FOOD CHAIN AND TROPIC LEVELS
TOPIC:
FOOD CHAIN AND TROPIC LEVELS
OBJECTIVE
The objective of this script is to know the basic concept of
food chain and trophic levels in various ecosystems.
Concept
Plants convert solar energy into chemical energy stored in
food material. The transfer of food energy from plants through a series of
organisms forms the food chain. A phytoplankton may be eaten by a copepod,
which is eaten by a small fish, the small fish is ultimately eaten by large
fish, and large fish by a swan.
This is a food chain that is generally observed in rivers. In
pasture grasses are eaten by grasshopper and grasshopper by a toad. In the two
examples the base of the food chain is formed by a plant, which is grazed on by
a herbivore, which is predated by a carnivore, which is eaten by another
carnivore.
In nature, food chain relationships are very complex, as one
organism may form the food source of many organisms. For example, grass may be
grazed on by grasshoppers as well as rabbits or cattle and each of these
herbivores may be eaten by many carnivores, such as toads, snakes, birds, or
hyena. Thus, instead of a simple food chain we find a web-like structure called
food web.
When the food is obtained from the same number of steps then
the organisms are said to belong to the same trophic level. Organism whose food
is obtained from plants are said to belong to the trophic level two or primary
consumer or herbivore. The green plants occupy the first trophic level and are
called the producers. Examples of herbivores are: plant grazing insects,
rabbits, rodents, deer, cattle and so on. Flesh eaters that eat herbivores form
the third trophic level or the secondary consumer or carnivore level one. The
next trophic level is the tertiary consumer or carnivore two, which eat the
fleshs of herbivores and secondary consumers. The trophic level can be expanded
taking the food habits of organisms into account.
Types of Food Chains
In ecosystems such as grassland, pond, or lake, a
substantial part of the primary production is grazed on by herbivores. Rodents
and cattle are the main grazers in grassland, whereas zooplanktons are the main
grazers in ponds and lakes. Usually up to 50% of the net primary production is
grazed on by these animals in their respective ecosystems and the remaining 50%
goes to the decomposer organisms as dead organic matter. Therefore, in these
ecosystems, the food chain is herbivore based and herbivores are considered
important consumers. This is classified as a grazing food chain.
In a forest ecosystem, the dominant primary consumers, that
is, herbivores are the insects, which usually consume less than 10% of the net
primary production. The rest 90% is consumed later as dead plant material by
the small detritus like protozoa, fungi, actinomycetes, and bacteria. The
animals consume the detritus and process it in their gut by reducing it into
small pieces and sometimes digesting it partially or fully, thus making organic
materials available for bacterial and fungal attack. These macro-organisms also
act as food for many soil animals. This food chain is classified as a detritus
or decomposer food chain. The detritus food chain exists in every ecosystem and
is very important for the circulation of materials. Even in a grazing food
chain, the faeces and urine of grazing herbivores ultimately come to the
decomposer pathway.
Significance of Food Chains
Food chain studies help understand feeding relationships and
the interaction between organisms in any ecosystem. They also help us
comprehend the energy flow mechanism and matter circulation in ecosystems, and
understand the movement of toxic substances in the ecosystem and the problem of
biological magnification. Certain harmful substances, usually ones not found in
nature but introduced by man, may get injected into plants and animals. These
poisonous substances like DDT may not be broken down in the body or excreted
easily and quickly. Instead, they accumulate in the tissues, and as the living
organism eats more, the concentration of these substances increases and they
pass from one trophic level to the next. As man is an omnivore (eat both herb
and flesh) and has access to all trophic levels for food gets large amounts of
toxic substances like DDT into the body.
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