Trophic Levels

Herbivores may be fed upon by other heterotrophs, which are known as secondary consumers. If the herbivore must be killed before it is eaten, the secondary consumer is known as a predator. However, if the herbivore does not have to be killed to be eaten, the secondary consumer is known as a parasite. Predators of the tiny zoo-plankton described in the previous section include somewhat larger, carnivorous zooplankton, as well as small fish. In terrestrial ecosystems, herbivorous mice may be fed upon by predatory weasels and hawks, while deer are killed and eaten by coyotes and cougars.

If the resource base of the ecosystem is large enough, the secondary consumers may be killed and eaten by higher-order consumers, which will generally be the top predators in the system. For example, mature lake trout may be at the top of the food web of a temperate-lake ecosystem, in which the trophic structure is organized as: algae...herbivorous zooplankton...predatory zooplankton and small fish...and the largest predatory fish, such as lake trout. If bald eagles or humans subsequently predate on the lake trout, they would be considered the top predators in the system as well as a trophic linkage to the terrestrial part of the larger ecosystem.