FOOD CHAINS, FOOD WEBS AND ECOLOGICAL PYRAMIDS
In an ecosystem, plants capture the sun's energy and use it
to convert inorganic compounds into energy-rich organic compounds. This process
of using the sun's energy to convert minerals (such as magnesium or nitrogen)
in the soil into green leaves, or carrots, or strawberries, is called photosynthesis.
Photosynthesis is only the beginning of a chain of energy
conversions. There are many types of animals that will eat the products of the
photosynthesis process. Examples are deer eating shrub leaves, rabbits eating
carrots, or worms eating grass. When these animals eat these plant products,
food energy and organic compounds are transferred from the plants to the
animals. These animals are in turn eaten by other animals, again transferring
energy and organic compounds from one animal to another. Examples would be
lions eating zebras, foxes eating rabbits, or birds eating worms.
This chain of energy
transferring from one species to another can continue several more times, but
it eventually ends. It ends with the dead animals that are broken down and used
as food or nutrition by bacteria and fungi. As these organisms, referred to as
decomposers, feed from the dead animals, they break down the complex organic
compounds into simple nutrients. Decomposers play a very important role in this
world because they take care of breaking down (cleaning) many dead material.
There are more than 100,000 different types of decomposer organisms! These
simpler nutrients are returned to the soil and can be used again by plants. The
energy transformation chain starts all over again.
Producers: Organisms, such as plants, that produce their own
food are called autotrophs. The autotrophs, as mentioned before, convert
inorganic compounds into organic compounds. They are called producers because
all of the species of the ecosystem depend on them.
Consumers: All the organisms that can not make their own
food (and need producers) are called heterotrophs. In an ecosystem heterotrophs
are called consumers because they depend on others. They obtain food by eating
other organisms. There are different levels of consumers. Those that feed
directly from producers, i.e. organisms that eat plant or plant products are
called primary consumers. In the figure above the grasshopper is a primary
consumer.
Organisms that feed on
primary consumers are called secondary consumers. Those who feed on secondary
consumers are tertiary consumers. In the figure above the snake acts as a
secondary consumer and the hawk as a tertiary consumer. Some organisms, like
the squirrel are at different levels. When the squirrel eats acorns or fruits
(which are plant product), it is a primary consumer; however, when it eats
insects or nestling birds, it is a tertiary consumer. Consumers are also classified depending on
what they eat; they can be herbivores, carnivores, omnivores or scavengers.
In looking at the previous picture, the concept of food
chain looks very simple, but in reality it is more complex. Think about it. How
many different animals eat grass? And from the Facts about Red-tailed Hawks
page, how many different foods does the hawk eat? One doesn't find simple
independent food chains in an ecosystem, but many interdependent and complex
food chains that look more like a web and are therefore called food webs.
We described in the previous sections how energy and organic
compounds are passed from one trophic level to the next. What was not mentioned
is the efficiency of the transfer. In a highly efficient transfer almost all of
the energy would be transferred -- 80% or more. In a low efficiency transfer
very little energy would be transferred -- less than 20%. In a typical food
chain, not all animals or plants are eaten by the next trophic level. In
addition, there are portions or materials (such as beaks, shells, bones, etc.)
that are also not eaten. That is why the transfer of matter and energy from one
trophic level to the next is not an efficient one.
One way to calculate the energy transfer is by measuring or
sizing the energy at one trophic level and then at the next. Calorie is a unit
of measure used for energy. The energy transfer from one trophic level to the
next is about 10%. For example, if there are 10,000 calories at one level, only
1,000 are transferred to the next. This 10% energy and material transfer rule
can be depicted with an ecological pyramid that looks like the one below.
This pyramid helps one visualize the fact that in an
ecological system there need to be many producing organisms at the bottom of
the pyramid to be able to sustain just a couple of organisms at the top. In
looking at the pyramid, can you guess how much larger the volume of each layer
is as compared to the one just above it? Take a guess. It might not look like
it but they are close to 10 times larger.
A basic pyramid shape
often represents a typical food chain or food web. The pyramid represents the decrease in the
amount of energy, the number of organisms and the biomass from the producer to
the high - order consumer levels. The
decrease in the numbers and in the biomass represent the fact that, due to
energy loss, fewer organisms can be supported at each successive trophic level.
Pyramid of Energy
Energy is lost between each link in a food chain. Much of the potential energy at each level
never reaches the next level. Where does
the energy go as it moves through a food chain?
Some of the energy that enters a food chain is used as each organism
carries out its life functions (i.e. foraging, metabolic processes,
reproduction, predator/prey behavior, etc.).
Producers manufacture their own food source directly from sunlight by
the process of photosynthesis. In order
to carry out life functions, consumers acquire energy through the 'burning' or
breaking down of food molecules they consume (eat). Thermal energy (heat) is produced as a result
of the burning of these food molecules.
More than half of the energy from each food molecule is lost as heat.
Only about 10% - 20% of energy at each trophic level is available to pass on to
the next level. In other words, at each
level there is only about 10% available energy to put on new biomass (growth).
The loss of energy at each trophic level also explains why
there are usually fewer organisms in each higher trophic level. The total number of plants in a particular
area would generally be higher then the number of herbivores that the plants
support and the number of herbivores would be higher than the number of higher
order carnivores.
Pyramid of Biomass
Biomass is the total mass of dry organic matter per unit of
area. Each higher trophic level contains
less biomass than the previous trophic level.
Therefore a drawing or graph that represents the amount of biomass at
each trophic level would also produce the basic pyramid shape. Biomass is related to the abundance of
organisms at each trophic level.
Human Impact on Food Chains and Webs
Humans have the ability to have a great impact on
ecosystems. Living organisms are a significant
portion of any ecosystem, therefore any activity that affects an ecosystem is
also likely to affect the organisms within that ecosystem. If organisms are affected the food chains
webs that the organisms are a part of will also feel the affects.
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