Algae, including microscopic phytoplankton and large seaweeds, form the foundation of many aquatic and some terrestrial food chains as primary producers. These photosynthetic organisms convert solar energy into biomass, making it available to the next level of life in the ecosystem. The organisms that consume algae are known as primary consumers. Identifying these diverse algae-eaters shows how energy is transferred throughout global ecosystems, supporting life from tiny copepods to massive whales.
Understanding the Primary Consumer Trophic Level
Primary consumers occupy the second trophic level, feeding directly on producers at the first level. In food webs based on algae or phytoplankton, these herbivores are the first recipients of the sun’s captured energy. The term herbivore applies broadly, encompassing any animal that obtains its nutritional requirements by eating plant material, including various forms of algae. This consumption is fundamental to the flow of energy, moving energy stored in the producer’s cells into the consumers’ living tissues.
Aquatic Algae Grazers
Most primary consumers that feed on algae are found in aquatic environments, where algae and phytoplankton dominate the producer base. Zooplankton are the most numerous of these grazers, including small, microscopic animals like copepods and rotifers that drift in the water column. These tiny organisms filter immense volumes of water daily, consuming single-celled phytoplankton, such as diatoms, and transferring that energy up the marine food web. Crustacean grazers, like amphipods, also play a significant role in nearshore environments by preventing nuisance algae from overgrowing seagrass meadows.
Larger invertebrates specialize in consuming algae, often using scraping mouthparts to remove attached growth from surfaces. Sea urchins are voracious grazers on rocky reefs, sometimes consuming large macroalgae like kelp. Aquatic snails and limpets use a rasping, tongue-like structure called a radula to graze on thin films of algae from rocks and submerged wood.
Various fish species have also evolved specifically to graze on algae. These include marine surgeonfish and freshwater catfish, such as the Plecostomus group. Surgeonfish are important reef grazers, with some species maintaining specialized gut bacteria to help them digest tough algal material. Tilapia are another example of herbivorous fish that graze on phytoplankton and filamentous algae in freshwater systems.
Terrestrial and Microscopic Algae Eaters
While most algae consumption occurs underwater, primary consumers also graze on algae found in damp soil, on wet surfaces, and within biofilms. Land snails are a common example, often seen grazing on the thin layer of green algae that grows on tree bark, rocks, and moist ground. Their feeding behavior relies on a radula to scrape the surface growth, similar to aquatic snails. Certain insects and their larvae, such as mayflies, are also specialized algae-eaters in semi-terrestrial niches, grazing on algae coating submerged rocks near stream edges.
On a microscopic level, the soil food web contains numerous algae consumers. These include specialized protozoa that ingest algae cells. Omnivorous soil nematodes, tiny unsegmented worms, also feed on algae found in the water films surrounding soil particles. Certain species of mites, such as Nanorchestes, feed on green algae in the ground, contributing to decomposition and nutrient cycling.
The Energy Transfer to Secondary Consumers
Primary consumers serve as the conduit for energy transfer to the next level of the food chain: the secondary consumers. This transfer is not perfectly efficient; only about ten percent of the energy stored in the primary consumer’s biomass is typically transferred to the secondary consumer that eats it. The remaining energy is lost as heat, waste, or used for the primary consumer’s metabolic processes.
Zooplankton, which consume phytoplankton, become a primary food source for secondary consumers like small fish, krill, and filter-feeding organisms such as clams and mussels. These small fish and invertebrates are then consumed by larger secondary consumers, including carnivorous fish species and seabirds. For example, the sea urchin, a primary consumer of kelp, is prey for sea otters, making the otters secondary consumers in that food chain. This flow of energy, starting with algae-eating primary consumers, supports the entire structure of the food web.