A second-level consumer, also called a secondary consumer, is an animal that eats plant-eating animals. It sits at the third trophic level of a food chain: producers (plants) are first, primary consumers (herbivores) are second, and secondary consumers come third. If a mouse eats seeds and a snake eats the mouse, the snake is the secondary consumer.
Where Secondary Consumers Fit in a Food Chain
Every ecosystem organizes its organisms into trophic levels based on how they get energy. Plants and algae form the base as producers. Herbivores like rabbits, snails, and insect larvae eat those producers and become primary consumers. Secondary consumers then feed on those herbivores.
Above secondary consumers sit tertiary consumers, which are carnivores that eat other carnivores. A simple chain might look like this: grass grows, a grasshopper eats the grass, a frog eats the grasshopper, and a hawk eats the frog. The frog is the secondary consumer, and the hawk is the tertiary consumer. The key distinction is straightforward: secondary consumers eat herbivores, while tertiary consumers eat other meat-eaters.
Carnivores, Omnivores, and Scavengers
Secondary consumers are mostly carnivores, but not exclusively. Any animal that feeds on a primary consumer qualifies, which means omnivores often fill this role too. A raccoon eating crayfish and frogs is acting as a secondary consumer, even though it also eats berries and acorns. Similarly, scavengers that feed on the remains of herbivores count as secondary consumers.
This flexibility matters because many animals don’t stick to one trophic level. Humans are a clear example. When you eat vegetables, you’re functioning as a primary consumer. When you eat beef, you’re a secondary consumer, because the cow ate plants. A 2013 study in the Proceedings of the National Academy of Sciences calculated the global human trophic level at 2.21, reflecting a diet that leans heavily toward plants but includes enough animal protein to push us above the herbivore level. In countries with almost entirely plant-based diets, like Burundi, the human trophic level drops to around 2.04.
Examples on Land and in Water
Secondary consumers show up in virtually every ecosystem. On land, common examples include frogs that eat insects, snakes that eat rodents, and birds like the great blue heron that feed on small fish, amphibians, and reptiles. Cricket frogs eat mosquitoes and other small insects along creek banks. Raccoons catch crayfish and frogs in addition to foraging for plant matter.
In aquatic ecosystems, the list is just as varied. In ocean food webs, fish like cod, mackerel, and dogfish are typical secondary consumers, feeding on smaller herbivorous fish and zooplankton. Freshwater examples include larger insects and small fish that prey on snails and insect larvae. Aquatic omnivores like sea turtles and crabs can also function as secondary consumers when they feed on herbivorous organisms.
The 10 Percent Rule
Energy transfer between trophic levels is remarkably inefficient. Only about 10% of the energy stored at one level makes it to the next. The rest is lost as heat through metabolism, used for movement, or passed out as waste. This is why ecosystems can support far fewer secondary consumers than primary consumers.
A well-studied aquatic ecosystem in Silver Springs, Florida, puts concrete numbers to this pattern. Primary consumers like snails and insect larvae stored energy at a rate of 1,103 kilocalories per square meter per year. Secondary consumers like fish and large insects stored just 111 kilocalories, almost exactly one-tenth. This consistent drop in available energy is what shapes the classic pyramid structure of ecosystems, with a broad base of producers and progressively smaller populations of consumers at each step up.
Why Biomass Pyramids Differ by Ecosystem
On land, the biomass pyramid follows an intuitive pattern: producers (plants and trees) vastly outweigh all the consumers combined, and secondary consumers make up a thin slice near the top. This is a direct consequence of the 10% energy rule compounding across levels.
Oceans tell a surprisingly different story. A comprehensive analysis published in PNAS found that roughly 1 gigaton of carbon in marine producers (mostly phytoplankton) supports about 5 gigatons of carbon in consumer biomass. This inverted pyramid exists because phytoplankton reproduce and get eaten so quickly that their standing population stays small, even though they produce enormous amounts of energy over time. The consumers accumulate more total mass at any given moment because they live longer and grow larger.
How Secondary Consumers Stabilize Ecosystems
Secondary consumers play a critical role in keeping herbivore populations in check. Without predators feeding on primary consumers, herbivore populations would grow unchecked and overgraze or overbrowse the plant life that forms the ecosystem’s foundation. This top-down pressure helps maintain equilibrium, the steady state where organisms remain in balance with their environment and with each other.
When secondary consumers are removed from an ecosystem, the effects cascade downward. Herbivore populations surge, plant communities decline, and the habitat shifts in ways that affect every other species. This is why the loss of mid-level predators like snakes, foxes, or predatory fish often causes more visible ecological disruption than people expect.