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A parasite is an organism benefiting from parasitism. Microparasite and macroparasite distinguish useful transmission-model descriptions of within-host multiplication and distributed parasite burden.

Parasitism is an interaction in which a parasite obtains resources from a host, reducing some component of the host's performance. Its demographic effects depend on exposure, within-host processes and transmission opportunities.

Predation is the consumption of prey by a predator, affecting the demographic rates of both populations.

In the usual animal-feeding sense, a predator kills and eats its prey. This distinguishes the interaction from feeding on already-dead material and from the prolonged exploitation of a living host in parasitism. In the illustrative food web, a small fish eating zooplankton is a predator; a large fish eating that small fish makes the same small fish its prey. These roles belong to a particular interaction. See Clark, Douglas and Choi, Biology 2e, 45.6: predation and community interactions.

For a toy count, suppose 50 small fish begin a day and 8 are eaten by large fish. With births, other deaths and movement explicitly omitted, 42 small fish remain, and the fraction removed by predation is $8/50=0.16$. This measures removal of prey, not production of new predators. Food intake can support survival and reproduction, but eight meals do not imply eight new predators; energy flow and life-history details intervene. Lotka-Volterra equations provide one idealized account of such population dynamics, rather than a guarantee that every interacting pair must cycle.