A predator is an animal or other organism that hunts and kills other organisms for food in an act called predation. Most predators are carnivores. Some predators are omnivores. The difference between a predator and a parasite is that for a predator killing the prey is necessary for consuming it, but for parasites it is not even desirable because a parasite lives on or in its host. Some might consider herbivores to be predators as well, but this is arguable as most herbivores only consume parts of their food species, leaving the remainder alive. However, where the "prey" consists of single-celled algae, the activities of the herbivorous grazer is generally of the same nature as that of a carnivore. As usual in ecology as most fields of study, there is seldom consensus on the distinctions; some ecologists prefer functional definitions like the one outlined above, others rather look at the ecological dynamics the relationships between the species create.