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In the literal sense, a carnivore is any flesh-eating organism. However, in the ecological usage of the word, carnivores kill animals before eating them (that is, they are predators), as opposed to feeding on animals that are already dead (the latter are called scavengers or detritivores).

Trophic ecology deals with the feeding and nutritional relationships within ecosystems, and this field has developed some specialized terminology. Carnivores, for example, are heterotrophs, which means that they must ingest other organisms to obtain energy and nutrition. (In contrast, autotrophs such as green plants can fix their own energy and synthesize biochemicals utilizing diffuse sources such as sunlight and simple inorganic molecules.) Animals that feed on plants are herbivores (or primary consumers), while animals that eat herbivores are known as primary carnivores (or secondary consumers), and carnivores that feed upon other A cougar (Felis concolor). JLM Visuals. Reproduced by permission.

carnivores are tertiary consumers. It is rare for an ecosystem to sustain carnivores of an order higher than tertiary. This is due to the pyramid-shaped structure of productivity in ecological food webs, which itself is caused by thermodynamic inefficiencies of energy transfer between levels. Therefore, the productivity of green plants is always much larger than that of herbivores, while carnivores sustain even less productivity. As a result of their trophic structure, ecosystems cannot sustain predators that feed upon, for example, lions, wolves, or killer whales.

Another consequence of the pyramidal structure of ecological webs is the tendency of top carnivores to bioconcentrate especially large residues of fat-soluble, persistent chemicals such as the chlorinated hydrocarbons, DDT, PCBs, and dioxins. This happens because organisms in successive levels of the trophic web absorb most of the chlorinated hydrocarbons that they ingest, storing these chemicals in fatty tissues. Consequently, top carnivores further concentrate the pre-concentrated residues of organisms lower in the ecological web. Therefore, the largest residues of these chemicals occur in peregrine falcons, polar bears, and seals, and these top predators have a disproportionate risk of being poisoned.

Almost all carnivores are animals. However, a few carnivores are specialized species of plants that trap, kill, and digest small animals, and then absorb some of their nutrients. Examples of these so-called carnivorous plants include Venus flytrap, sundews, and pitcher plants.

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