Whole Earth Curriculum WEC

Lesson 05 — One Population Ripples Through an Ecosystem

Learners analyze how a change in **one population ripples through an ecosystem** and can **reduce or restore balance**, using the return of wolves to Yellowstone and the sea-otter–urchin–kelp chain. They trace cause-and-effect through a food web.

D06 P3: Intellectual & Cognitive Awareness D06.S2 50 minutes Draft

Objectives

Essential question

How do changes in one population ripple through an ecosystem, and how can they reduce or restore balance?

ecosystempopulationfood webripplebalancerestore

Materials

Standard materials

Low-tech / no-cost

Enriched / lab & device

Works in different contexts

A chain diagram of a ripple through an ecosystem showing wolves reducing elk, elk browsing less, and willows and aspens recovering — with a downstream, more-debated step to beavers and birds marked as uncertain — with arrows labeled as the change moves through the web
A chain diagram of a ripple through an ecosystem showing wolves reducing elk, elk browsing less, and willows and aspens recovering — with a downstream, more-debated step to beavers and birds marked as uncertain — with arrows labeled as the change moves through the web

Lesson 5 — One Population Ripples Through an Ecosystem

Summary

Learners analyze how a change in one population ripples through an ecosystem and can reduce or restore balance, using the return of wolves to Yellowstone and the sea-otter–urchin–kelp chain. They trace cause and effect through a food web.

Objectives

  • Analyze how changes in one population ripple through an ecosystem and can reduce or restore balance. (D06.S2.08.02)

Connection

Nothing in a living place stands alone. If the fish in a lake suddenly become few, the birds that eat them go hungry; if a forest’s big trees are cut, the animals that lived in their shade move on. Living things are linked like a web — pull one thread, and the pull travels outward. Today you trace that ripple, and see how it can make a place poorer or richer, out of balance or back into it.

Materials

  • Ripple-chain sheet
  • Ripple cards

Preparation

  • Copy the ripple sheet and cards.
  • Retrieval: from Grade 5 and 7, recall food webs, and that matter and energy move through them; from Grade 7, recall predator and prey relationships.

Facilitator note

This lesson is written to the learner (“you”). The idea to land: ecosystems are webs, so a change in one population ripples outward — and that ripple can reduce balance (a species removed, its prey multiply, the plants suffer) or restore it (a species returned, the web reweaves). Worked example: Yellowstone wolves — wolves were removed by the early 1900s; elk grew numerous and browsed heavily; when wolves were reintroduced in 1995–1996, elk numbers and browsing fell, and some willow and aspen recovery followed (S-310). Be accurate and hedged: the real ecosystem response is complex and still debated — do not teach the oversimplified “wolves changed the rivers” story; state the documented elk-browsing reduction and partial plant recovery instead (S-310). Second example: sea otters, sea urchins, kelp — where otters were hunted away, urchins multiplied and ate the kelp forests; where otters returned, kelp recovered (S-176). The environment lens: the health of the whole place depends on its parts. The ethics lens: when people remove a species — for fur, for fear, for profit — the harm spreads; when people choose to restore it, the web can heal, and that healing is often slow and imperfect. The egalitarian lens: who benefits and who is harmed when an ecosystem changes is not distributed equally — fishing communities, herders, and farmers feel the ripple first. The global lens: this is the same process in a kelp forest off the Pacific coast, a savanna in Africa, or a reef in the tropics — for example, when rinderpest was driven out of the Serengeti in the mid-1900s, wildebeest numbers recovered and reshaped the savanna’s grass and fire. The critical-thinking lens: cause-and-effect in a web is rarely a single clean line — the lesson’s “because” chains are a model, and real webs have many threads (S-310).

Procedure

  1. Recall (5 min). From Grade 5 and 7: what is a food web, and what happens when one part changes? Today we trace the ripple precisely.
  2. Meet the ripple (10 min). A change in one population — a group of one kind of living thing — ripples through the ecosystem. Trace the worked example: wolves → elk → willow and aspen. When wolves returned, elk browsing fell, and some plants recovered. The steps that follow in the full story — beavers and birds — are real but more debated, so we hold them lightly (S-310).
  3. Read the cards (15 min). With a group, arrange the Yellowstone cards into a cause-and-effect chain, writing one “because” for each arrow. Then do the same for the sea-otter chain: otters → urchins → kelp → fish and shelter.
  4. Reduce or restore? (10 min). For each chain, ask: did the change reduce balance or restore it? Removing wolves reduced it; reintroducing them began to restore it. Removing otters reduced it; their return restored the kelp.
  5. Be honest about the model (5 min). Real webs are messier than a single chain. The chain is a model — useful and partly true, but the real ecosystem has many more threads (S-310).
  6. Close (5 min). Share one ripple you can now trace. A living place is a web, and every thread matters.

Differentiation

  • Support: Provide the chains pre-ordered and ask learners to write the “because” for each arrow.
  • Extension: Predict what happens if a different population changes in one of the two ecosystems, and trace your predicted ripple, naming the limits of your prediction.

Assessment

  • Formative (observation): Can the learner trace a ripple through a food web and say whether it reduced or restored balance?
  • Portfolio artifact: The completed ripple-chain sheet, kept in the portfolio.

Home connection

Think of a change near home — a pond cleaned, a field planted, a species that disappeared or returned — and trace what else might have changed because of it.

Resources