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August 1, 2002
A: FROM MENTOR Amy McMillan. To read bio. click
here.
Hi Kyndra. A food chain is a sequence of organisms within
a community
that "fill" successive trophic levels. Energy is
transferred through
this sequence by feeding and energy enters the "chain"
by primary
producers (the bottom of the food chain). Primary producers
are
generally green plants. This first energy is transferred to
herbivores
(primary consumers) then to carnivores (secondary and tertiary
consumers). A food web, on the other hand, is a network of
interconnected food chains within a given community.
The primary factor that limits a food chain is energy. The
primary
producers produce a given amount of energy. Primary consumers
can only
integrate 10% of this photo energy into chemical energy (useful
energy
for growth etc) - the other 90% is lost as waste heat (so,
in other
words, making food into useful stuff for the body is only
10%
efficient!). This loss of energy happens all the way up the
food chain;
secondary consumers only get 10% of the primary consumer energy,
etc.
The amount of energy in the food chain depends on the primary
consumer
base- there is a finite amount of energy available in any
given chain!
Lakes (and oceans)tend to have much longer food chains than
do
terrestrial systems because the primary producers (algae)
in lakes and
oceans have very short life spans and very large populations
so can
convert more of the available energy (sun) into energy for
the food
chain.
August 1, 2002
A: FROM MENTOR MARTY CHINTALA. To read bio. click
here.
Kyndra,
That is a good question. Ricklef's Ecology text defines a
food chain
as a representation of the passage of energy through populations
in the
community. A food web is defined as a representation of the
various
paths of energy flow through populations in the community.
Basically, a
food chain in a single line of who eats who or who chemically
processes
what. An example would be that inorganic nitrogen is processed
by
bacteria in the marine sediment, the bacteria in the marine
sediment is
eaten by a benthic invertebrate, the benthic invertebrate
is eaten by a
juvenile flounder, the juvenile flounder is eaten by a Crangon
shrimp,
and the Crangon shrimp is then eaten by a larger predatory
fish, and a
human then catches the large fish and eats it for dinner.
A food web is
like a collection of food chains to define the different ways
that
things can be eaten and processed within a community and ecosystem.
Food webs are much more complex and can include lots of arrows
and
pathways. Many organisms are part of more than one food chain
and eat a
few different things in order to satisfy their energy requirements.
Check out these webpages that talk about food chains and webs:
http://education.leeds.ac.uk/~edu/technology/epb97/forest/azfoodcw.htm
and http://www.arcytech.org/java/population/facts_foodchain.html.
One of the main factors limiting the number of levels in a
food chain is
the efficiency of energy transfer between the different trophic
levels.
Ecological efficiencies can be as low as 5 to 17% between
the producer
and herbivore levels, meaning that these levels do not contain
enough
energy to fully support a higher level predator population.
There
needs to be enough food for the animals at the highest level
to survive.
This helps to maintain the balance between the populations
at the
different levels, because if there is not enough food at a
lower level,
then there will be less animals at the higher level. If there
are less
animals at the higher level, then the animals (or plants)
at the lower
level will survive, providing more food, so that the number
of animals
at the higher level can then increase again. I hope this has
helped
clarify this for you.
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