Important words and concepts
from Chapter 54, Campbell & Reece, 2002 (3/25/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
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(1) Chapter title: Ecosystems
(a)
[ecosystem ecology (Google Search)]
[index]
FLOW OF ENERGY THROUGH ECOSYSTEMS
(a)
"An ecosystem consists of all the organisms living in a community
as well as all the abiotic factors with which they interact."
(b)
Note that the boundaries of ecosystems are typically not arbitrarily
defined, but instead are defined in some meaningful way: A pond, a field, a
forest, etc.
(c)
Ecosystems are typically understood in terms of
(i)
Energy flow through ecosystems
(ii)
Chemical cycling within (and through)
ecosystems
(d)
Note that both involve the movement of "stuff" through both
biotic and abiotic components of the ecosystem
(e)
(f)
“Ecosystems ecologists view ecosystems as energy machines and
matter processors. By grouping the species in a community into
trophic levels of feeding relationships, we can
follow the transformation of energy in the whole ecosystem and map the
movements of chemical elements as they are used by the biotic community.” (p.
1199, Campbell & Reece, 2002)
(g)
[ecosystems, ecosytem (Google Search)]
[index]
(a)
Energy does not cycle through ecosystems but instead enters ecosystems
and is used up within ecosystems
(b)
Ultimately energy is lost from ecosystems primarily as waste heat, the
most thermodynamically unavailable form of energy
(c)
"Energy enters most ecosystems in the form of sunlight. It is then
converted to chemical energy by autotrophic organisms, passed to heterotrophs
in the organic compounds of food, and dissipated in the form of heat . . . The
movements of energy and matter through ecosystems are related because both
occur by the transfer of substances through feeding relationships. However,
because energy, unlike matter, cannot be recycled, an ecosystem must be powered
by a continuous influx of new energy from an external source (the sun). Thus,
energy flows through ecosystems, while matter cycles within them."
(d)
Note that energy flows through ecosystems mostly as bonds between
carbon atoms and bonds between carbon and hydrogen atoms, e.g., as one finds in
carbohydrates and lipids; consequently, within and between organisms the
carbon cycle and the flow of energy are quite similar, at least
until the two are decoupled in the course of cellular respiration (i.e., the
separation of carbon atoms from their energy)
(e)
(f)
Note: See Figure 54.1, An overview of ecosystem dynamics as an
overview of trophic livels within ecosystems
(g)
[ecosystem "energy
flow" (Google Search)] [energy flow in ecosystems
(nice overview of subject) (Geography 210—Okanagan University College—Michael Pidwirny)] [environmental biology—ecoystems (lots of
nice, if crude, flowcharts of energy flow and nutrient cycling) (Dave McShaffrey—Marietta College)] [Life on Earth - Flow of Energy
and Entropy (Digital Recordings—Marek
Roland-Mieszkowski)] [index]
PRODUCTIVITY
(a)
Only a small fraction of the sunlight striking the earth is converted
to chemical energy by primary producers
(b)
That sunlight energy that is converted to chemical energy, over a given
period, is termed primary productivity
(c)
[primary productivity
(Google Search)]
[index]
(5) Gross primary
productivity
(a)
Gross primary productivity is all of the light energy that is converted
to chemical energy by producers
(b)
[gross primary productivity
(Google Search)]
[index]
(a)
Net primary productivity is all of the light energy that is converted
to chemical energy and that is subsequently stored by the primary producer
(i.e., the gross primary productivity minus that employed to run the primary
producer's metabolism)
(b)
The ratio of net primary productivity to gross primary productivity
gives an indication of the cost of keeping the organism going, with large
ratios indicative of relatively few costs (e.g., algae, ~50%) and smaller
ratios associated with many costs (e.g., complex plants such as trees, ~10%)
(c)
See Figure 54.10, Energy partitioning within a link of the food chain
(d)
See Figure 54.11, An idealized pyramid of net production
(e)
[net primary productivity
(Google Search)]
[index]
(a)
Net primary productivity is stored as biomass (dry mass of organisms)
(b)
[biomass (Google Search)]
[index]
(a)
Standing crop biomass is another way of saying accumulated net primary productivity
(b)
See Figure 54.12, Pyramids of biomass (standing crop)
(c)
[standing crop biomass
(Google Search)]
[index]
(a)
The productivity of an ecosystem is dependent on the primary productivity of the primary producers
within that ecosystem
(b)
Other than sunlight, primary productivity is limited by nutrient
availability
(c)
A limiting nutrient is that nutrient which is found in the lowest,
relative concentrations such that an increase in this nutrient will increase
primary productivity while a decrease in this nutrient will decrease primary
productivity (this is equivalent to the concept of limiting reagent in
chemistry)
(d)
Typically, either phosphorus or nitrogen serves as a limiting nutrient
within a given ecosystem, though water availability can (and often does) also
serve to limit the primary productivity of an ecosystem
(e)
[limiting nutrient (Google Search)]
[index]
(a)
"The rate at which an ecosystem's consumers convert the chemical
energy of the food they eat into their own new biomass is called the secondary
productivity."
