Trophic Level Energy Transfer Calculations

Energy Source

The source of energy for Earth is our sun. Sir Issac Newton thought of light as a steady stream of particles. His contemporary, Christian Huygens, thought that light had a wave motion. They were both correct. Light energy consists of moving quanta (packets) of photons (pure energy). In 1802 the English physicist Thomas Young confirmed Huygen' hypothesis by proving that light traveled in waves.

Visible light is measured in nanometers, from low frequency red, through orange, yellow, green, and blue, to high frequency violet. Rainbows are produced by the natural prism of raindrops in the atmosphere.

Electromagnetic wavelengths take approximately 8.34 minutes, traveling at the speed of light, to reach the earth. Estimate the distance of the sun from earth in miles if the speed of light is approximately 186 000 mi/sec.

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Energy Capture

Photosynthesis is the major process by which wavelengths are converted from light energy into a form of chemical energy which can be used by living organisms. This conversion process will be studied later in the semester. The energy from the sun is stored in the bonds that hold molecules together. One major storage source of this energy is the monosaccharide glucose which can be produced as a by-product of photosynthesis. When glucose is broken apart in the process of cellular respiration (studied later) it can be made available for use by organisms in all metabolic activities.

Energy Transfer

The purpose of this exercise is to understand how energy stored in molecules like glucose, proteins, and lipids can be transferred from one trophic level to another. To understand the concept of energy transfer one must first understand some of the laws of thermodynamics.

Laws of Thermodynamics (in layman's terms)

1st Law - Energy can neither be created nor destroyed.
2nd Law - No transfer of energy is 100% complete

The first law is self-explanatory. The second law tells us that it would not be possible for a deer eating grass to obtain 100% of the energy contained in the grass and incorporate it into deer tissue. Instead there are losses. In this exercise you will see that the total amount of energy with be accounted for in three ways.

1. Energy incorporated in tissues. This energy will be available for the next trophic transfer.
2. Energy of metabolism. This is the energy required for conscienceness, muscle contraction, nerve impulse transmission, movement of molecules across membranes, and any other activity requiring energy. You will see that most of the energy will go to metabolism.
3. Energy which remains at death [organic waste] will be harvested by bacteria and fungi, the major decomposer organisms.

Trophic Levels

When light energy strikes the earth three things happen. Either it is relfected back into space, it is transmitted through an object, or it can be absorbed. In biology we are most interested in that energy which is absorbed and captured in photosynthesis.

Autotrophic (photosynthetic) organisms are responsible for the initial conversion of light energy into chemical energy. Here is how that is possible?

Depending on a specific latitute and longitude a specific amount of light energy is captured per square meter per year (m2/yr) on average. In tropical regions, where there is the greatest input of light, measured in KCal of energy/m2, results in a high energy availablity. This expresses itself in stunning biodiversity. If one travels to polar regions, much less solar input results in fewer KCal of energy input/m2/yr. This is the reason that biodiversity is greater near the equator and sparse near the poles.

Once energy is captured by autotrophs (producers), it is available for cycling through other trophic levels. Here are the trophic levels beginning with the highest energy content to the lowest.

Primary consumers (herbivores)
Secondary consumers
Tertiary consumers
Quaternary consumers

10% Rule

In this exercise you will be working with statistical data which has been derived from studies made at particular geographic locations. Sometimes this kind of information is not available and an educated guess needs to be made on energy transfer from one level to the next. In this case the "Ten Percent Rule" is used. It is assummed that only 10% of the orginial energy will be completely transfered to the next trophic level and incorporated into tissue.

So, if we begin with 1 000 KCal of energy incorporated into plant tissue via photosynthesis, only 100 KCal would be available to the primary consumer, 10 KCal to the secondary consumer, 1 KCal to the tertiary consumer, and only 0.1 KCal to the quaternary level. One can quickly see why it is impossible for very many trophic level transfers to exist. There simply would not be enough energy available to support the next level. This is the reason that trophic levels typically do not go beyond secondary levels in the arctic, and could easily extend to the quaternary level in the tropics.


How to Calculate Trophic Level Energy Transfers

As discussed earlier (thermodynamics) three things happen to energy in a trophic level transfer. Some energy is incorporated into tissue at the next level, most is lost in metabolism, and some remains for harvesting by decomposers in the form of organic waste.

When calculating energy transfers always make certain that you can account for all of the KCal of energy. Also, remember that when you have finished with one trophic level you no longer have the original amount of energy - only what was transferred and incorporated into tissue which could be consumed at the next trophic level. If this still seems a little difficult to understand, consider the following example and do the calculations yourself to confirm each answer shown and it will soon become clear.


Assume that there are 1 620 000.0 KCal/m2 that strikes in Lodi, California when measured by scientific instruments for a year's time. Further assume that 98.7% of this energy is reflected light and that the remainder is captured as absorbed energy and processed by photosynthesis. For the purpose of this problem we will ignore transmitted light. Based on these assumptions answer the following questions:

1. How many KCal of energy is represented by reflected light?

2. How many KCal of energy is captured in photosynthesis?

Now you will do some calculations which demonstrate how energy is cycled.

You will be presented with three calculations which will add up to 100% of the energy input into the system based on our assumptions. The energy will be directed in three pathways. 1) % incorporated into tissue which can be passed to the next trophic level by predation. 2) % directed to metabolic activities. 3) % of energy remaining in the organism at death which will be harvested by decomposers (bacteria & fungi).

2 317.0 KCal = ______ % available for transfer to herbivore

This calculation for % transferred is done by dividing 2 317.0 KCal by the total available from photosynthesis which you calculated earlier - 21 060.0 KCal

14 110 .0 KCal = ______ % plant metabolism

4 633.0 KCal = _______ % plant organic waste

Make certain that your percentages add up to 100%

162.0 KCal = _______ % transferred to carnivore

All three of these calculations use 2 317.0 KCal as the divisor since this is the source of energy for this trophic level.

1 483.0 KCal = ______ % herbivore metabolism

672.0 KCal = ________ % herbivore organic waste

21.0 KCal = ______ % transferred to next carnivore

All three of these calculations use 162.0 KCal as the divisor since this is the source of energy for this trophic level.

113.0 KCal = ______ % carnivore metabolism

28.0 KCal = _______ % carnivore organic waste

0.0 KCal = __0____ % no trophic level exists above this one

All three of these calculations use 21.0 KCal as the divisor since this is the source of energy for this trophic level.

16.0 KCal = ______ % carnivore metabolism

5.0 KCal = _______ % carnivore organic waste

Now calculate the amount of energy in KCal which can be attributed to harvesting by decomposers.

To do this we need to understand the concept of where the energy came from for all of these calculations to begin with. Remember, the energy which was the source for all of the calculations that you have done so far was the amount converted in the process of photosynthesis. That was 21 060.0 KCal/m2. This then will be the divisor for these calculations.

Calculate the amount of energy converted by all decomposers in this problem.