PEE-Module-2.docx

**Module 2** **Energy Flow in the Ecosystem** **Learning Objectives** 1. Compare and contrast food chain with food web 2. Identify the feeding relationship of organisms 3. Draw a representative food web 4. Describe the energy pyramid **Introduction** The ultimate source of energy in the...

**Module 2** **Energy Flow in the Ecosystem** **Learning Objectives** 1. Compare and contrast food chain with food web 2. Identify the feeding relationship of organisms 3. Draw a representative food web 4. Describe the energy pyramid **Introduction** The ultimate source of energy in the biosphere is the sun. the energy that comes to us from the sun is in the form of radiation usually light and heat energy. Plants use light in photosynthesis while heat causes movement of molecules. The flow of energy in an ecosystem starts with the fixation of sunlight by plants through the process of photosynthesis. The energy accumulated by plants is called primary production since it is the first step in energy storage. The product of photosynthesis and minerals from the soil are used by plants to build biomass which represents the amount of energy and nutrients available for the macro and micro consumers in the ecosystem. **Food Chain** We discuss earlier the relationship of the different biotic factors in the ecosystem in terms of their role. Plants are the primary producers called photoautotrophs as they are capable of transforming light energy from the sun into chemical energy via photosynthesis. In the absence of light, chemosynthesis is used by chemoautotrophs. The stored energy by the autotrophs or primary producers is passed on the heterotrophs or consumers. The diagram below: The sequence of organisms feeding on each other constitutes what we call a food chain. Food chain is a simplified illustration of the path of energy and nutrient transfer in the ecosystem. Organisms rarely feed on only one species or type of food. Omnivores for example feed on both plants and animals. Most often, consumers feed on more than one species of food. Thus, species tend to have more predators. Starting from the producers, several food chains can be traced. Those that feed on primary consumers are called secondary consumers and so on to the topmost organism. **Food Web** As I mention earlier, consumers feed on more than one organism or species, thus each species tends to have more predators in the ecosystem. Thus, food chains are interlocking and woven into complex associations called food webs.  There are two types of food webs: the ground or grazing food web and soil-based or detritus food web. Grazing food web is usually composed of large plants and animals familiar to people. Detritus is a collective term for all organic debris or once part of living organisms, such as twigs, leaves, barks that had fallen off and decompose or break these matter down deriving nutrition from them, thus, they are called detritivores. **Energy Pyramid** The energy pyramid depicts the production, use and transfer of energy from one trophic level or another. It shows that as you go higher the trophic levels, the amount of energy decreases. This is generally, only about 10% to 30% of the total energy content of the lower trophic level is transferred to the next level. Much is lost as heat during respiration. The maintenance cost of the organism is about 70% to 90%. If these are the amount of energy removed from the lower trophic layer, then very small number of species can be supported at the top. Thus, the more levels that exist between a produce and the top-level consumer in an ecosystem, the less energy remains from the original amount. This explains why food chains are generally short.