The Big Picture

Biological productivity and energy flow are important quantities for ecologists to measure because they determine the rate at which biological resources such as trees are naturally replaced. For example, in the Case Study, two methods of forest harvest are discussed: (1) massive clearcutting was used in Michigan during 1860 - 1920, eventually causing nearly 99% of the original 7.7 million ha to become deforested permanently; and (2) the selective harvest on 14 % of a small 32.4 ha tract of woodland, which was maintained indefinitely in medieval England. The difference between the two methods was not only one of scale, but the degree to which the natural tree replacement rate was exceeded by the harvesting. An analogy can be drawn with a bank account, in which the harvest of tree biomass is equivalent to withdrawals, and the biological production is equivalent to increases due to interest accumulation on the biomass of trees present. In the Michigan situation, the account became overdrawn (withdrawals exceeded interest production). The result is a "stump forest" landscape that will not regrow into another forest. It is important to measure the rate of biological production so that we understand what sorts of biological limits will be placed on human harvests. In this chapter, the authors explain how ecologists estimate biological production and trace energy flow through an ecosystem. Other related topics to be discussed include: how much energy can be transferred between trophic levels; what the major physical laws of thermodynamics are and how they constrain energy flow in ecological systems; how ecosystems are one-way and open dynamic systems; how much energy is stored in biomass within the ecosystem; and how much energy is lost to randomness or heat in the system.

Frequently Asked Questions

• Simply stated, energy is the ability to do work or to move matter. It cannot be felt or observed, except as light or heat. Heat is a low quality (disordered) form of energy.
• Energy is a difficult concept to explain, yet everyone knows this word. In fact, most people expend a great deal of their income on energy in the form of electrical power, natural gas, or oil

• Energy contained in organic matter can be measured by burning it and measuring the heat released. This is often done by placing a sample of known mass in a bomb calorimeter, a device that can be completely sealed and is insulated to prevent heat loss. A thermometer is placed inside (but it can be read from the outside) and the increase in temperature after the sample is "bombed" or burnt completely is measured. From this device, a series of measurement standards have been developed.
• Here are some standard units of energy:

• kilocalorie (or kcal) = 1000 calories = the energy required to raise 1 kg of water 1.0 ^oC. (Note: most food cartons and diet books incorrectly use the term "calorie" when they are in fact referring to kcal).

• kilojoule (or kJ) = 1000 Joules = 0.24 kcal. (The kJ is preferred over kcal as a scientific unit of energy)

• Here are some typical organic materials in an ecosystem and their energy content per gram:

• Thus, we can calculate the energy content of everything in an ecosystem.

• Biomass is simply the mass of something that is or was alive. The mass is the quantity of matter in an object, or its weight. It is normally measured in grams (1 g = 0.035 oz.) or kilograms (1 kg = 1000 g = 2.2 lb.)
• Strictly speaking, mass and weight are different measurements. Weight is a function of gravity as well as mass; in space people are "weightless" but they still contain the same mass. On the surface of the Earth, mass and weight are identical numerical quantities.
• For ecological studies it is often easier to measure biomass when taking samples from an ecosystem. Later, biomass values can be converted to energy values using a table of conversion factors based on bomb calorimeter studies as described above.

• Photosynthesis is the process by which photoautotrophs make energy from sunlight. It can be written as a chemical equation:

• This equation can be stated in words: "Six molecules of carbon dioxide combined with six molecules of water produces one molecule of glucose and six molecules of oxygen gas".
• Photosynthesis must take place under direct sunlight in the autotroph’s tissues and chlorophyll or another photosynthetic pigment must be present for the reaction to occur. In this process, light energy is converted to chemical energy, which is stored in the bonds of the glucose molecule.

• Cellular respiration is the process by which cells breakdown the glucose needed to provide energy for the cells. It is essentially the reverse of photosynthesis:

C₆H₁₂O₆ + 6O₂ --------------> 6CO₂ + 6H₂O

• In this process, energy is released for the cell to do work.

• Biological production is the amount of biomass in g or energy in kJ produced in an ecosystem per unit area and per unit time.
• There are two types of biological production: primary production (autotroph production) and secondary production (heterotroph production).

• This is energy production by the autotrophs in an ecosystem. There are two kinds of primary production: gross primary production and net primary production.
• Gross primary production and net primary production are related in the following formula:

gross primary production - respiration = net primary production

or

gross primary production = respiration + net primary production

(These two equations are equivalent).

