Ecosystems Topic 4: Trophic Levels - Wikispaces

Topic 4: Trophic Levels
Part of the Local Ecosystems Module
Spotlight Biology Preliminary Text Chapter 4
Authors: D. Heffernan, J. Bastina, B. Grieve, K.
Humphreys, A. Sartor
Science Press 2002
Trophic Levels
Understanding the movement of energy through living systems
is essential to understanding ecology. Beginning with the
capture of sunlight by primary producers, energy passes
through food chains and webs from producer to consumer.
As it flows, energy is alternately stored and used to allow life
processes of all organisms to continue.
Trophic Levels
The biomass of an ecosystem is the mass of living material in an
area. This can be expressed in units of either energy (−2 )
or dry organic matter (−2 )
Trophic Levels
A better measure of the energy produced in an ecosystem is the rate at
which new biomass is produced per unit area. This is known as the net
primary productivity of a community and is measured in wither unites
of energy or mass of dry organic matter per unit area per unit of time.
Primary Productivity (−2 −)
Biomass (−2 )
Tropical Rainforest
Open Forest
Woodland and shrubland
Temperate grassland
Open Ocean
Continental Shelf
Lake and stream
Trophic Levels
The energy captured by producers and consumers is
temporarily stored until one organism eats another. Each of
these storage steps along a food chain or web is called a
trophic level. Primary producers represent the first trophic
level, herbivores occupy the second and carnivores the third.
Producers and
Autotrophs and
Herbivores and
Primary Producer
Primary consumer
Secondary consumer
Carnivore 1
Tertiary consumer
Carnivore 2
Trophic Levels
In theory there is no
limit to the number of
trophic levels but in
reality there is practical
limitations. Every time
one organism eats
another, only a small
fraction of the energy
present in the lower
trophic level is stored
and used in the next
higher level.
The percentage of energy transferred from one trophic level to
the next is known as the ecological efficiency. The values
are typically low, often between 5% and 20%.
This means that not all of the net primary productivity of a
trophic level is consumed by the next, and that not all of the
energy consumed is converted into body mass. A lot is used
during metabolism. Metabolism refers to the chemical
processes that go on continuously inside the body to allow life
and normal functioning.
On average only about 10% of the energy fixed by plants is
ultimately stored by herbivores. Only 10% of the energy
herbivores accumulate ends up being stored in the living tissues
of the carnivores that eat them. And only about 10% of that
energy is successfully converted into living tissue by carnivores
in the third trophic level.
Food Pyramids
The fact that only 10% is used at each trophic level is important
because the less energy available at any trophic level, the less
living tissue that trophic levels can support.
Food Pyramids
Inefficient energy chains thus
create what we call ecological
pyramids. In an ecological
energy period, each trophic
level contains only 1/10 as much
energy in living tissue as the layer
beneath it. If biomass is used to
construct the pyramid, it is called
an ecological biomass
Food Pyramids
This relationship also
produces a pyramid of
numbers, which means
that each successive
trophic level also
contains fewer individual
organisms than the one
below it.
Food Pyramids
In some cases the consumers are much smaller than the organism
the consume so the pyramid numbers are turned upside down. For
example, countless numbers of mosquitoes may feed off a one or a
few animals. The pyramid biomass still applies to this situation
Why Big Animals Eat Plants
The ecological rules we’ve just talked
about have dramatic social and ecological
consequences. For example, if you
choose to only eat red meat to gain
1 kilogram of weight, you need to
eat 10 kilograms of beef. The cow
that produced the 10 kilograms of
flesh must have originally eaten at least
10 times that weight (100kg) of grass or feed.
For us to obtain one unit of energy from beef required the storage of at
least 100 units of energy in cattle feed. (this doesn’t include the energy
spent to process, store, transport and sell the meat)
Why Big Animals Eat Plants
That’s why it make good
ecological sense that the
largest land animals are
vegetarians and that the
largest marine animals are
plankton feeders. Such
large animals need huge
quantities of food to build
their living tissue.
Why Big Animals Eat Plants
If they were to feed on large animals, those already on the 4th or
5th trophic levels, the total amount of primary production needed
to support them would be enormous! A given ecosystem could
support very few if any of them. By feeding at or near the base of
the ecological pyramid, whales and elephants make much more
efficient use of energy so that a given ecosystem can support many
more of them.
Students to complete the mid-topic quiz

similar documents