The organisms of most ecosystems obtain in three basic ways: producing, consuming, and decomposing.
19.3.1 Producers
Organisms that make their own food from inorganic molecules and solar energy are called producers. Plants are the most important producers in land ecosystems (fig 19. 6a). In an aquatic ecosystem, photosynthetic protists are the most important producers (fig 19.6b). Nearly all the energy entering the biosphere comes from the sun through photosynthesis.
Fig. 19.6 major producers in land ecosystems are plants (a). Diatoms are important producers in the oceans (b).
19. 3.2 Consumers
Organisms that cannot make their own food and use the food prepared by the producers are called consumers. All animals are consumers, as are fungi and many protists and many bacteria. Consumers obtain energy by ingesting other organisms or by absorbing food molecules. Consumers obtain energy by ingesting other organisms. Consumers obtain food in three basic ways.
Organisms that eat only plants are called herbivores. Because herbivores eat producers, they are also called primary consumer. Many insects and birds are herbivores, as are grazing animals such as cows, buffalos, goats and giraffes. The bodies of herbivores are adapted to gathering, grinding and digesting plants or other producers.
19.7 consumers may be (a) herbivores (b) carnivores or (c) omnivores
Carnivores capture and eat herbivores or other carnivores. Carnivores that eat primary consumers are called secondary consumers, and those that eat other carnivores are called tertiary are adapted to hunting, capturing, and eating prey.
Some consumers, including human beings are omnivore that is; they eat both producers and other consumers. Omnivores act as primary. Secondary, or tertiary consumers depending upon what they eat (fig 19.7).
19.3.3 Decomposers
Bacteria and fungi that consume the bodies of dead organisms and other organic wastes are called decomposers. They break down a variety of dead organic matter, from the fallen leaves of a tree to the bodies of herbivores and carnivores. Decomposers are crucial to the ecosystem because they recycle nutrients from the organisms back into the environment. Without decomposers, the producers in an ecosystem would quickly run out of nutrients.
Fig. 19.8 energy flows when organisms eat one another, the organisms that obtain their energy from a common source constitute a trophic level.
Trophic levels
The different feeding levels of organisms in an ecosystem are called trophic levels. Producers make up the first trophic level in all ecosystems. Because they make their own food, producers are called autotrophs. Autotrophs are the sole point of entry for new energy into the ecosystem.
Consumers from the second and higher trophic level in the ecosystem. Because they cannot produce their own food and must obtain nourishment by eating other organisms, consumers are called heterotrophs. Primary consumers that eat producers form the second trophic level, and secondary consumers form the third trophic level. Omnivores and decomposers feed at all trophic levels. Most ecosystems have three, four, or five trophic levels. Each trophic level depends completely on the level below it (fig. 19.8)