a) Producers:
These are the most important components of ecosystems. Producers of an ecosystem may be defined as those individuals who depend directly on the abiotic component for their survival and production of nutrients. Green plants are the chief producers of a negotiable energy source in the nature which could be utilized by all other living organisms. By their photosynthetic activity, the green plants trap the solar energy and convert it into chemical energy which is available in their body tissues. Apart from the green plants, photosynthetic and chemosynthetic bacteria are the two types of autotrophic organisms in the ecosystem.
b) Consumers:
The living components of the ecosystem which depend on producers for their nutrition are called consumers. All the animals and certain plants are included in this category. There are different types of consumers. These are:
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a) Primary consumers
b) Secondary consumers
c) Teritary consumers, and
d) Parasites and saprophytes
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Primary consumers:
All the herbivorous animals like rodents, cow, elephants, deer, goats etc, which directly consume the plants are called primary consumers. Among the aquatic animals, certain kinds of fish, crustaceans, molluscs etc which survive on phytoplankton are also primary consumers. According to Elton (1939), herbivorous animals are key industry animals, because all other animal’s life is dependent on these primary consumers.
Secondary consumers:
Carnivorous and omnivorous animals belong to this category. These animals predate on herbivorous animals. Omnivorous animals eat herbivorous animals as well as plants. Sparrow, Crow, Fox, Wolves, Cat, Dogs, Snakes etc belong to this category.
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Tertiary consumers:
They are strictly carnivorous animals that prey upon carnivores, herbivores, and omnivorous organisms. Lions, Tigers, Vultures etc are regarded at tertiary consumers.
Decomposers:
These are also called transformers as they transform organic compounds into inorganic or simple compounds. Saprophytic fungi and bacteria belong to this category. They act upon the dead bodies of plants and animals and decompose them to their elemental stage. These in turn can be used by producers for their existence and photosynthetic activity. The decomposers occupy a pivotal place in the ecosystem as they indirectly support the producers.
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Parasites:
Plants and animals that infect other living components of the ecosystem and survive on them are regarded as parasites. Various types of fungi, bacteria and a few flowering plants are parasitic. Several protozoans, insects and nematodes are also parasitic.