• Respiration is the energy used by cells in the autotrophic organism. The autotrophs produce more than enough energy for their cells respiratory needs, so gross primary production (GPP) is normally greater than respiration (if it is less than respiratory energy needs, the autotroph will not grow and will eventually die).
• Thus, the net primary production (NPP) is the difference between the energy produced in GPP and the energy used in respiration. An analogy can be made with a business, in which the gross production is like the gross income for the business and the respiration is analogous to the fixed costs of conducting the business (rent, labor, taxes, etc.). The difference between gross income and the fixed costs for any period is the net income (or profit), which is analogous to net primary production in our ecosystem.
• In practice, ecologists measure GPP using the second equation above by calculating the net primary production, which is measured as a change in biomass per unit time, and adding that to a measurement of respiration:

net primary production = B₂ - B₁

where

B₁ is the autotroph biomass at time 1

B₂ is the autotroph biomass at time 2

• Respiration measurements can be obtained by conducting carbon dioxide or oxygen uptake studies.

• Secondary production is the amount of production per unit area and per unit time for heterotrophs. When heterotrophs consume autotrophs, the population and individual bodies of the heterotrophs will grow.
• The growth rate of heterotrophs is a good measure of net secondary production.
• Because heterotrophs respire as well as autotrophs, net secondary production and gross secondary production are related in a similar way as in the primary production equations:

gross secondary production = net secondary production + respiration

net secondary production = B₂ - B₁

where

B₁ is the biomass of heterotrophs at time 1

B₂ is the biomass of heterotrophs at time 2

• Carbon storage is another unit for measuring biological production. Because carbon is included in molecules in the photosynthesis and respiration equations given above, and because of the fact that all organisms consume glucose to power there cells and make various other biomolecules from carbon in their bodies, carbon is a universal atom associated with life on Earth.
• Every organism stores carbon as it grows. It is thus a good measure of the photosynthesis and respiration rates, or the balance between them. Carbon can be easily determined, because it remains behind after drying a sample in a furnace and weighing the remaining ashes. Often, production rates are given as g of carbon/m²/year.

• Energy flow is the movement of energy through an ecosystem.
• Ecosystems are open systems, that is, they receive energy input from the sunlight, but that energy is eventually lost from the system. The energy is passed on to the autotrophs, to the heterotrophs and eventually the energy is dissipated as heat out into space.
• Organisms (including dead and partially decomposed ones that are responsible for the oil, coal, and natural gas reserves on Earth) can store this energy for a while, but eventually it all flows out into space again.

• Thermodynamics is the study of the movement of energy and heat through systems, and the conversion of light, chemical, and mechanical energy into heat and vice versa.
• This science is responsible for most of the technology we call heating, refrigeration and air conditioning systems. It is one of the most fundamental of all sciences, because it unifies physical, chemical, and biological principles.

• Energy can neither be created nor destroyed, but it may be converted from one form to another.
• For example, when the sun hits the sandy beach, it turns the sand warm. This is a thermodynamic conversion of light energy to heat energy. None of the sun’s energy is destroyed, but some of the sun’s rays are reflected back into space, some are converted to heat, some of the heat warms the air above the sand, etc.
• This law is also known as the Law of Conservation of Energy.

• In any conversion of energy from one form to another (say, light energy to chemical energy), some amount of energy will be dissipated as heat.
• Thus no energy conversion is 100 % efficient. This is why there are no perpetual motion machines.
• If we took a 1000 kJ candy bar and burned it in a closed bomb calorimeter, there would be more heat in the chamber after burning (the internal temperature would rise), but there would still be 1000 kJ of energy in the system. Because the total energy is constant, and the amount of energy in the form of heat is greater than before, there must be less chemical energy left in the candy bar. The candy would have been burned away, heat produced, and the chemical bonds in the candy would be broken. The candy bar itself would have less mass as well, but the mass would be conserved in the entire system because of the next law of physics.

• In any physical or chemical change, matter is neither created nor destroyed, but it may be changed from one form to another.
• In our candy bar example above, the molecules of sugar (glucose) in the candy would be broken apart by the burning, but they would end up as CO₂ in the air in the chamber. In fact, for every one glucose molecule (which has six carbon atoms) we would find six carbon dioxide molecules in the air (each of which has one carbon atom). So the carbon atoms would be conserved, and so would every other atom. Thus, no matter would be loss during this conversion of the candy bar into heat.

• Light energy and chemical energy are high quality energy, because the energy is concentrated in a small space. A little bit of light or chemical energy can do a great deal of work. The molecules or particles that store these forms of energy are highly ordered and compact, thus it is high quality energy.

