Energy Flow

<h1>Text Preview</h1> <p>Within an ecosystem, the primary producers support all other organisms. Without autotrophs, there would be no way to convert solar energy into chemical energy, and all heterotrophic organisms would die. We've discussed how food moves through an ecosystem. Now let's see how this dictates energy flow through an ecosystem.</p> <p>In almost all ecosystems, the energy originally enters as light from the sun. Energy can't be recycled, so it must be constantly supplied. Primary producers convert the solar energy to chemical energy through photosynthesis. The total energy in an ecosystem depends on the photosynthetic activity of the primary producers.</p> <p>Each day, around 10 to the 22nd joules of solar energy reach the Earth's atmosphere. Most of the solar radiation is absorbed or reflected by the atmosphere. The small amount that reaches the planet is mostly absorbed or reflected by bare ground and water. Only a small fraction of energy reaches photosynthetic organisms. Of that, only 1 to 2 percent of the solar energy is converted to chemical energy. Even with all of the lost energy, Earth's primary producers produce around 170 billion tons of organic material each year.</p> <p>Some ecosystems convert more solar energy than others. As an indication of how efficient a particular ecosystem is at converting solar energy, we can calculate its primary productivity. Primary productivity is the amount of light energy converted to chemical energy in a given amount of time. </p> <p>Not all of the chemical energy produced is available to be passed on to primary consumers. Primary producers use some of the energy themselves for cellular respiration. The total primary productivity of primary producers is called the gross primary productivity. Gross primary productivity minus the energy used by the primary producers is the energy available, or net primary productivity. As we can see, energy is lost as it passes through the primary producers.</p> <p>Primary productivity is often expressed as a rate: the mass of vegetation produced per unit time, grams per square meter per year, which is known as biomass. Biomass is usually calculated as dry mass, because the energy in water is not usable energy. It is important to remember that biomass is a rate. Don't confuse it with standing crop biomass, which is the dry crop mass at a particular time.</p> <p>Different ecosystems have differences in primary productivity and how much they contribute to the total primary productivity on Earth. An ecosystem's contribution depends on both the biomass and the size of the ecosystem. For example, beds of algae have a very high net primary productivity, but they only cover a tiny portion of the Earth, so their overall contribution is small. The open ocean has a very small net primary productivity, but it covers around 65% of the Earth, so its overall contribution is great.</p> <p>Now that we've looked at the energy transfer by primary producers, let's explore what happens when the energy is transferred to consumers. The rate at which consumers convert chemical energy from food into new body mass is called secondary productivity. </p> <p>Energy is lost between each trophic level. Organisms use some of the energy they get from their food for cellular respiration. This energy is lost from the ecosystem. Energy is also lost in the feces. This energy remains in the ecosystem, thanks to the detritivores. Only the energy stored as new body mass at each trophic level is available for the next level. Most of the chemical energy from primary producers is never consumed by primary consumers. This is why when you look at most ecosystems, they are green from the plant material that isn't consumed.</p> <p>To keep track of how much energy is lost between each level, we can look at ecological efficiency, the percentage of energy transferred from one trophic level to the next. Most organisms have an ecological efficiency between 5 and 20%. This means that 80 to 95% of the total energy is not transferred from level to level. We can show the energy loss between levels graphically.</p> <p>In a pyramid of productivity, the biomass of each level is shown as a block. The blocks for the different levels are stacked to form a pyramid. A pyramid of productivity is generally very bottom-heavy because of the inefficient transfer of energy between levels. If we assume an ecological efficiency of 10% at each level, we see how quickly the biomass decreases. Tertiary consumers have only 0.1% of the biomass of primary producers.</p> <p>The energy loss can also be represented by the standing crop biomass of each level, called a biomass pyramid. A biomass pyramid has the same basic shape as a pyramid of productivity. A third type of pyramid can be used to demonstrate energy loss between levels.</p> <p>In a pyramid of numbers, each block represents the number of individuals in each level. Since energy is lost between levels, fewer organisms are found in each level as we move up the pyramid. In an ecosystem, there are far more primary producers than high level consumers.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>