Second-order consumers, also known as secondary consumers, are organisms that occupy a position in a food chain. They are often referred to simply as predators, as they get their food by eating first-order consumers, or herbivores.
A food web, which consists of multiple food chains linked together, has identifiable niches for organisms based on the way they get their energy. Producers sit at the bottom of the web and extract energy directly from an abiotic source, usually the sun. Producers are eaten by first-order consumers, which are almost always herbivorous animals. Herbivores are, in turn, eaten by the second-order consumers, the predators.
A typical food chain runs from grass, which is a producer, to the gazelle that eats the grass as a first-order consumer, to the cheetah, which is the second-order consumer, in this example. While there is no theoretical limit to how many layers a food web can support, as predators themselves can always be eaten by other predators, second-order consumers can be the practical limit for unproductive ecosystems, as energy is lost with each transfer up the chain.
To support a single second-order consumer, an environment must be able to support 10 times as much biomass in primary consumers, and 100 times as much among the producers at the bottom of the web.