Diploma of Environmental Monitoring & Technology
Study module 3
Energy in ecosystems
MSS024003A
Environmental
principles (Ecology)
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Ecological Principles (Ecology)
Study module 3 – Energy in ecosystems
INTRODUCTION TO ECOSYSTEMS
2
THE THERMODYNAMICS OF ECOSYSTEMS
2
Sources of Energy in Ecosystems
PRIMARY PRODUCTION
Factors affecting primary productivity
3
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5
SECONDARY PRODUCTION
5
DECOMPOSITION (THE DETRITUS CYCLE)
7
FOOD CHAINS, WEBS & PYRAMIDS
8
Food Chains
Food Webs
Food Pyramids
ASSESSMENT & SUBMISSION
Knowledge questions
Assessor feedback
Assessment & submission rules
References & resources
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Study module 3 – Energy in ecosystems
Introduction to ecosystems
When we look at any natural environment, what we see is a collection (termed an
assemblage) of organised levels of structure. This may take the romance out of nature, but
this is what we see. The structures referred to here are the following;
◗
Individuals
◗
Populations
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Communities
◗
Ecosystems
An individual refers to one organism of one species. An individual organism is defined by the
genetic makeup of that individual, and the species, and is a topic that is best dealt with by
biologists. We shall only refer to individuals in these notes when describing consequences of
various changes to an environment. We shall therefore leave the individuals alone.
Populations are groups of individuals of a species. This is dealt with as a separate topic in
another chapter.
Communities are an assemblage of populations. This is dealt with in another chapter.
An ecosystem is defined as the highest level of organisation in an environment. The
organisation refers to the organisation of all the individuals of a species, in all of the
populations, in all of the communities in an environment, and how they interact with the
physical environment (abiotic factors). The ecosystem concept is of critical concern to the
environmental scientist and technicians alike because it is the ecosystem as a whole that is
affected by pollution and it is the separate parts of an ecosystem that are monitored by the
environmental field technician. Figure 3.1 depicts the major components and general flows
and cycles within a common ecosystem.
The Thermodynamics of Ecosystems
Before we delve into the topic of ecosystems in detail, it is important that you be introduced
to the concept of thermodynamics. Now, although the topic of thermodynamics is not
generally student friendly, we only need to understand the basic concepts – nothing more.
So here they are;
One ‘proper’ definition of thermodynamics could be;
“the science of the relationships between the different forms of energy and the
conversions between these different forms”
It is often easier for ecology students to view thermodynamics as;
“how energy and matter is captured and moved around the environment”
This is much more user friendly definition for us.
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Study module 3 – Energy in ecosystems
The first law of thermodynamics is also called the law of conservation which states that
energy cannot be created nor destroyed, but rather simply changes form. As applied to
ecology this means that the change in an ecosystem’s form of energy is related to the
ecosystems heat (by the growing and dying of organisms) and work (also by the growing and
dying of organisms), so that overall, as an ecosystem grows and dies, the form of energy in
the ecosystem changes. The same principle applies to matter, which is constant on Earth,
and can only change form, such as a plant growing and dying).
Of more importance to you is the second law of thermodynamics which states that in any
energy transfer, some energy is lost as heat. This is enormously important (and easy to
understand) when we examine the various food chains later on.
Figure 3.1 – Generalised depiction of an ecosystem. Energy (dashed lines) flows through an
ecosystem whereas matter (solid lines) is cycled around an ecosystem. The energy flow and
loss follows the Second Law of Thermodynamics. Matter is (universally) constant and follows
the principles of the First Law of Thermodynamics. (Krebs 2001)
Sources of Energy in Ecosystems
One way by which scientists can look at ecosystems is to start with the basic functions and
work their way up from there. This thinking process would lead to an inevitable question: if
an organism requires energy to live, what energy is available for organism to use?
In answering this question, scientists discovered three main sources of energy in the
environment for organisms to use;
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Study module 3 – Energy in ecosystems
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Sunlight
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Organic chemicals (e.g. wood like carbohydrates and meat like proteins etc)
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Inorganic chemicals (e.g. carbon dioxide, CO2, oxygen gas O2 and water H2O)
Evolution resulted in two basic classes of organism, heterotrophs and autotrophs, each of
which capture and use the above sources of energy in completely different ways. The Greek
term trophic literally means food, yet the meaning can encompass the terms energy or
nutrient, as both energy and nutrients are derived from food.Heterotrophs use organic
chemicals (or other organisms) as sources of energy. It is believed that the earliest life forms
on Earth were all heterotrophic microbes (single celled organisms).
