Consumers, Ecological

The term ECOLOGICAL CONSUMERS refers to species that cannot produce their own food, and so get energy and nutrients by eating other organisms. Properly termed heterotrophs, they are distinguished from autotrophs, or producers, that produce their own food via photosynthesis (plants and cyanobacteria) and chemosynthesis (carried out by bacteria near deep ocean hydrothermal vents). Consumers occupy the highest levels of the trophic hierarchy, a system of classification in which species are grouped according to their position in food chains and webs. Other levels of the trophic hierarchy involve the conversion of energy (solar and heat), gases and inorganic gases into biotic carbohydrates and proteins (via producers), and back into the abiotic components (via decomposers, such as fungi), whereas consumers strictly cycle biotic compounds and energy.

Consumers are further subclassified according to the nature of predator–prey interactions. Primary consumers engage in herbivory (considered to be a form of predation). Secondary consumers prey directly upon primary consumers, and include carnivores and insectivores. Tertiary consumers include top carnivores and omnivores, and include all of those species that prey upon multiple trophic levels. For example, bears are tertiary consumers, as are humans, because they eat fruits (producers), herbivores (primary consumers) and secondary consumers (such as salmon). Consumers play an important ecological role in maintaining biodiversity in that they keep the populations of prey species in check, and cycle nutrients and energy through the ecosystem.

Between each trophic level, some energy is lost to heat. Due to these inefficiencies, the biomass of each successive trophic level decreases. A given mass of vegetation in an ecosystem will support a smaller mass of herbivores, which in turn support successively smaller mass of secondary and tertiary consumers. The reduced mass of consumers across each of the levels typically translates into decreased populations of respective species. A given landscape will typically support a higher number of primary consumers than secondary consumers, with tertiary consumers being fewest in number. A general rule of thumb cites a 90 percent loss of energy across each trophic level, but this figure varies greatly between differing species and systems when analyzed empirically.

Large populations of tertiary consumers thus need a larger productive base to support them. Similarly, human societies have differing levels of impact on the landscape based on consumption habits and affluence. For example, ecologists often compare the number of people a certain amount of land can support through grain production, if those people consume the grain directly or if the grain is [Page 356]first fed to cattle, and the cattle subsequently eaten by people. Due to the inefficiencies across multiple trophic linkages, the land supports a smaller human population through beef consumption than if people consumed the grain themselves; that society requires substantially more land and has a higher ecological impact.

Alternatively, pastoral nomadic societies survive in semi-arid grasslands precisely because of these same inefficiencies across multiple trophic levels. Despite being unable to directly digest grass, people are able to survive in these inhospitable climates by keeping livestock that digest the grass, then consuming the livestock's productivity. Similarly, range-fed beef is considered by many to be a more ecologically sound choice than grain-fed beef.

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