Text from Kendall Hunt Global Science 7^th Ed.

Energy Flow in Nature and Human-Built Systems:

The energy flow in natural ecosystems is governed by the same laws that operate in human-built systems. One of these laws is that energy can be changed from one form to another. Energy must be transformed (changed) to drive either living or human-built systems. The chemical energy stored in carbon compounds must be changed into mechanical energy of movement. (You can’t see chemical energy because it is stored in the chemical bonds between atoms. This energy is released when the bonds are broken). This is true whether we are talking about the food that moves a person or the gasoline that powers a car. A second law is that the total amount of energy never changes. With living things, energy is lost to the environment as heat. This is much like burning fuel to run an engine, or using electricity to light a bulb.

In addition, efficiency is just as important in natural systems as it is in the energy systems people have created. The same principles apply. No conversion is ever 100% efficient. Some energy is always lost (as heat). The final efficiency of a series of energy changes is the product of all the intermediate efficiencies. Efficiency refers to how much of the energy that is used goes toward doing something useful. If a lot of energy does useful tasks, it is an efficient system. If a lot of energy is just lost as heat, the system is not efficient.

Figure 3.2 and 3.3 have more in common than may seem apparent at first. Can you see how they show the same idea? Figure 3.3 starts with a certain amount of the energy that arrives on the surface of a green leaf. It is captured in the sugar molecules that plants produce through the process of photosynthesis. Through photosynthesis, a plant converts solar energy to chemical energy. The chemical energy is stored in the plant’s cells. Plants (and animals) can use this energy to perform their daily functions. Special enzymes break down the energy compounds, which release the energy for the body’s needs.

Figure 3.2: Diagram of an electric power plant showing how energy flows from coal to alternating current. (source: http://holbert.faculty.asu.edu/eee463/coalart.gif)

Figure 3.3: Simple food chain showing how energy flowers through various levels. (source: http://www.field-studies-council.org/urbaneco/images/011-food-chain.jpg) **At each step, heat is transferred out of the system.

But photosynthesis is nowhere near 100% efficient. Whatever energy is trapped in this way is all the energy that the ecosystem has. It is the living equivalent of a full tank of gas. If the tank were not constantly refilled by the sun and the action of green plants, the ecosystem would “run out of gas” very quickly.

When a primary consumer eats a plant, energy is transferred and transformed. The chemical energy stored in the plant food materials is changed to kinetic energy. This energy helps move the organism, and heat energy is released into the environment. Some of the plant’s energy is stored in the bones and tissues of the animal.

If that animal is eaten, its energy becomes available to the secondary consumer (or carnivore) that ate it. But energy is lost at each transfer. As a result, the total amount of energy available to secondary consumers is always less than what was available to primary consumers. Likewise, the total amount of energy available to primary consumers is always less than what came from the sun, or even what the plants trapped through photosynthesis.

A food chain is the transfer of energy form one organism to another. Green plants, the producers, transfer a certain portion of their energy to the herbivores or primary consumers such as rabbits or cows. However, a major portion of the energy (often as much as 90%) is lost in the transfer, usually in the form of heat energy. At each of the steps that follow in the food chain, more energy is lost. This is one of the reasons there are not very many animals at the top of the food chain. It takes a lot of energy to support them. This also explains why animals spend much of their time eating and behaving in ways that conserve energy.

These relationships are sometimes drawn as an energy pyramid. An energy pyramid shows producers on the bottom and secondary or tertiary consumers on top (see Figure 3.4). Food chains usually become diverse and weblike instead of linear as in Figure 3.3. Secondary consumers may have many choices and sometimes become primary consumers as well. When that happens, we have a food web. Figure 3.5 illustrates a food web. As the animals die, their remains are converted to nutrients, gases, and heat by decomposers that break them down. Much of the decomposition energy is lost to adjacent material or to the atmosphere. The heat created in a compost pile is one example of this energy release.

