PYTR Class

Learning Objectives

1. Construct a food web by identifying food chains
2. Identify organisms in each trophic level
3. Summarise ecological pyramids

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Part 1: Food Chains and Food Web

A trophic level is a stage of position of a organism in a food chain. From the above example, the organisms can be placed into these trophic levels:

Trophic level 1 = grass

Trophic level 2 = grasshopper

Trophic level 3 = toad

Trophic level 4 = snake

Trophic level 5 hawk

Part 2: Ecological Pyramids

Types of Ecological Pyramids

1. Pyramid of Numbers
   • Represents the number of producers and consumers in an ecosystem.
   • Constructed by counting organisms at each trophic level.
2. Pyramid of Biomass
   • Measures the amount of biomass at each trophic level at a specific point in time.
   • Expressed in grams per square meter (g m⁻²) or joules per square meter (J m⁻²).
   • Typically pyramidal in shape but can be inverted if higher trophic levels have more biomass.
   • Biomass pyramids reflect standing stock, not productivity, making them a snapshot of an ecosystem.
   • Example: Intensively grazed pastures may show lower grass biomass but higher productivity than ungrazed ones.
   • Pond ecosystems often have inverted pyramids because fast-reproducing phytoplankton have less standing biomass than consumers.
   • Seasonal variations can also cause inversions in biomass pyramids.
3. Pyramid of Energy (Productivity)
   • Represents the rate of energy production over a period of time.
   • Each level indicates energy per unit area per unit time.
   • Shows the total energy flow through an ecosystem over a year.
   • Never inverted, as energy decreases at each successive trophic level.
   • Helps compare energy flow across different ecosystems.