Introduction to Energy Flow
Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
Plants and photosynthetic bacteria (autotrophs) fix the sun’s radiant energy to make food from simple inorganic materials. Plants capture only 2-10% of the PAR, and this small amount of energy sustains the entire living world.
Energy Flow in Ecosystems
All organisms are dependent on producers for their food, either directly or indirectly. This creates a unidirectional flow of energy from the sun to producers and then to consumers.
Thermodynamics and Ecosystems
Ecosystems follow the Second Law of Thermodynamics and require a constant supply of energy to counteract universal disorderliness. This energy is needed to synthesize molecules and maintain system function.
Detritus Food Chain
The detritus food chain begins with dead organic matter. Decomposers such as fungi and bacteria break down this matter into simple inorganic substances, which are then used by other organisms:
- Dead Leaves (Detritus)
- Fungi/Bacteria (Decomposers)
- Earthworms (Detritivores)
- Small Animals (Secondary Consumers)
In terrestrial ecosystems, a significant amount of energy flows through this chain due to the high amount of organic matter. In aquatic ecosystems, the grazing food chain usually has a more prominent role.
Producers in Ecosystems
In terrestrial ecosystems, major producers are herbaceous and woody plants. In aquatic ecosystems, producers include phytoplankton, algae, and higher plants.
Food Chains and Webs
Food chains and webs are formed as animals feed on plants or other animals, creating interdependencies. Energy trapped in producers is passed to consumers or converted to detritus upon death, forming the detritus food chain/web.
Grazing Food Chain
A simple grazing food chain (GFC) example is: Grass → Goat → Man. The GFC is the major conduit for energy flow in aquatic ecosystems.
Detritus Food Chain
The detritus food chain (DFC) starts with dead organic matter. Decomposers like fungi and bacteria degrade this detritus, obtaining nutrients and energy. The DFC is crucial in terrestrial ecosystems, where it often represents a major energy flow pathway.
Comparison of Energy Flow
Energy flow differs between ecosystems:
- Terrestrial Ecosystems: A larger portion of energy flows through the detritus food chain. Decomposition of organic matter is significant for nutrient cycling.
- Aquatic Ecosystems: The grazing food chain often dominates, with primary producers like phytoplankton being the key energy source.
This variation is due to the differences in organic matter availability and the rate of decomposition between terrestrial and aquatic environments.
Summary and Conclusions
Energy flow through ecosystems is governed by the principles of thermodynamics and ecological interactions. The comparison between terrestrial and aquatic ecosystems highlights the varying roles of detritus and grazing food chains in energy transfer.

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