consumers). A fourth trophic level would be a group of organisms that feed off of the
third trophic level—these organisms would be considered tertiary consumers. These
relationships are depicted in Figure 6.
Figure 6. Trophic levels for a simple food chain.
We have already seen that not all the energy available at one level (e.g., the producer
level) is available to higher levels (the consumer level). The second law of
thermodynamics, which states that we cannot transfer energy from one form to another
without loss, is a formal way of saying that some of the energy is released along the chain
due to waste and respiration. Just as in a steam plant, as energy goes from one stage (or
organism) the next (another organism) in the form of food, a large part is degraded as
heat through metabolic activity. As another example of this energy degradation Figure 7
shows the energy flow in a grassland ecosystem.
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Figure 7. Energy flow for a grassland system.
6. The energy pyramid
If you study Figures 6 and 7, you will notice that one implication is that higher trophic
levels have less energy available to them than lower trophic levels. Because energy is
degraded as it moves from one trophic level to the next, the amount that is stored in
animal tissue at high trophic levels is much less than that received by producers. This
concept is reflected in the energy pyramid. An energy pyramid is shown in Figure 8,
where there is a 90% loss in energy as it flows from one trophic level to another. Notice
that the producers start with 10,000 units of available energy, but due to energy loss only
10 units get to the tertiary consumers. This is one reason why most ecosystems have
lower populations of high-level consumers compared to low-level consumers.
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Figure 8. The energy pyramid.
In nature, the efficiency of energy flows from one trophic level to the next varies from
5% to 20% (that is, we expect a 95% to 80% loss of energy). Figure 9 depicts energy
flows for another system where top carnivores receive only about 21 units of energy from
a total input of 1,700,000!
Figure 9. The energy pyramid with numbers
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7. Questions for thought
1) Consider Figure 6 above. Assume that the producers capture 1000 units of energy from the
sun. Given 15% efficiency at each trophic level, how many units of energy are available to
support each trophic level?
2) For problem 1 above, plot your results with energy units on the Y-axis and trophic levels on
the X-axis. What kind of relationship do you see? Can you develop an equation that expresses
this relationship mathematically?
3) Why can the total amount of animal flesh on Earth never exceed the total amount of plant
flesh, even if all animals were vegetarians?
4) The figure below depicts a herbivore (the Australian box rabbit!) consuming a plant's energy
stored in its biomass.2 All units are in kcal. Of the 1000 kcal available to the herbivore, only
200 kcal are ingested (herbivores tend to be wasteful eaters). Of this 200 kcal, 130 kcal are
excreted as waste and 70 kcal are assimilated into the herbivore. Of that 70 kcal, 56 kcal are
released during respiration. The remaining 14 kcal are used for growth, and hence are
available for the next trophic level.
Given these numbers, determine the assimilation efficiency, the production efficiency, and
the production to ingestion ratio.
14
56
70
130
200
800
1000
2
Data from Smith and Smith (2001).
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