Trophic Pyramids & Diagrams (College Board AP Biology)

• A species can be classified as an autotroph, consumer, detritivore, or saprotroph on the basis of its mode of nutrition

Autotrophs

• An autotroph synthesizes, or produces, its own organic molecules from simple inorganic substances in its environment
  • Photosynthetic organisms use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates
  • Some autotrophs use energy from the oxidation of inorganic compounds instead of light energy
  • Autotrophs that use light energy are known as photoautotrophs, while those that use energy from oxidation of chemicals are known as chemoautotrophs (or chemosynthetic organisms)
• Because autotrophs make their own organic molecules without relying on other organisms, they are known as producers
• Most green plants are autotrophs, with few exceptions
  • Some unusual plants are parasitic, gaining their nutrients from the roots of host plants, or via networks of fungi in the soil

Heterotrophs

• Heterotrophic organisms gain their organic molecules from other organisms
  • These organic molecules (including carbohydrates, lipids and proteins) are then metabolized by hydrolysis to release energy
• There are several types of heterotroph, including consumers, detritivores, and saprotrophs

Consumers

• Consumers gain their organic molecules by ingesting the tissues of other living or recently dead organisms
• The consumers that eat plants are known as herbivores, and are the primary consumers in a food chain
• The consumers that eat other animals are carnivores, and those that eat the primary consumers are secondary consumers
• Carnivores that eat secondary consumers are tertiary consumers

Detritivores

• Detritivores gain organic molecules by ingesting the tissues of dead organisms or ingesting animal waste
• Detritivores digest their food inside their bodies
• Examples of detritivores include earthworms, woodlice and dung beetles

Saprotrophs

• Saprotrophs also gain their organic molecules from dead matter, but they digest their food externally
• Saprotrophic organisms secrete enzymes onto dead matter, and these enzymes break down the food before nutrients are absorbed
• Saprotrophs include fungi and bacteria

Classifying Species as Autotrophs, Consumers, Detritivores, or Saprotroph Table

• Pyramids of number and pyramids of biomass have been used to portray trophic levels within ecosystems
  • Both have their faults and can be misleading
  • So a more meaningful representation of trophic levels can be used : a pyramid of energy
• Pyramids of energy illustrate the amount of energy contained within the biomass of organisms at each trophic level
• The length of each box, or bar, represents the quantity of energy present
  • Pyramids of energy should be drawn to scale so that each bar is proportional in size to the amount of energy that it represents
  • In some situations, however, a pyramid of energy may be an approximate sketch where each bar is a rough representation of the energy contained
• These pyramids are always widest at the base and decrease in size as they go up
  • The base is wide due to the large amount of energy contained within the biomass of producers
• Pyramids of energy show a stepped decrease in the energy contained at each level of the food chain rather than appearing as pyramid with smooth sides
• The levels of a pyramid of energy are labeled producer, first consumer, second consumer, and so on
• The units used should be the amount of energy, per unit area, per year e.g. kJ m^-2 year^-1
• As you move up the pyramid to higher trophic levels, the quantity of energy decreases as not all energy is transferred to the biomass of the next trophic level (roughly 10 % of the energy is passed on)
• Energy is lost at each trophic level due to
  • Incomplete consumption
  • Incomplete digestion
  • Loss of heat energy to the environment during respiration
  • Excretion of the waste products of metabolism e.g. carbon dioxide, water, and urea
• As a result of this, the biomass at each trophic level will also decrease as energy availability decreases

Pyramid of Energy Diagram

The energy stored in the biomass of organisms can be represented by a pyramid of energy