1.9 Trophic Levels

What are trophic levels?

  • Trophic levels show feeding positions in a food chain or food web

  • They explain how energy flows and matter cycles through ecosystems

  • Each level represents a step farther from the original energy source (the Sun)

Energy vs. Matter

Energy

  • Enters ecosystems as sunlight

  • Flows in one direction: Sun → producers → consumers → heat

  • Not recycled

  • Only ~10% of energy transfers to the next level (10% Rule)

  • Most energy is lost as heat (2nd Law of Thermodynamics)

Matter

  • Conserved (not created or destroyed)

  • Recycled through biogeochemical cycles

  • Key cycles: carbon, nitrogen, phosphorus, water

The Main Trophic Levels

1st Trophic Level: Producers (Autotrophs)

  • Make their own food using photosynthesis (or chemosynthesis)

  • Convert solar energy into chemical energy (glucose)

  • Form the base of all food webs

  • Examples: plants, algae, phytoplankton, cyanobacteria

2nd Trophic Level: Primary Consumers (Herbivores)

  • Eat producers

  • First level of heterotrophs

  • Transfer energy from plants to animals

  • Examples: deer, grasshoppers, rabbits, zooplankton

3rd Trophic Level: Secondary Consumers

  • Eat primary consumers

  • Usually carnivores or omnivores

  • Examples: frogs, small fish, snakes, foxes

4th Trophic Level: Tertiary Consumers

  • Eat secondary consumers

  • Often top predators

  • Receive the least energy

  • Examples: eagles, hawks, wolves, lions, orcas

(Some food chains include quaternary consumers, but APES usually groups them as apex predators.)

Decomposers and Related Groups (Not a Single Trophic Level)

Decomposers

  • Break down organic matter chemically

  • Recycle nutrients back to soil or water

  • Examples: bacteria, fungi

Detritivores

  • Physically break down dead matter

  • Examples: earthworms, dung beetles, millipedes

Scavengers

  • Eat dead animals before decomposition

  • Examples: vultures, raccoons

Key idea:

  • These organisms work across all trophic levels

  • Essential for nutrient recycling

Food Chains vs. Food Webs

  • Food chain: simple, linear energy path

  • Food web: complex, realistic network of feeding relationships

  • Food webs show ecosystem stability better than food chains

Energy Pyramids

  • Show decreasing energy at higher trophic levels

  • Lower levels have:

    • More energy

    • More biomass

    • Larger populations

  • Higher levels have fewer organisms due to energy loss

Productivity (Important Link to Trophic Levels)

  • GPP (Gross Primary Productivity): total energy captured by producers

  • NPP (Net Primary Productivity): energy available to consumers

    • NPP = GPP − respiration

  • Higher NPP = more energy to support higher trophic levels

Key Reminders for Exams

  • Energy flows one way, matter cycles

  • Only ~10% of energy transfers between trophic levels

  • Decomposers are essential, not optional

  • Higher trophic levels = less energy, smaller populations

  • Producers support all life