Why is there a loss of energy at each trophic level

Understanding:

•  Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels

    
When energy transformations take place in living organisms the process is never 100% efficient

• Most of the energy is lost to the organism – either used in respiration, released as heat, excreted in faeces or unconsumed
• Typically energy transformations are ~10% efficient, with about 90% of available energy lost between trophic levels
• The amount of energy transferred depends on how efficiently organisms can capture and use energy (usually between 5 – 20%)

As energy is lost between trophic levels, higher trophic levels store less energy as carbon compounds and so have less biomass

• Biomass is the total mass of a group of organisms – consisting of the carbon compounds contained in the cells and tissues
• Because carbon compounds store energy, scientists can measure the amount of energy added to organisms as biomass
• Biomass diminishes along food chains with the loss of carbon dioxide, water and waste products (e.g. urea) to the environment 

Because energy and biomass is lost between each level of a food chain, the number of potential trophic levels are limited

• Higher trophic levels receive less energy / biomass from feeding and so need to eat larger quantities to obtain sufficient amounts
• Because higher trophic levels need to eat more, they expend more energy (and biomass) hunting for food
• If the energy required to hunt food exceeds the energy available from the food eaten, the trophic level becomes unviable

Representation of Energy Flow Along a Food Chain

Living things need energy to grow, breathe, reproduce, and move. Energy cannot be created from nothing, so it must be transferred through the ecosystem. The primary source of energy for almost every ecosystem on Earth is the sun. Primary producers use energy from the sun to produce their own food in the form of glucose, and then primary producers are eaten by primary consumers who are in turn eaten by secondary consumers, and so on, so that energy flows from one trophic level, or level of the food chain, to the next. The easiest way to demonstrate this energy flow is with a food chain. Each link in the chain represents a new trophic level, and the arrows show energy being passed along the chain. At the bottom of a food chain is always the primary producer. In terrestrial ecosystems most primary producers are plants, and in marine ecosystems, most primary producers are phytoplankton. Both produce most the nutrients and energy needed to support the rest of the food chain in their respective ecosystems.

All the biomass generated by primary producers is called gross primary productivity. Net primary productivity is what is left over after the primary producer has used the energy it needs for respiration. This is the portion that is available to be consumed by the primary consumers and passed up the food chain. In terrestrial ecosystems, primary productivity is highest in warm, wet places with plenty of sunlight, like tropical forest regions. In contrast, deserts have the lowest primary productivity. In marine ecosystems, primary productivity is highest in shallow, nutrient rich waters, such as coral reefs and algal beds.

To show the flow of energy through ecosystems, food chains are sometimes drawn as energy pyramids. Each step of the pyramid represents a different trophic level, starting with primary producers at the bottom. The width of each step represents the rate of energy flow through each trophic level. The steps get smaller further up the pyramid because some of that energy is changed to a form that cannot be consumed by organism at the next higher step in the food chain. This happens at every step of the pyramid.

Not all of the energy generated or consumed in one trophic level will be available to the organisms in the next higher trophic level. At each level, some of the biomass consumed is excreted as waste, some energy is changed to heat (and therefore unavailable for consumption) during respiration, and some plants and animals die without being eaten (meaning their biomass is not passed on to the next consumer). The waste and dead matter are broken down by decomposers and the nutrients are recycled into the soil to be taken up again by plants, but most of the energy is changed to heat during this process. On average, only about 10 percent of energy stored as biomass in a trophic level is passed from one level to the next. This is known as “the 10 percent rule” and it limits the number of trophic levels an ecosystem can support.

There is a loss of energy as we go from one trophic level to the next; this limits the number of trophic levels in a food chain.

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