3.  An ecosystem is a definable area containing a
relatively self-sustained community of organisms
interacting with their non-living surroundings.
 A Habitat is the area in which an individual lives.
 An ecological niche consists of a community of
interacting populations in an ecosystem. Each
population is a collection of individuals belonging
to the same species that play a particular role in
the community. This role of the population called
the ecological niche.

4.  In an ecosystem energy flow is all ways in one
direction. This one-way flow of energy can be
represented in several ways including food chains,
food webs, and pyramids of numbers, biomass,
and energy.
 A food chain is single sequence of energy flow
usually starting with a producer and ending with a
top consumer, in which each organism is the food
of the next one in the chain.
 A food web shows the interconnection of food
chains in an ecosystem

8.  Ecological efficiency describes the efficiency
with which energy is transferred from one
trophic level to the next. It is determined by a
combination of efficiencies relating to
organismic resource acquisition and
assimilation in an ecosystem

9.  Energy transfer between trophic levels is generally
inefficient, such that net production at one trophic
level is generally only 10% of the net production at
the preceding trophic level. This means that 90% of
the energy available at one trophic level never
transfers to the next. Due to non-predatory
death, egestion, and respiration, a significant
amount of energy is lost to the environment
instead of being absorbed for production by
consumers, as depicted in the figure below. The
figure approximates the fraction of energy
available after each stage of energy loss in a typical
ecosystem, although these fractions vary greatly
from ecosystem to ecosystem and from trophic
level to trophic level.

10.  The loss of energy by a factor of 1/2 from each
of the steps of non-predatory
death, defecation, and respiration is typical of
many living systems. Thus, the net production
at one trophic level is 1 / 2 * 1 / 2 * 1 / 2 = 1 / 8
or approximately 10% that of the trophic level
before it.

11.  Example: Assume 500 units of energy are
produced by trophic level 1. One half of that is lost
to non-predatory death, while the other half (250
units) is ingested by trophic level 2. One half of the
amount ingested is expelled through
defecation, leaving the other half (125 units) to be
assimilated by the organism. Finally one half of the
remaining energy is lost through respiration while
the rest (63 units) is used for growth and
reproduction. This energy expended for growth
and reproduction constitutes to the net production
of trophic level 1, which is equal to 500 * 1 / 2 * 1 /
2 * 1 / 2 = 63 units

15.  Ecological efficiency is a combination of several
related efficiences that describe resource
utilization and the extent to which resources
are converted into biomass.

16.  Exploitation efficiency is the amount of food ingested
divided by the amount of prey production (I / Pn)
 Assimilation efficiency is the amount of assimilation
divided by the amount of food ingestion (A / I)
 Net Production efficiency is the amount of consumer
production divided by the amount of assimilation (Pn +
1 / A)
 Gross Production efficiency is the assimilation
efficiency multiplied by the net production
efficiency, which is equivalent to the amount of
consumer production divided by amount of ingestion
(Pn + 1 / I)

17.  Ecological efficiency is the exploitation
efficiency multiplied by the assimilation
efficiency multiplied by the net production
efficiency, which is equivalent to the amount of
consumer production divided by the amount of
prey production (Pn + 1 / Pn)

18.  Theoretically, it is easy to calculate ecological efficiency
using the mathematical relationships above. It is often
difficult, however, to obtain accurate measurements of the
values involved in the calculation. Assessing ingestion, for
example, requires knowledge of the gross amount of food
consumed in an ecosystem as well as its caloric content.
Such a measurement is rarely better than an educated
estimate, particularly with relation to ecosystems that are
largely inaccessible to ecologists and tools of measurement.
The ecological efficiency of an ecosystem is as a result often
no better than an approximation. On the other hand, an
approximation may be enough for most ecosystems, where
it is important not to get an exact measure of efficiency, but
rather a general idea of how energy is moving through its
trophic levels.

20. Consumption
efficiency =
200/1000
Assimilation
efficiency
70/200
Production
efficiency =
14/70
Amt produced by
trophic level n-1
Amt ingested by
trophic level n
Amt egested as
feces (waste) by
trophic level n
Amt assimilated (i.e.
absorbed into body)
by trophic level n
Amt respired by
trophic level n
Secondary production
by trophic level n
Efficiency of
energy transfer

21. Energy loss between trophic levels

22. Efficiency of
production

23.  traces energy flow through ecosystems
 • Primary Production --
 Gross Primary Production = gpp: amount of light energy
 converted to chemical energy by PS per unit time in a given area.
 Net Primary Production = npp: gpp minus amount energy
used by primary produces for respiration.
 • J/m 2/ yr or as weight g/m 2 /yr = biomass
 • NOT TOTAL BIOMASS but new BIOMASS added
 to ecosystem in given amount of time
 • Measured as dry weight of organic material
 • Standing crop - total biomass of photosynthetic autotrophs at given time

25.  A pyramids of numbers is a graphical method
of showing the quantitative relationships
between organisms at each trophic level.
 While a pyramid of biomass shows the dry
mass at each trophic level.
 A pyramid of energy shows the energy
entering each trophic levelin an ecosystem over
one year.

26.  A pyramid of biomass is a representation of
the amount of energy contained in biomass, at
different trophic levels for a given point in
time. The amount of energy available to one
trophic level is limited by the amount stored by
the level below. Because energy is lost in the
transfer from one level to the next, there is
successively less total energy as you move up
trophic levels. In general, we would expect that
higher trophic levels would have less total
biomass than those below, because less energy
is available to them.

27.  We could also construct a pyramid of
numbers, which as its name implies represents
the number of organisms in each trophic level.
For the oceans, the bottom level would be quite
large, due to the enormous number of small
algae. For other ecosystems, the pyramid of
numbers might be inverted: for instance, if a
forest's plant community was composed of
only a handful of very large trees, and yet there
were many millions of insect grazers which ate
the plant material.

28. Just as with the inverted pyramid of numbers, in some
rare exceptions, there could be an inverted pyramid of
biomass, where the biomass of the lower trophic level is
less than the biomass of the next higher trophic level.
The oceans are such an exception because at any point
in time the total amount of biomass in microscopic algae
is small. Thus a pyramid of biomass for the oceans can
appear inverted (see Figure 4b). You should now ask
"how can that be?" If the amount of energy in biomass
at one level sets the limit of energy in biomass at the
next level, as was the case with the hares and foxes,
how can you have less energy at the lower trophic level?
This is a good question, and can be answered by
considering, as we discussed in the last lecture, the all
important aspect of "time". Even though the biomass
may be small, the RATE at which new biomass is
produced may be very large. Thus over time it is the
amount of new biomass that is produced, from whatever
the standing stock of biomass might be, that is
important for the next trophic level.

29.  We can examine this further by constructing a
pyramid of energy, which shows rates of
production rather than standing crop. Once
done, the figure for the ocean would have the
characteristic pyramid shape (see Figure 4c).
Algal populations can double in a few
days, whereas the zooplankton that feed on
them reproduce more slowly and might double
in numbers in a few months, and the fish
feeding on zooplankton might only reproduce
once a year. Thus, a pyramid of energy takes
into account the turnover rate of the
organisms, and can never be inverted.

31.  Standing crop biomass – amount of
accumulated organic matter found in an area at a
given time [g/m2]
 Productivity – rate at which organic matter is
created by photosynthesis [g/m2/yr]
 Primary productivity – autotrophs
 Secondary - heterotrophs
 Gross versus net primary productivity