Figure 3.4: Energy Pyramid

Figure 3.5: Food web (source: http://www.champaignschools.org/science/images/foodweb.gif)

What is the system efficiency of an ecosystem? The answer varies depending on the type, variety, and number of plants and animals in the system and how they interact with nonliving factors. In general, the more plant life present, the greater the energy flow through an ecosystem. Warmer climates typically support more plant life than cooler climates. Lower elevations are usually more productive than higher elevations.

Questions:

1. Draw out an example of a food chain. Include at least 4 organisms. Label each organism according to the type of consumer or producer they are. Make notes about the type of energy transfers that are occurring (forms of energy). Include ALL types.

1. What are the two laws nature obeys regarding energy?
2. Explain why vegetarians use food energy more efficiently than people who are not vegetarians.
3. Why don’t lions hunt mice?
4. Elephants and blue whales are the largest animals on Earth. Both are vegetarians. Why don’t meat-eaters get as large as elephants and blue whales?
5. Why are there more rabbits than coyotes? I.E. Explain in terms of energy availability why there are few individuals represented at the top trophic level of any food web
6. What does the system efficiency of an ecosystem depend on?
7. You move regularly every day (walking, talking, breathing, etc). What is the fuel that provides this kinetic energy of movement for you on a daily basis?
8. What provides the “starting” energy for ALL food webs?
9. When a grasshopper consumes grass, what percentage of energy actually is consumed by the grasshopper?
10. Where does the remainder of the energy from #10 go?
11. Joules (a unit of energy) is used in the energy pyramid, but can be easily compared to the amount of biomass at each trophic level. As joules of energy decrease at each level, the amount of biomass at each level decreases (by the same percentage). So, how many kilograms of tertiary consumer biomass can 1000 kg of grass biomass support in the ecosystem?
12. Efficiency is described as the amount of desirable energy one receives as a result of energy conversion(s). Example: Burning coal is not very efficient because energy is converted several times with heat loss throughout each conversion before electricity is produced (at a power plant). So, compare energy efficiency between a vegetarian and a human that consumes mostly meat products. Which is more energy efficient and why?

ANSWER KEY:

1. Answers will vary but should include a producer, primary consumer(herbivore), secondary consumer(carnivore/omnivore), and tertiary consumer(carnivore/omnivore). Energy transfer notes should include sun energy converted into chemical energy in the plant, then heat energy between transfers. Students will also note that chemical energy is converted into mechanical energy when the organism moves.
2. 1) energy cannot be created nor destroyed, only transferred from one form to another

2) the total amount of energy never changes.

1. vegetarians consume food at the source of the food chain, directly from the garden or field. Only on energy tranfer is involved in change the plant energy to energy the human can use. Eating meat involves intermediate steps from the plant to what is placed in the mouth. The amount of useful energy that becomes unavailable for further use in a food chain is directly related to the number of transfers involved.
2. lions don’t hunt mice because the energy they would expend in the process would be more than the nutritional energy they would gain by eating the mice. Hence, exclusively mice-eating lions would soon starve.
3. more energy is available in plant than in animal material. Large amounts of energy are consumed in hunting, more than in grazing or browsing.
4. rabbits are herbivores. Herbivores are closer to the bottom of the food chain where most of the useful energy is; therefore, the greater food source supports a larger population. Less energy is available at the top levels, so that can only support a small amount of biomass. Much energy is lost along all of the transfers between levels.
5. the type, variety, and number of plants and animals in the system and how they interact with the nonliving factors.
6. the food that you consume contains chemical energy that you convert through your metabolism (respiration) into kinetic energy.
7. sunlight (some students may say plants through photosynthesis)
8. only 10%
9. (90% is lost to heat, and metabolic processes that the plant goes through…growth, reproduction, protein production, etc)
10. 1 kg ---each trophic level only gets 10% so…primary consumer are 100 kg, secondary consumers are 10 kg

13. Vegetarians consume food at the source of the food chain, directly from the garden or field. Only one energy transformation is involved in changing the plant energy to energy the human can use. Eating meat involves intermediate steps from the plant to what is placed in the mouth. The amount of useful energy that becomes unavailable for further use in a food chain is directly related to the number of transformations involved.