Autotrophs are organisms that obtain their energy from sunlight and/or inorganic
chemicals, of which there are two main types; photosynthetic autotrophs, which use both
sunlight and inorganic chemicals, and chemo-autotrophs, which only use inorganic
chemicals (such as iron). Photosynthetic autotrophs play an enormous role in today’s
ecology (as opposed to 3.5 billion years ago), as they are the species that produce the first
trophic layer in the food chain – biomass (plant material), from which most other life forms
gain there energy (as heterotrophs). For this reason, they are termed primary producers.
Primary Production
Apart from being one of the many great wonders of biochemistry, the capture and use of
sunlight and inorganic chemicals by autotrophic organisms is without doubt the most
fundamental of life’s energy related processes. Autotrophs use two processes;
◗
Photosynthesis – which is the ‘capturing’ stage of the autotrophic process, and
◗
Respiration – this is the ‘using’ stage of the autotrophic process.
The result of these two processes (both of which are chemical reactions) is the creation of
high energy complex sugars called carbohydrates (note that the rate at which the energy is
captured is called primary productivity, whereas the mass of carbohydrate produced is
termed biomass). In the second process, respiration, the carbohydrate is ‘burnt’ by the
organism, which releases energy required by the organism to perform life functions.
Photosynthesis is expressed by the following chemical reaction;
carbon dioxide + water + sunlight >>> biomass + oxygen
6CO2
 6 H 2O
energy
 C6 H12O2
 6O2
Respiration is effectively the reverse of photosynthesis, and is expressed as;
biomass + oxygen >>> carbon dioxide + water + energy
C6 H12O6
 6O2

 6CO2
 6 H 2O  energy
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Study module 3 – Energy in ecosystems
The overall process is termed ecosystem metabolism, which is the chemical reactions that
occur in living organisms that maintain life and allow all the life functions to be performed.
Metabolism is usually divided into two categories.
Catabolism breaks down large molecules, for example, the burning of carbohydrates in
order to harvest the stored chemical energy, with respiration being the obvious example.
Anabolism, on the other hand, uses energy to construct components of cells such as
proteins and nucleic acids. Photosynthesis is a close analogue of this process.
A special situation exists in ecosystem’s metabolism when the rate of photosynthesis is
equal to the rate of respiration: equilibrium. We use the term compensation point when
this equilibrium is achieved. The metabolic equilibrium of primary productivity involves two
parts, gross primary productivity, which is the amount of energy or biomass fixed by
photosynthesis, and net primary productivity, which is the gross primary productivity minus
the amount of respiration that has occurred.
Factors affecting primary productivity
The key question at this point regarding ecosystem metabolism is: “How does primary
productivity vary over the Earth, and between the various types of vegetation?” It turns out
of the two major ecosystem types (terrestrial and marine) have very similar primary
productivities of approximately 46% and 54% net primary productivity respectively. Tropical
rainforests and savannahs are the major contributors to terrestrial productivity, yet, there is
a productivity gradient that decreases toward the poles (phenomena observed with many
abiotic factors). In all ecosystems, there are two key factors that affect the primary
productivity;
◗
Temperature – cold climates are less productive than warm climates
◗
Nutrient availability – different ecosystems have different nutrient ratios
Secondary Production
Energy fixed by green plants (that is not lost to respiration), flows either to herbivores or to
detritus feeders, and from herbivores to carnivores, and then to other carnivores (until it all
ends up as detritus). In forest ecosystems, most primary production goes to detritus, and
only 4% goes to herbivores. Compared to about 20% in aquatic environments.
Secondary production is defined as the rate at which consumers accumulate their biomass.
Generally there are two types of consumer;
◗
Primary consumers – which are all herbivores
◗
Secondary consumers – which are carnivores and omnivores
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Study module 3 – Energy in ecosystems
There are higher order consumers, it just depends on the ecosystem in questions as to how
many higher order consumers there are, yet there will always be a top carnivore (other than
humans). Secondary productivity is the sum of all consumers – primary and secondary.
Secondary production is more complex than primary production, as, even though secondary
production it the sum of the two parts, it is inherently more dynamic as there are different
mechanisms of energy assimilation and use used by the various types of herbivores and
carnivores.