(b)
Note that secondary productivity is dependent, in part, on the efficiency
of transfer of chemical energy between trophic levels.
(c)
The transfer between trophic levels, however, is typically not highly
efficient because of inefficiencies involved in energy transfers in general,
and the fact that the consumer must use acquired energy to respire (i.e., keep
their metabolism going, reproduce, repair themselves, etc.)
(d)
The more energy required to keep the consumer going (e.g., endotherms =
“warm blooded” = more versus ectotherms = “cold blooded” = less), the less
efficiently primary productivity will be converted to secondary productivity
(e)
"Of course, the energy contained in the feces is not lost from the
ecosystem; it can still be consumed by decomposers. However, the energy used
for respiration is lost from the ecosystem; thus, while solar radiation is the
ultimate source of energy for most ecosystems, respiratory heat loss is the
ultimate sink. This is why energy is said to flow through, not cycle within,
ecosystems."
(f)
See Figure 54.10, Energy partitioning within a link of the food chain
(g)
[secondary productivity
(Google Search)]
[index]
(a)
Trophic efficiency refers to the transfer of energy up trophic levels,
e.g., the ratio of secondary productivity to primary productivity consumed
(b)
Trophic efficiencies generally range from 5% to 20%; that is, only 5%
to 20% of primary producer biomass consumed is converted into new consumer
biomass
(c)
Note that trophic inefficiencies arise note just due to the second law of thermodynamics but because of
inefficiencies in digestion (i.e., not everything is assimilated but instead is
pooped out); in addition, it is always important when looking at food pyramids
to keep in mind that not everything at the lower trophic levels is eaten, i.e.,
there is a reason that much of the terrestrial world is green, animals do not
consume all of the plant material; on the other hand, there is a reason that
many aquatic environments are not quite as green, animals do consume most of
the planktonic photosynthesizers within aquatic systems
(d)
[ecological efficiency
(Google Search)]
[index]
(a)
A common way of illustrating ecological efficiency is via pyramids of
productivity
(b)
In these, productivity consumed is compared to productivity acquired,
going up trophic levels, e.g., each level represents a drop of net productivity
of approximately 90% (95% to 80%)
(c)
Note that this is the reason that eating "lower on the food
chain" is more consistent with being a good world citizen than eating
higher on the food chain, i.e., vegetarians make a substantially smaller per
capita impact on our planet than do meat eaters
(d)
[A generalization exists among
ecologists that on average, about 10% of the energy available in one trophic
level will be passed on to the next; this is primarily due to the 3 reasons
given above. Therefore, it is also reasonable to assume that in terms of
biomass, each trophic level will weigh only about 10% of the level below it,
and 10x as much as the level above it. It also seems, however, that every time
I go to measure, test, or model this assumption I run into an inconsistency, so
take this generalization with a big grain of salt. Still, it comes in useful in
terms of human diet and feeding the world's population, consider this. If we
all ate corn, there would be enough food for 10x as many of us as compared to a
world where we all eat beef (or chicken, fish, pork, etc.). Another way of
looking at it is this. Every time you eat meat, you are taking food out of the
mouths of 9 other people, who could be fed with the plant material that was fed
to the animal you are eating. Of course, it's not quite that simple, but you
get the general idea. – Mr. Kousen’s Biology]
(e)
See Figure 54.11, An idealized pyramid of net production
(f)
[pyramid of productivity
(Google Search)]
[index]
(a)
Similar to the pyramid of productivity, pyramids can be constructed
using biomass
(b)
Again, the variable associated with the primary producer is placed on
the bottom with blocks associated with trophic levels stacked one upon the
other
(c)
Just as with pyramids of productivity, biomass pyramids can show