• Heat is low quality energy. It can still be used to do work (think of a heater boiling water), but is much less compact and it rapidly dissipates. The molecules in which this kind of energy is stored (air and water molecules) are more randomly distributed than the molecules of glucose in our candy bar. This disordered state of the molecules and the dissipated energy requires a classification of low quality energy.

• Entropy is a measure of the randomness and disorder of any system.
• Entropy will increase over time, unless energy is expended to reverse the disorder. Think of the laundry in your room. Unless you expend energy (do work) to wash it and fold it, it will become more disordered over time. In a similar way, all systems, including ecosystems, are becoming more disordered over time.
• Ecosystems only retain an order at all because of the energy input from sunlight and the work done by the autotrophs to make glucose. If the sun were to stop shining (as it is predicted to do in 5 billion years), then the ecosystems on Earth would become totally disordered and cease to function.
• The increase in randomness and disorder over time is called "an increase in entropy".

• Energy efficiency is the ratio of output to input, or the amount of useful work obtained from a given amount of energy. After every energy conversion, some energy is converted into low quality heat energy and the entropy of the system increases.

What is trophic level efficiency?

• It would be interesting to measure the efficiency of the transfer of energy between trophic levels. To do this, ecologists measure trophic level efficiency, which is the ratio of amount of biological production at one trophic level divided by the biological production at the trophic level right below it in a food chain. Usually this is expressed as a percentage of the lower trophic level. A good formula to use is as follows:

Trophic level efficiency (%) =

Production at trophic level n-1

• For example, if the production of herbivorous rabbits is 1100 kJ/m²/year, and the production of the grasses they feed upon is 12,000 kJ/m²/year, then the % trophic level efficiency is:

12,000 kJ/m2/year

= 9.16 %

• Most studies of trophic level efficiency indicate efficiencies of 10 % or lower. The efficiency of transfer of solar energy to green plants is 1-3 %. Wolves feeding on moose in Isle Royale National Park have a trophic level efficiency of 0.01 %. In managed agricultural ecosystems, the trophic level efficiency may approach or exceed 10 %.

• Sometimes ecologists wish to know "What is the efficiency of the transfer of energy within a trophic level?". To estimate this, they use a measure known as growth efficiency. There are two kinds: gross and net growth efficiency.
• Gross growth efficiency (also called the gross production efficiency) is the ratio of the growth of a consumer (in g) to the food consumption by that consumer (also in g).
• For instance, it takes an average of 7 lb. of grains and human-edible plant material to produce 1 lb. of all livestock; this is a growth efficiency of approximately 14 %. For cattle, this growth efficiency is even lower: 6 % (16 lb. of feed for 1 lb. of meat). For chickens and eggs, it is 33 % (3 lb. of feed for every 1 lb. of eggs or meat). (Your textbook refers to these as trophic level efficiencies, but they are also referred to as growth efficiencies).
• Net growth efficiency (also called the net production efficiency) is the ratio of the growth of a consumer to the amount of food assimilated by the consumer (this is less than the amount consumed, because some food is never assimilated and is passed out as feces).

• Growth efficiencies are typically greater than net growth efficiencies (see Table 8.2). For example, whereas gross growth efficiencies are 8-27 % for terrestrial invertebrate herbivores (insects), net growth efficiencies are 20-40 %. Microorganisms have the highest net growth efficiency (~40 %), whereas terrestrial vertebrates have the lowest net growth efficiencies (2-10 %).

• The biological production and biomass at each higher trophic level decreases. This is due to several factors:
• 1) some of the energy is dissipated as heat and entropy increases at each energy transfer (the Second Law of Thermodynamics);
• 2) biomass is lost through respiration as organic carbon compounds like glucose are converted to carbon dioxide, which escapes into the atmosphere;
• 3) not all the available food at one trophic level is consumed by organisms at the next level, much of it instead going to the decomposers;
• 4) not all the food consumed is assimilated by the consumer, some of it passing out as feces.

