A secondary consumer is an animal that eats herbivores. It sits on the third trophic level of a food chain, one step above the plant-eaters (primary consumers) and two steps above the plants themselves (producers). Snakes eating mice, frogs eating insects, and cod eating herring are all examples of secondary consumers in action.
Where Secondary Consumers Fit in a Food Chain
Every ecosystem organizes its organisms into trophic levels based on where they get their energy. The first level belongs to producers like plants and algae, which make their own food from sunlight. The second level is primary consumers, the herbivores that eat those plants. Secondary consumers occupy the third level by feeding on those herbivores.
Above secondary consumers, you may find tertiary consumers, the top predators that eat secondary consumers and often have no natural predators of their own. A wolf eating a snake that ate a mouse is a tertiary consumer in that chain. The key distinction: secondary consumers are predators, but they are also prey. Tertiary consumers typically are not.
They Don’t Have to Be Pure Carnivores
The term “secondary consumer” describes a position in a food chain, not a strict diet. Many secondary consumers are omnivores that eat both plants and animals. Raccoons, bears, many songbirds, sea turtles, and crabs all qualify as secondary consumers when they eat herbivores or other primary consumers, even though they also eat plants. Humans fall into this category too. A study published in the Proceedings of the National Academy of Sciences calculated the global human trophic level at 2.21, roughly the same as an anchovy or a pig. That’s well short of an apex predator and reflects the large share of plant-based food in most human diets worldwide.
Pure carnivores like snakes, foxes, and eagles can also be secondary consumers when their prey is herbivorous. The same animal can even shift trophic levels depending on what it eats. A fox eating a rabbit (a herbivore) is acting as a secondary consumer. A fox eating a frog that ate insects could be considered a tertiary consumer in that particular chain.
Common Examples on Land and in Water
On land, secondary consumers include a wide range of animals across size and species. Bullfrogs eat insects and other invertebrates. Box turtles feed on small invertebrates, amphibians, and even small mammals. Mink, little brown bats, and deer mice all occupy this trophic level by preying on insects or small herbivores. Many familiar birds, including mallards and various songbirds, eat insects and small aquatic herbivores that place them squarely at the secondary consumer level.
In aquatic ecosystems, the pattern is similar. Dogfish, cod, and mackerel are secondary consumers in ocean food webs, feeding on herbivorous fish like herring or on krill and scallops. Common aquatic omnivores that function as secondary consumers include snails, sea turtles, zooplankton, and crabs. Above them, animals like tuna and seals act as tertiary consumers.
The Energy Problem at the Third Level
One of the most important things about secondary consumers is how little energy actually reaches them. Each time energy passes from one trophic level to the next, roughly 90% of it is lost, mostly as heat from the metabolic processes of the organisms being eaten. The transfer rate between levels averages about 10%, though it can range from 3% to 20% depending on the ecosystem.
In practical terms, producing 1 kilogram of a secondary consumer’s body tissue requires about 100 kilograms of plant material at the base of the food chain. That’s because 10 kg of plants feeds 1 kg of herbivore, and 10 kg of herbivore feeds 1 kg of secondary consumer. This inefficiency is why food chains rarely stretch beyond four or five levels. There simply isn’t enough energy left to support additional layers of predators.
Warm-blooded secondary consumers lose even more energy than cold-blooded ones because they burn calories to maintain body temperature. A squirrel, for instance, converts as little as 1.6% of the energy in acorns into body mass, while a caterpillar can convert about 18%. This is why large warm-blooded predators need to eat far more frequently than a snake or frog of comparable size.
Why Ecosystems Need Them
Secondary consumers regulate herbivore populations. Without them, primary consumers would multiply unchecked and overgraze or overbrowse the plant life that forms the foundation of the entire food web. This top-down pressure keeps ecosystems in balance.
When secondary consumers disappear, the consequences can cascade through the entire system. Research in Baltic Sea coastal ecosystems showed what happens when larger predatory fish are removed: medium-sized predators (like three-spined sticklebacks) exploded in number, a phenomenon called meso-predator release. Those medium predators then selectively wiped out certain types of grazers, reducing amphipod populations by 40 to 60%. The surviving grazers were less effective at eating algae, and under nutrient-rich conditions, plant biomass surged to 23 times its normal level. The removal of one predator level reshaped every level below it.
This kind of trophic cascade illustrates why secondary consumers are not just links in a chain but active regulators. The strength of their effect depends on how large a share of their prey’s population they consume. When a predator takes a large fraction of its prey’s total reproduction, it exerts strong control over that population. Remove it, and the system can shift dramatically.