4.2 – Energy losses
lesson one: Energy losses
Objectives: Understand how energy losses occur in food chains; both within and between trophic levels. Understand how this limits the length of food chains. Recall that energy is lost in food chains, while nutrients can be recycled.
- Entropy: the concept that systems tend to become disorganised if left to their own devices. This applies to the flow of energy in food chains, that begins as chemical energy in a highly organised bio-molecules and ends up as highly scattered heat energy moving by convection and radiation.
- Pyramid of energy: a graphical representation of the energy level in each trophic level.
- Trophic level: the feeding level eg. producer
Introduction: the image shows the thermal profile of a human body. It can be concluded that heat loss by radiation occurs constantly, and is particularly associated with the ‘core of the body’ including the thoracic cavity where many vital organs are found. This energy is generated by aerobic respiration, and in fact one of the functions of cellular respiration is to maintain a constant body temperature – even it means wasting a little heat energy.
Energy flows in a food chain from producers to consumers to top consumers. Ultimately energy is released to the environment and escapes into the Earth’s atmosphere as heat energy.
This is in contrast to nutrients, which are recycled through the work of decomposers and may pass through the same food chain many times. Producers convert light energy into chemical energy through the process of photosynthesis (some other producers also harness the energy in chemical reactions, they are called chemoautotrophs). Chemical energy flows through food chains through the result of feeding.
Although this varies in different ecosystems, around 90% of energy is often lost in a feeding relationship between trophic levels. So if a person subsists on apples, only 10% of the energy in the apple trees ends up being available to the human.
WHY SO MUCH ?
Energy losses in ecosystems may occur:
- 1. Between trophic levels (during feeding)
- 2. Within trophic levels (energy is consumed by the organism)
1. Between trophic levels
Energy losses that occur between trophic levels are due to the incomplete ingestion, incomplete digestion, and uneaten organisms.
Incomplete ingestion – using the context of the apple trees, the human does not eat the whole apple tree. Only the apple. Therefore most of the energy available in the apple trees is not accessed.
Incomplete digestion – Using the context of the apple trees, some parts of an apple cannot be digested, such as the seeds. They would count as fibre. Therefore the energy in them is not available to the human.
Uneaten organisms – The human does not feed from every apple tree. Some apple trees are not fed upon, and of course even on those that are, some apples are not eaten.
2. Energy loss within trophic levels
Energy losses that occur within trophic levels are due to the organism digesting the food and then using the energy in respiration for cellular processes eg. muscle contraction. When this happens the chemical energy stored in the food is converted into ATP in the process of respiration. The reason this is necessary is that the energy in chemicals like glucose is not readily available. Adenosine Tri-Phosphate reality breaks down into Adenosine Di-phosphate, yielding a packet of energy that can be instantly used for muscle contraction, molecule synthesis, cellular movement e.t.c.
The second law of thermodynamics states that energy changes (from one form to another), are never 100% efficient. Ultimately this means that the energy is lost as heat energy ( mostly as radiation).
The energy obtained by an organism tends to be stored as chemical energy, in the body. This not only includes storage carbohydrates such as glycogen (animals), or starch (plants), but proteins and lipids as well, as well structural components like bones and hair. This means that the mass of the organisms in a trophic level, can represent the energy stored at that trophic level.
P= Energy stored at a trophic level, in grams per year.
Pyramids of Energy
A pyramid of energy represents the energy found at a particular trophic level. This consists of a kind of horizontal bar chart, where the energy at each trophic level is shown by the area (length) of the bar. The units of a pyramid of energy are often kilojoules / m2 / year.
Class activities A:
Problem: Should salmon farming be carried out using fish feed, or soy as a source of food for the salmon
- Draw the two possible food chains
- Using graph paper, make two pyramids of energy to show each food chain
- Discuss the problem, using your pyramids of energy to defend your answer.
Food chain soy
- Energy in Soy 50,000 KJ m-2yr-1
- Energy in Salmon 7,000 KJ m-2yr-1
Food chain fish meal
- Energy in plankton 80,000 KJm-2yr-1
- Energy in fish meal 2,500 KJm-2yr-1
- Energy in salmon 200 KJm-2yr-1
Class activities B:
Some students were asked Demonstration of Energy losses using the ‘blind paper toss model’.They stood in four rows, facing the same way. The front row were given paper balls and asked to toss them over their shoulders at the group behind. Any students in the second row who caught them, were considered to have been successful feeders. Any who did not catch a paper ball are effectively eliminated, and have to sit down. This continues until the fourth row.
Discussion of model.
- Does this model produce a pyramid of energy?
- Why does this occur?
- How does this accurately model energy flow through food chains, and therefore explain the pyramid shape shown by ecological pyramids.
- Conversely, what are the limitations of this model.
- Can you make suggested refinements to this model, perhaps including energy loss within a trophic level? or the inclusion of decomposers?
Recommended questions IB Oxford
Page 214 questions 1, 2, 3
Page 216 questions a, b, c
Page 219 questions 1, 2, 4, 5
Socratic discussion question: Discuss: If a Cappucin monkey eats a fig, at what point will it actually access the energy in the fruit?
MESOCOSM PROJECT NOTES
Introduction from recent news: the world’s biggest ever Mesocosm. Biosphere 2.
A mesocosm is a mini-ecosystem in closed system. A closed system means that only energy can be exchanged across the boundary, and not matter. This means that light energy can come in, and heat energy out, but you can’t add food or water to a mecososm once it has been set up.
Mesocosms can be aquatic, or terrestrial.
Your challenge? Make a successful mesocosm in the lab. I suggest that you:
-choose the type of mecocosm you want to make (terrestrial or aquatic)
-choose the community you want to include (which species)
-think carefully about how those species might interact in a stable way
-ethics: how can you prevent an organism from suffering as a result of a mesocosm experiment.
-choose the abiotic components
-Hint: what will the community need in order to survive?
Download this document for the mesocosm Project:
Check out this strange story of one of the world’s most successful mecososms.
Figure below: A conceptualised energy flow diagram can be developed (note that energy is ultimately lost to space and not recycled)