• Some environmentalists argue that humans (especially many overweight Americans) eat too high on the food chain and that by eating more fruits, vegetables and grains we could feed more people in the world than we do now.
• In order to prove this to yourself, consider that trophic level efficiencies are typically less than 10 %. This means that 90 % (and usually much more than this) of the food at each trophic level is not transferred to higher levels.
• If you eat nothing but tuna (a top carnivore in marine food webs at level 5), then you are consuming a lot of your energy at trophic level 6 or higher. If instead you consumed catfish (an omnivore at level 3), you would get the same amount of energy at trophic level 4 or higher. How much more energy is unavailable for other humans by eating the tuna? Consider the following data:

• Thus, you have consumed 100 catfish dinners by consuming your one tuna dinner, because the primary production energy required to produce that tuna was 100 times greater than the primary production energy to produce the catfish (compare the energy levels at trophic level 1 in the table above).
• Thus, if you want to make more food available for others, you might choose catfish rather than tuna, because 99 more meals can be obtained with the same amount of primary production. This illustrates the concept of "eating lower on the food chain".
• However, there are other considerations, such a market price (tuna generally costs more than catfish), palatability (what tastes good to you), cultural factors (such as whether catfish or tuna has been a traditional food item in your culture), and availability (maybe there isn’t any catfish on the menu in the restaurant where you’re dining). You may wish to lower your intake of toxic chemicals such as mercury, which tends to biomagnify (see Chapter 14), and will be in a higher concentration in the tuna. In addition, eating less tuna does not automatically result in more catfish biomass -- the primary production of the ocean where the tuna feeds is not easily moved into a catfish farm (but some of it is, because catfish are being fed soybeans plus menhaden oil, from another kind of ocean fish. This is why catfish are considered to be at trophic level 3).
• What should you do? The choice is up to you, but at least consider the food chain level at which you eat. (To determine the trophic level at which you feed most of the time based on your own diet, please see Ecology in Your Backyard in Chapter 6).

Ecology In Your Backyard

• Measure primary production in your backyard. This procedure involves "destructive sampling", as ecologists refer to it, but you probably call it "mowing the grass"! In order to estimate net primary production on your lawn, we will have to measure the change in biomass over time. A good period over which to measure growth is two weeks. Cordon off a 1.0 m² area of your lawn (the area can be any size, but you should measure the area and the divide your biomass estimates by the area you select). Cut down the grass as close to the earth as possible (don’t worry, tell your landlord or parents that it will grow back). Use scissors, hand-held garden clippers, or if you are careful to save all the grass blades, a power mower. Weigh the grass blades you have cut on a postal balance or some other accurate balance (a bathroom scale can work well here if the area is large and your clippings weigh a lot). This represents the standing (above-ground) biomass on that plot of lawn. (We will ignore below-ground biomass and productivity for this experiment, because you would have to dig up all the roots.) Now wait two weeks for the grass to grow back and cut it again close to the ground to the same approximate level as before (you may choose a longer or shorter time interval depending on the rate of growth on your lawn, fertilizers used, soil nutrients, sunlight, water, etc.). Weigh the clippings again. This represents the net primary production in g/m²/2 weeks for your lawn (if you weighed in ounces, multiply by 28.35 to obtain the weight in g; if you weighed it in pounds, multiply by 453.6 to get g). You can multiply this value by 21 kJ/g to get the total energy in the grass clippings. You could also multiply these productivity values by 26 to get an annual production rate (there are 26 two-week periods in a year), but this would assume that growth you measured was constant across all seasons, which we suspect is not true. A better way would be to repeat the measurement on a new plot of lawn every two weeks for an entire year. You would find that the primary production slows down in the winter and is the greatest in the summer. You could also use this measure of net primary production to test the impact of various light levels, watering regimes, fertilizers, herbicides, pesticides, etc.

• Measure the growth efficiency of your dog, cat, or other pet. This is a harmless procedure which involves feeding your pet (you do that anyway!), and keeping careful notes about the pet’s changes in weight or biomass. You will need a balance or bathroom scale to weigh the food and the pet. A food scale used by people who diet or a postal scale can be used for weighing the food. You do not have to weigh every meal eaten by your pet, but try to feed standard servings (1/2 can of food, one scoop or bowl full of kibble, etc.), and weigh that once the first time you feed (you may wish to improve your accuracy by weighing several standard servings and take the average). Then later during the growth study you can simply count servings and multiply by this "average" serving weight. Try and weigh or estimate the weight of any uneaten food left behind. Let your animal feed on as much as they want, i.e., don’t restrict their diet other than you normally would. Your veterinarian may be able to help you get an accurate weight of your pet at the beginning and end of the study, so you can calibrate your home scale. This works best with puppies and kittens or other animals that are in their major period of growth; adult animals will show little growth, and so your growth efficiencies will be very small or zero (if they are significantly less than zero, i.e., -10 %, your pet might be sick and you should take them in to see a veterinarian). Fill out the following table (be sure to use the same biomass units throughout or convert to common units. If your animal is small, the use oz. or g; if it is large, then use lb. or kg):

