Energy Flow

Introduction

• • Energy flow is the flow of energy through living things within an ecosystem
  • The flow of energy in ecosystems is vitally important to the thriving of life on Earth
  • Nearly all of the energy in Earth’s ecosystems originates within the Sun. Once this solar energy reaches Earth, it is distributed among ecosystems in an extremely complex manner
  • Energy is the basic force responsible for all metabolic activities; and this flow from producer to top consumers is called energy flow, and is unidirectional in nature
  • The unidirectional flow of energy and the successive loss of energy as it travels up the food web, are patterns in energy flow that are governed by thermodynamics, which is the theory of energy exchange between systems

Producers are the energy gateway

• • Plants, algae, and photosynthetic bacteria act as producers
  • Producers are autotrophs, or “self-feeding” organisms, that make their own organic molecules from carbon dioxide
  • The energy stored in organic molecules can be passed to other organisms in the ecosystem when those organisms eat plants (or eat other organisms that have previously eaten plants)
  • In this way, all the consumers, or heterotrophs (“other-feeding” organisms) of an ecosystem, including herbivores, carnivores, and decomposers, rely on the ecosystem’s producers for energy.
  • If the plants or other producers of an ecosystem were removed, there would be no way for energy to enter the food web, and the ecological community would collapse
    • That’s because energy isn’t recycled: instead, it’s dissipated as heat as it moves through the ecosystem, and must be constantly replenished.

Energy Flow Illustration

Terms associated to understand the flow of Energy

• • In ecology, productivity is the rate at which energy is added to the bodies of organisms in the form of biomass
  • Biomass is simply the amount of matter that’s stored in the bodies of a group of organisms
  • Productivity can be defined for any trophic level or other group, and it may take units of either energy or biomass.
    • Gross primary productivity, or GPP, is the rate at which solar energy is captured in sugar molecules during photosynthesis (energy captured per unit area per unit time). Producers such as plants use some of this energy for metabolism/cellular respiration and some for growth (building tissues)
    • Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it’s the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem

How does energy move between trophic levels?

• • Energy can pass from one trophic level to the next when organic molecules from an organism’s body are eaten by another organism.
    • However, the transfer of energy between trophic levels is not usually very efficient
  • Plants typically capture and convert about 1.3-1.6% of the solar energy that reaches Earth’s surface and use about a quarter of the captured energy for metabolism and maintenance
    • So, around 1% of the solar energy reaching Earth’s surface(per unit area and time) ends up as net primary productivity
  • On average, only about 10% of the energy stored as biomass in one trophic level (e.g., primary producers) gets stored as biomass in the next trophic level (e.g., primary consumers). Put another way, net productivity usually drops by a factor of ten from one trophic level to the next
  • Why is energy transfer inefficient?
    • There are several reasons
      • One is that not all the organisms at a lower trophic level get eaten by those at a higher trophic level.
      • Another is that some molecules in the bodies of organisms that do get eaten are not digestible by predators and are lost in the predators’ faeces. The dead organisms and faeces become dinner for decomposers.
      • Finally, of the energy-carrying molecules that do get absorbed by predators, some are used in cellular respiration (instead of being stored as biomass)