Food Chains and Trophic Cascades
Food chains illustrate the transfer of energy through an ecosystem—intricately interconnected into complex networks called food webs.
Food webs are often represented in pyramid format. Each trophic level, or feeding level, is represented as its own horizontal slice of the pyramid. The size of each slice provides a relative scale of the abundance of organisms in that trophic level.
Producers:
Powered by sunlight, these organisms (including plants, algae, and phytoplankton) produce their own energy in the process of photosynthesis. In a lake ecosystem, algae are the producers, uptaking nutrients in the water and using sunlight to produce energy.
Primary Consumers:
Primary consumers get their energy by eating producers. In a lake ecosystem, the primary consumers are small organisms such as zooplankton (e.g. Daphnia), free floating in the water column that feed directly on producers (algae).
Secondary Consumers:
Secondary consumers get their energy by eating primary consumers. They can be carnivores (exclusively meat-eaters) or omnivores (eating meat and plants). In a lake ecosystem, the secondary consumers are planktivorous fish—smaller fish that consume zooplankton such as Daphnia.
Tertiary Consumers:
Tertiary consumers get their energy by eating secondary consumers. These organisms are apex predators. In a lake ecosystem, the tertiary consumers are piscivorous fish—large fish that prey on other fish.
Decomposers:
Decomposers, including fungi, worms, and some bacteria, bring the whole pyramid full circle, consuming dead and decaying plant and animal material (from all trophic levels above), releasing nutrients and organic matter.
IMAGE: WOW. 2004. Water on the Web - Monitoring Minnesota Lakes on the Internet and Training Water Science Technicians for the Future - A National On-line Curriculum using Advanced Technologies and Real-Time Data.(www.waterontheweb.org).
Biodiversity is all about balance. If one species’ population increases or decreases, this can cause a cascading effect across the whole pyramid.
Trophic cascades are ecosystem interactions that occur when predators limit the density and/or behavior of their prey and thereby enhance survival of the next lower trophic level. For example, in a lake ecosystem that experiences high levels of eutrophication (nutrient enrichment), strategic fish stocking can aid in providing top-down control—the piscivorous fish prey on the planktivorous fish, suppressing their population. In turn, the zooplankton (e.g. Daphnia) grow abundantly and feed on algae, helping to control the nuisance algae population, overgrown due to nutrient enrichment. Trophic cascades can be an effective ecosystem management tool, but are most fruitful when also paired with nutrient control measures in the surrounding watershed land area, as well.
Additional Resources
The Food Web – Water on the Web | University of Minnesota
Trophic Cascades Across Diverse Plant Ecosystems – Nature Education
The Big Scientific Debate: Trophic Cascades – National Park Service