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The goal of ecology is to explain where organisms live, how many there are, and how their interactions with one another and their surroundings change those patterns. Living partners include prey, predators and parasites; nonliving conditions include light, temperature and water. An ecosystem brings these interacting parts into one study system. See Clark, Douglas and Choi, Biology 2e, 44.1: the scope of ecology.

Start with an invented aquatic food web: photosynthetic algae feed zooplankton, which feed small fish, which feed large fish. That food chain has four feeding positions and three feeding arrows. Dead material also reaches decomposers through detritus, so the food web branches rather than forming a single ladder.

The appeal of ecology is that an organism's surroundings include other organisms that change those surroundings. Photosynthesis links light to living material; predation transfers that material between organisms; decomposers return its constituent chemical elements to forms that can be used again. These connections help explain why changing one part of an ecosystem can affect distant parts of its food web, although an arrow alone does not predict the size or even the direction of an indirect change. Population dynamics supplies quantitative tools when the question concerns changing counts.

In 1866, Ernst Haeckel's Generelle Morphologie der Organismen included a section titled Oecologie und Chorologie in volume 2, beginning on page 286. This places the terminology of ecology in an early systematic treatment of organisms and their relationships. See Haeckel, Generelle Morphologie der Organismen, volume 2 (1866), contents and p. 286. A later quantitative landmark is Raymond L. Lindeman's 1942 treatment of ecosystems through feeding relationships and energy flow. See Lindeman, The Trophic-Dynamic Aspect of Ecology, Ecology 23 (1942), 399–417.

A biological host provides the environment or resources used by a parasite or other associated organism.

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.