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Within an ecological food chain, consumers are categorized into primary consumers, secondary consumers, and tertiary consumers. [3] Primary consumers are herbivores, feeding on plants or algae. Caterpillars, insects, grasshoppers, termites and hummingbirds are all examples of primary consumers because they only eat autotrophs (plants).
The primary consumer may be eaten by a secondary consumer, which in turn may be consumed by a tertiary consumer. The tertiary consumers may sometimes become prey to the top predators known as the quaternary consumers. For example, a food chain might start with a green plant as the producer, which is eaten by a snail, the primary consumer.
At the lowest trophic level (the bottom of the food chain), plants convert about 1% of the sunlight they receive into chemical energy. It follows from this that the total energy originally present in the incident sunlight that is finally embodied in a tertiary consumer is about 0.001% [7]
Biomass represents stored energy. However, concentration and quality of nutrients and energy is variable. Many plant fibers, for example, are indigestible to many herbivores leaving grazer community food webs more nutrient limited than detrital food webs where bacteria are able to access and release the nutrient and energy stores.
In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. [3] [4] Living organisms that are heterotrophic include all animals and fungi, some bacteria and protists, [5] and many parasitic plants.
[1] [7] Some examples of primary producers are algae, mosses, and other plants such as grasses, trees, and shrubs. [1] Chemosynthetic bacteria perform a process similar to photosynthesis, but instead of energy from the sun they use energy stored in chemicals like hydrogen sulfide.
For example, cyanobacteria and many purple sulfur bacteria can be photolithoautotrophic, using light for energy, H 2 O or sulfide as electron/hydrogen donors, and CO 2 as carbon source, whereas green non-sulfur bacteria can be photoorganoheterotrophic, using organic molecules as both electron/hydrogen donors and carbon sources.
In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. [5] [6] Living organisms that are heterotrophic include all animals and fungi, some bacteria and protists, [7] and many parasitic plants.