Figure 3.2 – Flowchart of ecological production and consumption. Secondary production is
the sum of all consumption in an ecosystem.
The rate of biomass accumulation in secondary productivity is governed by the efficiency of
the accumulation, and can be described using the Lindeman efficiency classification (which
also apply to primary productivity), but in secondary productivity there are three;
Ingestion efficiency - this is the proportion of available energy eaten by a heterotroph. For
example, not all grasshoppers are eaten by the birds, therefore proportionality applies here.
Assimilation efficiency - once a proportion of available food has been ingested, only some is
digested and absorbed, while the rest is excreted as wastes. Carnivores have higher
assimilation of energy than herbivores, which have poor digestion due to the poor quality of
the food they eat.
Production efficiency – this is simply the proportion of the assimilated energy that can
eventually be stored by an organism and used when necessary, i.e. the biomass
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Study module 3 – Energy in ecosystems
Putting the concept of secondary production in perspective we can use ourselves as an
example. We don’t eat everything on offer (like dolphins), nor do we eat everything that is
on offer (like tomatoes), therefore ingestion efficiency applies. We don’t digest everything
we eat, evidenced by the waste we produce, nor do we use all the energy we get. You could
predict that the laws of thermodynamics come into play here as well, because secondary
production is limited by both primary production and the second law of thermodynamics.
Decomposition (the detritus cycle)
During the life of an organism (plant or animal), a variety of situations occur that generate
non-living organic (and some inorganic) material to be deposited into the ecosystem. This
material is called detritus, examples include;
◗
Cast-off (leaves or moulted exoskeletons)
◗
Excreta (such as faeces)
◗
Food scraps
◗
Dead organisms
All of these different types of materials gradually get broken down into their most basic
forms (organic materials to carbon dioxide or methane, inorganic materials to the various
metals and non-metals) due to both physical processes (such as ultraviolet sunlight – similar
to the fading of clothing) and the action of decomposers, such as certain mollusca and
insects (termed detrivores), as well as bacteria and fungi, in a process called the detritus
cycle.
Decomposition is the process through which organic (and inorganic) matter is decomposed.
Materials like proteins, lipids and sugars are rapidly decomposed and assimilated by microorganisms and organisms that feed on dead matter. Larger and more complex (stable)
organic materials (such as tannins) are broken down more slowly. The insects, bacteria and
fungi use the detritus materials as food, which they obviously need for their own survival,
and the breaking down of the material is simply a part of that process.
Terrestrial ecosystems find the detritus deposited on the surface of the ground, taking
forms such as the humic soil beneath a layer of fallen leaves. In aquatic ecosystems, most
detritus is suspended in water and much of the material settles still areas of water which
gradually settles becoming part of the sediment.
What is left behind by the detrivores is then further broken down and recycled by
decomposers, such as bacteria and fungi. The entire process extracts all of the nutrients out
of the detritus and returns it back to the rest of the ecosystem, making the nutrients
available for primary and secondary production until the whole process starts again.
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Food Chains, Webs & Pyramids
We are always considerate of the energy (as trophic structures or layers) in an ecosystem
for many reasons. It is the availability of energy that determines the length and complexity
of any ecosystem. Ecologists are always eager to both determine and explain the energy
flow in a system, and food chains, webs and pyramids are one of many ways to describe the
trophic structure in an ecosystem.
Food Chains
A food chain is the most simple of the three common trophic descriptors, and is probably a
concept quite familiar to you already. With primary and secondary productivity explained,
we need to look at how energy flows through the system from an efficiency point of view
once more. When we follow the path of energy from the initial rays of sunlight right through
to the top carnivore, we find that the rule of thumb known as the 1%/10% rule applies.
This rule states, firstly, that of all the sunlight that falls on the Earth, only 1% is used by
primary producers. Secondly, only 10% of the primary biomass is used by primary
consumers, and only 10% of that energy is used by secondary consumers etcetera. The
trend in energy reduction is linear (in the sense that it is not cyclic) between the trophic
layers, yet approaches a somewhat logarithmic or exponential decay pattern in the loss
between each trophic layer
Figure 3.3 – Hypothetical example of a food chain showing the approximate percentage of
energy that is transferred between each link in the chain. Food chains depict the shortest
chain in an ecosystem with the average length of a food chain being about 4 or 5 trophic
levels long.
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Study module 3 – Energy in ecosystems
Overall, the transfer of energy through the environment is not very efficient – which is in
perfect accordance with the Second Law. Efficiency is related to many other factors and is
very much dependant upon the type of ecosystem in question.
