A trophic level is a
level on the
food chain: producers
(
plants,
aglae, etc.),
primary consumers, secondary
consumers, etc.
An
example for a food chain would be:
Grass-->
Insect-->
Mouse-->
Snake-->
Hawk
Often, the real world isn't quite a simple
linear path for the food chain,
so a
food web is a more realistic diagram, showing various routes through
which energy (in the form of
food) travels to various
animals (there
can be several
organisms per trophic level,
animals can have multiple
trophic levels, etc.)
There are three main components to an ecosystem, into which all trophic
levels fit:
- producers: such as plants and aglae
- consumers:(whether herbivores, carnivores, or omnivores) such as cows, fox, humans.
- decomposers: such as bacteria
The producers are
autotrophic, while the consumers are
heteratrophic,
relying on lower levels of the food chain for preproduced
energy.
Generally, a pyramidal structure arises when the
biomass, or bulk weight
of a particular species per
kilmoter, is compared, with the higher trophic
levels (the high-number consumers, for example), making the smaller, upper
part of the
pyramid.
Generally, the number of organism follows this trend (less
individual consumers,
more
producers).
The energy, of course, starts from the
producers, which use
photosynthesis
to create the energy, sucrose, etc. used by all the other levels. Without
a supply of producers, any system would fail. The energy then flows up, as
consumers, well, consume. Of course, there is energy lost in this process
(not all the grass's energy is passed up to the insects, for example). Only
about 10% of the
energy from one level passes to the next. The other 90%
is used for
growth, lost as
heat, animal
movement,
anabolic processes,
etc.
Several factors limit the number of trophic levels (you aren't going to find
a 100th level consumer, for example). One is the lack of energy, due to the
10% rule. Much higher then 4th or 5th level and the
animal uses to much
energy catching
food to be able to simply survive. The other is the pyramidal
structure: as the
biomass decreases with each level, pretty soon you don't
have much
biomass to make a species with. Similarly, the declining number
of
individuals per level narrows down to a dangerously low number or zero
altogether. Only a limited number of
producers can survive in a given area,
which also limites the size of a
pyramid, with affects from top to bottom.
And again, high level organisms are innefficient: spending all your time
hunting for little energy is not a good
survival strategy.