When you are calculating the amount of energy that is transferred from one trophic level to the next, you will need to calculate a percentage of the energy at the first tropic level to calculate how much is transferred to the next level. Before we do an example, let's explore percent. PercentPercent, literally translated, means per 100. So if you scored 50% on your Biology assignment, you got 50 out of every 100 questions correct. You probably would have no trouble also identifying this as half. We can write this in a variety of ways. We can also express this as a decimal by dividing 50 by 100. What if your test had 379 questions on it. How many did you get correct? NOW! How can we use this to calculate the energy passed on to each successive tropic level?! Let's say that an there is 56790 kJ of energy help in the first tropic level of an ecosystem. In this ecosystem, 12% of the energy gets conserved or passed on to the next level. (Remember the rule of 10 is a guide, when a percentage of conserved energy is not in the question, use 10%). So in the second trophic level, there is 6814.8 kJ of energy. To calculate how much is in the third level, we would calculate 12 % of 6814.8 kJ and so on.
Ecologists have a number of ways of measuring and representing ecosystems. You will see a few of those in Biology 20. Conveniently, many of them end up showing an ecosystem in the form of a pyramid. What are each of these pyramids and what are they showing about the ecosystem?
By Thompsma (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons EnergyAn energy pyramid shows the available energy at each tropic level. Notice that although some of the pyramids of numbers and biomass are inverted (have a small base), the pyramid of energy can never be inverted. This is due to the first law of thermodynamics and the rule of 10.
Firstly, the Rule of 10 is not a rule! It is really more of a trend that scientists noticed. The trend is that the available energy to a tropic level is usually about 10% of the total energy available to the tropic level before it. Why? Well, all organisms have to use some of the energy that is available to maintain life. Plants use energy to grow and reproduce, as do animals. In addition, animals use energy to move and maintain body functions. As they use the energy they consumed to do useful things, they also produce heat. Heat is not a usable form of energy for living organisms, so it is lost to the surroundings. The important thing to remember is that this 'rule' applies ONLY to energy. Mass does not demonstrate the same 'rule'. We know this because there is no such thing as an inverted energy pyramid but it is possible to have an inverted biomass pyramid. Although it is possible to calculate the mass that an organism would gain due to consumption of a specified mass of food, it is a much more complicated calculation than you have been taught. You will only be required to use the basic calculation as shown here.
Lastly, remember that 10% is a generalization. Each energy relationship has its own percentage of energy transfer that can be calculated, it is just around 10%. However, some questions will ask you to calculate the energy available at the next tropic level if the energy transfer is 13%. So now you need to calculate 13% of the original energy rather than 10. Do the math! |
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