dramatically decreasing biomass with increasing trophic levels
(d)
However, this is not always the case and the reason for exceptions has
to do with biomass pyramids being constructed from standing-crop biomass rather
than from consumed-biomass data
(e)
Consequently, aquatic biomass pyramids can seemingly be upside down if
net primary productivity does not accumulate in the ecosystem within primary
producers (i.e., primary producers are eaten as fast as they grow/reproduce)
(f)
See Figure 54.12, Pyramids of biomass (standing crop)
(g)
[biomass pyramid (Google Search)]
[index]
(a)
Just as with productivity, total numbers of individual organisms tend
to decline as one goes up trophic levels
(b)
All else held constant, this decline is a consequence of ecological
efficiencies being less than 100%
(c)
A consequence of the pyramid of numbers is that top predator numbers
tend to be small, thus making top predators both slow to evolve (also because
they tend to be long lived and have long generation times) and relatively easy
to drive to extinction
(d)
See Figure 54.13, A pyramid
of numbers
(e)
[pyramid of numbers
(Google Search)]
[index]
CHEMICAL CYCLING
(a)
"Chemical elements such as carbon and nitrogen are cycled between
abiotic and biotic components of the ecosystem. Photosynthetic organisms
acquire these elements in inorganic form from the air, soil, and water and
assimilate them into organic molecules, some of which are consumed by animals.
The elements are returned in inorganic form to the air, soil, and water by the
metabolism of plants and animals and by other organisms, such as bacteria and
fungi, that break down organic wastes and dead organisms."
(b)
Chemical cycles may be divided into two broad categories
(i)
Those elements that have a gaseous form
(ii)
Those elements that do not have a gaseous form
(c)
We will consider particularly
(i)
The carbon cycle
(ii)
The nitrogen cycle
(iii)
The phosphorus cycle
(d)
See Figure 54.15, A general model of nutrient cycling
(e)
See Figure 54.16, The water cycle
(f)
See Figure 54.20, Review: Generalized scheme for biogeochemical cycles
(g)
[chemical cycling (Google Search)]
[The Hubbard Brook Site
(the web page to the experimental site described by your text)] [index]
(a)
"Because nutrient cycles involve both biotic and abiotic
components of ecosystems, they are also called biogeochemical cycles."
(b)
[The inorganic nutrients cycle
through more than the organisms… they also enter into the atmosphere, the
oceans, and even rocks. Since these chemicals cycle through both the biological
and the geological world, we call the overall cycles biogeochemical
cycles. Each chemical has its own unique cycle, but all of the cycles do have
some things in common. Reservoirs are those parts of the cycle where the
chemical is held in large quantities for long periods of time. In exchange
pools, on the other hand, the chemical is held for only a short time. The
length of time a chemical is held in an exchange pool or a reservoir is termed
its residence time. The oceans are a reservoir for water, while a cloud
is an exchange pool. Water may reside in an ocean for thousands of years, but
in a cloud for a few days at best. The biotic community includes all living
organisms. This community may serve as an exchange pool (although for some
chemicals like carbon, bound in a sequoia for a thousand years, it may seem
more like a reservoir), and also serve to move chemicals from one stage of the
cycle to another. For instance, the trees of the tropical rain forest bring
water up from the forest floor to be evaporated into the atmosphere… The energy
for most of the transportation of chemicals from one place to another is
provided either by the sun or by the heat released from the mantle and core of
the Earth. – Mr. Kousen’s Biology]
(c)
[biogeochemical cycling
(Google Search)]
[index]
(a)
See Figure 54.17, The carbon
cycle
(b)
The carbon cycle is an example of biogeochemical cycle
in which the element (carbon) has a gaseous form, i.e., CO2, carbon dioxide
(c)
Carbon dioxide is converted to organic forms of carbon in the Calvin cycle of primary producers
(d)
