Food webs, biomes, and the 10% rule — the systems view of the living planet.
Suggested pace: 4 weeks. Mastery looks like:
Ecology zooms out. After units spent inside cells and molecules, your student now studies how whole organisms interact with each other and their environment. The vocabulary scales up in layers: a population is all the members of one species in an area; a community is all the populations interacting there; an ecosystem is the community plus its nonliving environment — soil, water, climate; and the biosphere is every ecosystem on Earth. Ecology is the original systems-thinking course, and its lessons transfer to economics, engineering, and every other field where everything touches everything.
Two rules organize every ecosystem. First, energy flows one way: it enters as sunlight, is captured by producers — the photosynthesizers — and passes to the herbivores that eat them, the carnivores that eat the herbivores, and finally the decomposers, the fungi and bacteria that break down the dead and recycle their nutrients. At each step roughly 90% of the energy is lost as heat, leaving only about 10% for the next level — the 10% rule, which explains why food chains are short and why a field can feed many mice but only a few hawks. Second, matter cycles: the same carbon, nitrogen, and water atoms circulate endlessly, with photosynthesis pulling carbon dioxide from the atmosphere and respiration and decay returning it.
A food chain is a useful simplification — grass to grasshopper to frog to hawk — but real ecosystems are food webs: many interlocking chains, because most organisms eat and are eaten by multiple species. Webs explain resilience and fragility at once. Remove one species and others may compensate; remove a keystone species — one whose role is disproportionately large, like the sea otters that keep urchins from mowing down kelp forests — and the whole structure can shift. Mapping a local food web from direct observation makes an outstanding multi-week homeschool project.
Populations grow when resources allow and stall when they don't. Every environment has a carrying capacity: the maximum population it can sustainably support, set by limiting factors such as food, water, space, and predation. Species also live in tangled partnerships called symbiosis: mutualism, where both benefit (bees and flowers); commensalism, where one benefits and the other is unaffected; and parasitism, where one benefits at the other's expense. Your student should be able to find all three within a hundred feet of the front door.
At the planetary scale, climate sorts the land into biomes — large regions with characteristic life: tundra with its permafrost, taiga's conifer belts, temperate forests, grasslands, deserts, and tropical rainforests, with their aquatic counterparts in freshwater and marine systems. Each biome is a different answer to the same questions of temperature and water. Ecology ends, properly, in stewardship: a student who understands energy flow, nutrient cycles, and carrying capacity is equipped to think clearly — rather than emotionally — about conservation, agriculture, and the human place in the systems that feed us.
Every correct answer climbs one rung. Climb forever. Badges at 10, 25, 50, and 100 lifetime rungs.
GENO — a robot you can actually TALK to — has studied this entire unit and is available day or night, in 32 languages, at no cost. Ask him to re-explain any idea on this page, quiz you out loud, or go deeper than the lesson goes.