• Please respond to these questions or send your thoughtful examples and comments to:

BackYard@wiley.com

Hardcopy Links In The Library

• Cousins, S. 1985. Ecologists build pyramids again. New Scientist. 4 July 1985. 106:
• Lindeman, R. L. 1942. The trophic-dynamic aspect of ecology. Ecology 23: 399-419. A classic paper that began the movement to study food webs and trophic interactions.
• Pimm, S.L., J. H. Lawton. and J. E. Cohen. 1991. Food web patterns and their consequences. Nature 25 April 1991, 350 (6320): 669.
• Polis, G. A. and K. O. Winemiller. 1996. Food Webs: Integration of Patterns and Dynamics. Chapman & Hall, New York. 472 pp.

Ecolinks On The Web

• http://www.cbl.cees.edu/~ulan/ntwk/network.html - Bob Ulanowicz’s Ecological Network Models website. Here you can view the computer network model of the food web found in Chesapeake Bay. The model can be downloaded and run on personal computer.

• http://ecuvax.cis.ecu.edu/~biluczko/stmarks/stmarks.htm - Ecological Network Analysis of The St. Marks National Wildlife Refuge Seagrass Food Web. A webpage about this foodweb research project conducted in Florida by Joe Luczkovich and Bob Christian at East Carolina University, with assistance from Dan Baird (University of Port Elizabeth, South Africa) and funded by the National Biological Survey.

• http://sfbay.wr.usgs.gov/ - This is the USGS San Francisco Bay and Delta Ecosystem Program website. Click on the Biology selection for information on primary production in the Bay.

• http://www-eosdis.ornl.gov/npp/npp_home.html - This is the Oak Ridge National Laboratory, Net Primary Production Database website. Site maps, sampling methods, data summaries, and program descriptions are provided.

• http://www.dieoff.org/page83.htm - This website consists of a paper on the influence of humans on global primary production.

• Note: If any of these links are not working, please see if alternative links are available at the Ecolinks Update Site.

Ecotest Online

1. The fact that energy losses from one trophic level to the next are about 90 percent is explained by the:

a. second law of thermodynamics.

b. law of conservation of matter.

c. first law of thermodynamics.

d. None of these choices are correct.

2. Of the following types of heterotrophs, which has the lowest net growth efficiency, according to the textbook?

a. microorganisms

b. invertebrates

c. vertebrates

d. All of these have similar net growth efficiencies

3. Calculate the trophic level efficiency of energy transfer between a population of rabbits that has a secondary production (or growth rate) of 2000 kJ/m²/year and a population of wolves that has a tertiary production of 10 kJ/m²/year.

a. 5 %

b. 10 %

c. 0.01 %

d. 0.5 %

e. 0.005 %

4. If the Gross Primary Production (GPP) for a marine ecosystem is 100,000 kcal/m²/year and the respiration for this ecosystem is 90,000 kcal/m²/year, what is the Net Primary Production (NPP) for this ecosystem?

a. 10,000 kcal/m2/year

b. 100 kcal/m2/year

c. 0.9 kcal/m2/year

d. 0.1 kcal/m2/year

5. Calculate the gross growth efficiency for snook population (snook are carnivorous fish that eat shrimp) that consumes 5000 kJ/m²/year of shrimp and grows 250 kJ/m²/year.

a. 25 %

b. 10 %

c. 5 %

d. 1 %

6. Why is biomass at the second trophic level in most ecosystems not as great at as biomass at the first trophic level?

a. all of these are correct

b. Not all plant material is eaten by herbivores.

c. Not all of the biomass eaten by herbivores is digested.

d. Some of the biomass is used to make cellular energy and ultimately is respired

7. If you have a choice between consuming a beef hamburger and a tunafish sandwich, and you want to eat "lower on the food chain", which would you choose?

a. hamburger

b. tunafish sandwich

c. neither one, because the are at the same trophic level

d. none of these are correct

8. The amount of randomness and disorder increases as ___________________ increases in a system.

a. energy

b. biomass

c. entropy

9. Which of the following contains the lowest quality of energy?

a. gasoline

b. sugar

c. a car battery

d. a mug of hot coffee, with no sugar

10. You have a group of rabbits and their total weight at the start of the year is 10,000 g. After the year ends, you weigh them again and find that they now weigh 15,000 g. What is the secondary production for the group during the year?

a. 66.7 %

b. 33.3 %

c. 50.0 %

d. 5,000 g/yr