b - Food Webs, Trophic Levels, and Biogeochemical Cycles

Food Webs and Trophic Levels

Food webs illustrate trophic interactions within communities, focusing on how organisms obtain nutrients.

Functional Roles

  • Primary Producers: Autotrophic organisms (e.g., plants, algae) that get energy from abiotic factors like the sun and nutrients from the environment.
  • Consumers: Heterotrophic organisms that feed on other organisms.
    • Primary Consumers: Herbivores that eat primary producers.
    • Secondary Consumers: Feed on primary consumers.
    • Tertiary Consumers: Feed on secondary consumers.
  • Decomposers and Detritivores: Heterotrophic recyclers that break down dead organisms, returning nutrients to the ecosystem.

Trophic Levels

Trophic levels categorize organisms by their rank in the food web:

  • Primary producers
  • Primary consumers (herbivores)
  • Secondary consumers
  • Tertiary consumers

Consumers can feed from multiple trophic levels, like omnivores.

Energy Transfer and Trophic Pyramids

Energy does not cycle through an ecosystem; it requires continuous input. Photosynthesis is at the core of ecosystem function in most ecosystems. Energy stored in organic molecules by primary producers is available to higher trophic levels.

Trophic Pyramid

Biomass and energy decrease from one trophic level to the next, forming a trophic pyramid. Biomass measures the overall weight of a trophic group.

Biomass{\text{primary producers}} > Biomass{\text{primary consumers}} > Biomass_{\text{secondary consumers}}

  • Approximately 10% of energy in biomass is transferred to the next level due to waste, work, and heat dissipation.
  • Energy transfer is one-directional, unlike the cycling of carbon and other elements.

Biogeochemical Cycles

Biogeochemical cycles link organisms and their environment through the chemical cycling of raw materials. Photosynthetic rate depends on water availability because photosynthesis requires both carbon dioxide and water.

Carbon Cycling

  • Terrestrial: Vascular plants absorb CO_2, herbivores consume plants, predators eat herbivores, and decomposers break down organic waste.
  • Oceanic: Algae and cyanobacteria convert CO_2 into organic molecules, consumers feed on organic matter, and decomposers recycle nutrients.