Predators are often another organism's prey, and likewise prey are often predators. Though blue jays prey on insects, they may in turn be prey for cats and snakes, and snakes may be the prey of hawks. One way of classifying predators is by trophic level. Organisms that feed on autotrophs, the producers of the trophic pyramid, are known as herbivores or primary consumers; those that feed on heterotrophs such as animals are known as secondary consumers. Secondary consumers are a type of carnivore, but there are also tertiary consumers eating these carnivores, quartary consumers eating them, and so forth. Because only a fraction of energy is passed on to the next level, this hierarchy of predation must end somewhere, and very seldom goes higher than five or six levels, and may go only as high as three trophic levels (for example, a lion that preys upon large herbivores such as wildebeest, which in turn eat grasses). A predator at the top of any food chain (that is, one that is preyed upon by no organism) is called an apex predator; examples include the orca, sperm whale, anaconda, Komodo dragon, tiger, lion, tiger shark, Nile crocodile, and most eagles and owls—and even omnivorous humans and grizzly bears. An apex predator in one environment may not retain this position as a top predator if introduced to another habitat, such as a dog among alligators, a skunk in the presence of the great horned owl immune to skunk spray, or a snapping turtle among jaguars; a predatory species introduced into an area where it faces no predators, such as a domestic cat or a dog in some insular environments, can become an apex predator by default.