Ecosystems Lesson 3 Background Information

In Lesson 3, students develop complete explanations for what causes the organic matter pyramid, the key pattern in ecosystems they examined in Lesson 2. Using carbon pools as a context, students trace matter through representations that connect carbon-transforming processes at the atomic-molecular, organismal, and ecosystem scales, and they consider the differences between matter cycling and energy flow through ecosystems.

Activity 3.1 Large-Scale Four Questions introduces students to the carbon pools: atmosphere, soil, producer, herbivore, and carnivore. They further simplify these into two groups: “organic matter” and “inorganic matter” pools to help distinguish between pools with CO₂ and pools with C-C and C-H bonds. These pools become the context through which matter and energy move throughout the rest of the unit. This forms a foundation for answering the Four Questions for the Large Scale: The Carbon Pools Question, the Carbon Cycling Question, the Energy Flow Question, and the Stability and Change Question. It is important to note that additional pools exist in ecosystems, but this ecosystem is intentionally simplified to focus on the organic matter pyramid as a theme in the unit.

Activity 3.2, Carbon Dice Game, helps students think about how carbon is cycled through different organisms in an ecosystem. The Meadow Simulation showed that the pattern of the organic matter pyramid emerges repeatedly in ecosystems but did not provide evidence for what drives this pattern. Through the dice game students can see that the organic matter pyramid is a natural consequence of the carbon-transforming processes that take place in all organisms: photosynthesis, digestion/biosynthesis, cellular respiration, being eaten, or death/defecation. Students play the role of individual carbon atoms, and rolls of the dice represent the likelihood of which process will happen to them inside an organism. Students can see the pattern in their visits to different carbon pools: They go through plants, soil carbon, and the atmosphere often, through herbivores less often, and through carnivores hardly ever.

In Activity 3.3, students explain the reason for the pattern of size of carbon pools that was observed in the Meadows Simulation and the Carbon Dice Game. When they followed individual carbon atoms through an ecosystem in the Carbon Dice Game, they observed that the carbon atoms visited some pools more often than others. In this Activity they consider the implications of this observation for the size of the pools.

In order to explain the organic matter pyramid, students need to think about movement of carbon in semi-quantitative ways. Most of organic carbon created through photosynthesis is used for cellular respiration (energy needs) is used by organisms and returns to the atmosphere. Because most of the organic carbon in an organism is used for cellular respiration (and much of it is lost during death and defecation), very little organic carbon is available to be passed from one level in a food chain to another. Thus, in a “steady state” ecosystem, carbon atoms are constantly in motion, but the relative size of the large-scale carbon pools stays about the same.

In Activity 3.4, which is optional, students are further introduced to the soil carbon pool.

In Activity 3.5, students connect the dice game with the Large-Scale Four Questions to describe how carbon cycles and energy flows in ecosystems:

• Carbon cycles: carbon atoms move between pools via carbon transforming processes
• Energy flows: in describing energy flow students must add the energy source for all energy in ecosystems: sunlight, which is transformed into chemical energy and ultimately heat. That heat energy is ultimately radiated into space in the form of infrared light (this is why the earth cools down at night). So, while heat energy may move through the atmosphere (and some of it may temporarily be trapped in the atmosphere due to the greenhouse effect), eventually all of it will be lost from ecosystems, and radiate into outer space.
• It is especially important for students to understand that while matter and energy move through an ecosystem in tandem when they are combined in organic matter, they follow different pathways at the beginning and end of food chains and food webs:
  • Producers get matter ONLY from carbon dioxide, water, and minerals and energy ONLY from sunlight.
  • When organisms use organic matter for cellular respiration, ALL the matter goes back into carbon dioxide, water, and minerals, while ALL the energy leaves the ecosystem as heat (which is ultimately radiated out into space). So matter cycles, energy flows through ecosystems.

In Activity 3.6 students use the Explanations Tool for Ecosystems to explain the reason for the key pattern in this unit: the organic matter pyramid. Students use their knowledge of energy flow and matter cycling to explain why the producer pool has more organic matter than the herbivore pool, and why the herbivore pool has more organic matter than the carnivore pool.

Key carbon-transforming processes: photosynthesis, cellular respiration, combustion