"...a big idea is not 'big' merely by virtue of its intellectual scope. It has to have pedagogical power: It must enable the learner to make sense of what has come before; and, most notably, be helpful in making new, unfamiliar ideas seem more familiar." (Understanding by Design, 2nd Ed., p.70)
This chapter took a closer look at the so-called "Stage 1" concepts for unit planning: key questions, key understandings, big ideas, and core tasks. All of these are related, though not synonymous, and they're all different ways of getting at the heart of the question: what do I want my students to understand, know, and be able to do when they complete this unit?
The chapter is full of a lot of good advice on how to screen through lists of material -- whether a textbook or a set of state-imposed content standards -- and filter out the big ideas and core tasks from among the less-crucial concepts. The models presented here help to distinguish the crucial from the important, and the important from the incidental, and the incidental from the trivial. It's something I'm going to have to put into use for myself as I screen through the California science standards to figure out which points are most important for my students to understand.
Reading through chapter 3 inspired a number of thoughts about the "big ideas" of ecology, the first unit for my upcoming biology course. The book points out that big ideas are usually counter-intuitive and susceptible to misunderstanding. This got me thinking about food webs, nutrient cycles and energy flow in ecosystems, all of which involve the central "big idea" that thermodynamics limit the possibilities in biological systems. But the first law of thermodynamics -- the law of conservation of energy -- can easily be misunderstood by students in this context, because energy is constantly being lost from the ecosystem in the form of heat. Energy isn't being destroyed, but it's no longer in a useful form. Likewise, the second law -- the law of increasing universal entropy -- often seems like it's being violated by living systems, in which organisms that are higher on the food chain often appear more "advanced" or complex than the creatures they feed on. Yet the constraints placed on ecosystems by thermodynamics -- a fixed amount of energy entering the system (1st law), and every energy transfer leading to a loss of energy to heat (2nd law) -- are essential to grasping why ecological communities are structured the way that they are. The key misconception is that students might be fooled into thinking that the biosphere is a closed system, energetically speaking. It isn't; it's a closed system for nutrients, which is why we speak of nutrient cycling, but it's an open system for energy, which is why we speak of energy flow.
I think I have a good way of modeling energy flow for the students: an analogy to money. Suppose American shoppers buy products from Company A, leading to a gross income for the company. The money flowing into Company A from the shoppers represents the maximum amount of money available in the "system;" the Company has no other way of acquiring more money. The Company then pays its employees, but it can't pay them everything that it got from the shoppers; it has to pay for the electricity, the water, maintenance of the equipment, the raw materials to make its products, and various regulatory costs in the form of taxes. Only a small portion of its gross income gets passed on to the employees. Employee B thus gets a small fraction of the money Company A had; that's his gross income. But that isn't pure profit, either; he has to pay for upkeep on his house, gas for his car, food for himself and his family, and his own income taxes. Only a little bit of money is left over for the next step in the monetary "food chain": his kids. Child C gets an allowance that is only a tiny fraction of what Employee B got; it's such a small amount, in fact, that the child doesn't have enough money to support anyone "higher" on the chain.
By the same token, the "gross income" of an ecosystem from the sun leads to a lot of energy going through the producers (A), with less being passed on to the primary consumers (B) and still less going to the secondary consumers (C). My students will probably find the analogy of themselves as "apex predators" to be an amusing one -- though perhaps "parasite" would be a more accurate analogy. ;-)
I think this will be a good way to explain energy flow, but I'm having more trouble finding a way to explain nutrient cycling. I need something to represent a commodity that can be passed around from one group to another, modified repeatedly into different forms but ultimately recycled back to the beginning again, unchanged in what it essentially is. The carbon, nitrogen and water cycles are key examples, all important for illustrating how ecosystems function -- but I'm having a hard time finding something similar to compare to that these students would be familiar with. I thought about the example of a commodity (such as a bicycle or a CD) being passed around from one person to another, but that analogy misses one of the key elements (no pun intended) of nutrient cycling: that these basic nutrients are often repackaged in radically different forms and used for very different purposes as they make their way around the ecosystem. The sugar made by the plant, the fat stored in the human, the carbon dioxide breathed out when the human exercises -- all of these contain the same carbon atoms, passed on from one place to the next but showing up in very different chemical forms.
I'd love to hear if anyone has any suggestions on a better analogy for this difficult concept. I recognize that this is only one example among several "big ideas" that I'll have to tackle in this unit, but it's one that I'm going to have to wrestle with soon enough, and I think it's a useful "field test" of the UbD process to start thinking about this now.