Food chains (which were first described by Elton in 1927) are often depicted graphically, as
seen in the Figure 3.3 above, which is a simple food chain for grassland. Food chains are
used to depict the depth (or length) of the ecosystems trophic structure (layers) and are
usually quite short with maximum chain-lengths of between 5 and 8. The food chain stops
when there is insufficient energy to maintain another trophic layer, which may be brought
about by two possible mechanisms; the initial quality of primary productivity, or by the
effectiveness of the Lindeman efficiencies in the secondary productivity stage of the
ecosystem, at this stage though, ecologist are not certain how the length of a food chain is
controlled.
Although this is very simple, it is not difficult to relate the concepts of energy transfer along
the chain. Unfortunately it is too simple, as the chain concept ignores many of the
important complexities associated with ecosystems and we need to find another way to
describe the transfer of energy through an ecosystem – the food web.
Food Webs
Food webs are exactly the same as food chains, but are more representative of what is
actually happening in the ecosystem. As mentioned, food chains are too simple, and neglect
important factors such as an organism’s competitors, or other food sources. A food web
allows us to incorporate such information, and can best be viewed as many individual food
chains strung together.
Figure 3.4 – Example of possible food web interactions in Australia. Figure modified from
http://openlearn.open.ac.uk/file.php/1656/formats/SK220_2_rss.xml (accessed 2/3/08)
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Food webs provide a very good tool by which ecological and environmental management
decisions can be made, especially when it comes to preventing extinction. if the food web is
comprehensive, then we can predict what will happen if we remove one or more species. An
example of a food web is seen in Figure 3.4 above;
Food webs are derived from food web theory, which is simply the set of rules and associated
language for constructing and interpreting food webs. It is a complex art that is best learnt
by a worked example, and therefore will not be included in the text of these notes, but a
comprehensive assessment for food web construction can be found in Appendix A.
Food Pyramids
The final way by which the trophic structure of ecosystems can be portrayed is by use of
Eltonian pyramids, and is perhaps the most useful, primarily because it can be made a
quantitative method (which means that we can attach numbers to it).
Figure 3.5 – Eltonian food pyramids for a forest and a grassland ecosystem, showing both
numbers of organisms per trophic layer (all organisms for that layer) in the top row, and
energy of the trophic layers in the bottom row.
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The quantitative nature of pyramids means that we can construct them using several types
of ecological data. The three types of food pyramids that are commonly encountered are;
◗
Numbers
◗
Energy
◗
Biomass
Each type has its own uses, pros and cons, of which the main points are discussed below.
The numbers pyramids involve counting the relative abundance for each functional trophic
layer of the community, for example, counting the numbers of photosynthetic species,
herbivores and carnivores. Obviously there would be some ecosystems in which it would be
easy to do this, and there are some that are practically impossible. Using numbers of
organisms can provide a unique insight into the trophic structure though, as seen in figure
3.5 above.
Note that the forest pyramid has a smaller primary production layer than does the grassland
pyramid for numbers. The reason for this is simple, there are fewer trees in a forest than
there are grasses in grassland, yet the equivalent energy and biomass pyramids for a forest
would look ‘normal’.
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References & resources
Resources
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http://www.physicalgeography.net/fundamentals/chapter6.html
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http://www.barrameda.com.ar/ecology/the-ecosystem.htm
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http://www.springerlink.com/content/101552/
References
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Study module 3 – Energy in ecosystems
Note that some of these resources might be available from your teacher or library
Krohne.D.T. 2001. General Ecology 2nd Ed. Brooks Cole Publishing. Pacific Grove CA USA.
Manahan, S.E. 1999. Environmental Chemistry 7th Ed. CRC Press LLC. Boca Raton. USA.
Sturman. A. Tapper. N. The Weather & Climate of Australia & New Zealand. Oxford
Publishing, Melbourne, Australia.
Carlton, C. Chalson J. 2002. Plant Survey Methods (Comprehensive). NPWS (National Parks
Association of NSW inc. Canberra. Australia.
Krebs, C.J. 2001. Ecology: The Experimental Analysis of Distribution and Abundance 5th Ed.
Benjamin Cummings Publishing. San Francisco. USA.
Lavelle, P., Spain, A,V. 2001. Soil Ecology. Kluwer Academic Publishers. Boston. USA.
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