Organic carbon is converted back to carbon dioxide during respiration
(e)
Not all fixed carbon is converted back to CO2 over
medium-term time scales since some ultimately is buried as oil, coal, or
limestone (the latter is calcium carbonate)
(f)
(The only error that I immediately notice in the image above is that at
least some of the CO2 released by volcanoes must, ultimately, have a
biotic source, and certainly all of the carbon released from volcanoes can
trace itself back to the rectangle in the lower right, i.e., the Earth)
(g)
[carbon cycle (Google Search)]
[index]
(18)
The Nitrogen cycle [ammonification, nitrogen
assimilation, denitrification, nitrification, nitrogen
fixing]
(a)
See Figure 54.18, The
nitrogen cycle
(b)
The nitrogen cycle, like the carbon cycle,
involves a gaseous form, i.e., N2 or nitrogen gas
(c)
Nitrogen gas may be removed from the atmosphere, particularly by
bacteria, in a process called nitrogen fixing [which is relatively
expensive since nitrogen gas is quite stable]
(d)
Nitrogen gas may be returned to the atmosphere, again particularly by
bacteria, in a process called denitrification (a form of anaerobic
respiration);
(e)
More typically, bioavailable nitrogen is found as ammonium ion (NH4+),
nitrate ion (NO32-), and various organic,
nitrogen-containing compounds (e.g., amino acids and nucleic acids)
(f)
Nitrate and ammonium ion are converted back and forth between each
other (and nitrite, NO22-), also by various bacteria via
processes termed nitrification and ammonification
(g)
The nitrogen cycle thus involves
(i)
Nitrogen fixing, the fixing of nitrogen
from the atmosphere [typically by free-living or plant-associated
nitrogen-fixing bacteria]
(ii)
Assimilation, the uptake of ammonium ion
and nitrate ion from soil by plants and the uptake of organic nitrogen by
animals from plants (amino acids, nucleic acids)
(iii)
Ammonification, the conversion of organic
nitrogen back to ammonium ion by decomposers (nitrogenous waste) (4NH4+); “The
decomposition of organic nitrogen back to ammonium, a process called
ammonification, is carried out mainly by bacterial and fungal decomposers.” (p.
1211, Campbell & Reece, 2002)
(iv)
Nitrification, the various conversions of
nitrogen within the soil from ammonium ion (NH4+ 4NO22- 4NO32- ; note that this
represents an oxidation of nitrogen); “Although plants can use ammonium
directly, most of the ammonium in soil is used by certain aerobic bacteria as
an energy source; their activity oxidizes ammonium to nitrite (NO22-)
and then to nitrate (NO32-).” (p. 1211, Campbell &
Reece, 2002)
(v)
Denitrification, also by soil bacteria (4N2; note that this process
involves the reduction of nitrogen); “Some bacteria can
obtain the oxygen they need for metabolism from nitrate (NO32-)
rather than from O2 under anaerobic conditions.” (p. 1211, Campbell
& Reece, 2002)
(h)
[A
portion of the nitrogen cycle as it occurs within fish tanks (here
nitrification is a good thing and ammonification, the production and subsequent
build up of ammonium ion, is a bad thing):
The nitrogen cycle has some
important practical considerations, as anyone who has ever set up a saltwater
fish tank has found out. It takes several weeks to set up such a tank, because
you must have sufficient numbers of nitrite and nitrate bacteria present to
detoxify the ammonia produced by the fish and decomposers in the tank.
Otherwise, the ammonia levels in the tank will build up and kill the fish. This
is usually not a problem in freshwater tanks for two reasons. One, the pH in a
freshwater tank is at a different level than in a saltwater tank. At the pH of
a freshwater tank, ammonia is not as toxic. Second, there are more
multicellular plant forms that can grow in freshwater, and these plants remove
the ammonia from the water very efficiently. It is hard to get enough plants
growing in a saltwater tank to detoxify the water in the same way. –
Mr. Kousen’s Biology]
(i)
[nitrogen cycle, ammonification, nitrogen assimilation,
denitrification, nitrification, nitrogen fixing (Google Search)]
[index]
(a)
See Figure 54.19, The
phosphorous cycle
(b)
Unlike the nitrogen and carbon cycles, the
phosphorus cycle does not involve a gaseous phase
(c)