Food Chains and Trophic Pyramids in Ecosystems

Food Chains and Trophic Levels

Living beings require material and energy, obtained from nutrients in food, to perform vital functions such as nutrition, relationships, and reproduction. Living things are classified into three groups or trophic levels:

• Producers: These are autotrophic organisms that produce organic matter from inorganic substances. Most are photosynthetic, using sunlight to convert water, mineral salts from the soil, and carbon dioxide from the atmosphere into organic material. This level is formed by plants, algae, and certain bacteria.
• Consumers: These are heterotrophic organisms that feed on organic matter from other living beings. Consumers are categorized into types:
  • Primary Consumers: They feed on producers. Herbivores are primary consumers.
  • Secondary Consumers: They feed on primary consumers. Carnivores are secondary consumers.
  • Tertiary Consumers: They feed on both primary and secondary consumers.
• Decomposers: These organisms utilize organic matter from the previous trophic levels, transforming it into inorganic nutrients. These inorganic nutrients are then reused by producers. Fungi and bacteria are examples of decomposers.

Trophic Relationships

Trophic relationships are established between organisms that feed on each other. Within an ecosystem, these relationships can be represented by food chains and food webs, which illustrate the sources used by different organisms to obtain the matter and energy they need.

Trophic Pyramids

A trophic pyramid is a graphical representation of the variation between different trophic levels for a particular characteristic.

• Pyramid of Numbers

  Each level represents the number of individuals within that trophic level, per unit area or volume. These pyramids provide limited information as they consider very different organisms within a trophic level equally. In some cases, inverted pyramids can occur, where the number of individuals at one level may be greater than at the lower level.

• Pyramid of Biomass

  On each level, this pyramid represents the biomass of each trophic level at a given time. Biomass is the total amount of organic matter that forms an individual, a trophic level, or an ecosystem. Biomass is measured in grams or kilograms of dry organic matter per unit area or volume. This type of pyramid provides useful information on the structure and functioning of the ecosystem. In some marine ecosystems, inverted pyramids can occur, where the biomass of producers is lower than that of higher levels.

• Pyramid of Energy

  In these pyramids, each level represents the energy stored in one trophic level at a given time, and which is available for higher trophic levels. Their values are expressed in units of energy (kJ or kcal) per unit area or volume per unit time. These are the pyramids that provide the most information and clearly show the flow of energy between different trophic levels.

  In the food chain, there is always a loss of usable energy. Each trophic level typically transfers only about 10% of the energy available from the lower trophic levels. Energy pyramids are usually not inverted, as the energy at a lower level must always be greater than that at a higher level.