Three ecosystems run on the same bargain. A partner does a job for its host. The host pays for that work. Visit a savanna, the soil under a grassland, and a coral reef, and push on each partnership.
Data format
When a run finishes, the simulation asks whether to keep it. A kept run goes to the data tools, one row per season for each side of the comparison. All three ecosystems share one table, so students can compare them in one graph. Every row starts with Ecosystem, Run, Season, Partner (With partner or Without partner), Partner name, and Host health (% of best). Host health is on one scale in every ecosystem, where 100 is the healthiest that host gets: canopy left on the savanna, growth in the soil, coral energy on the reef. After those come each ecosystem's own columns, such as Heat (%), Heat pattern, Algae in tissue (%), and Coral energy (% of healthy) on the reef. A table that holds one ecosystem shows only that ecosystem's own columns. One ecosystem on its own works well: graph Host health against Season, colored by Partner. The cross-ecosystem graph is the extension, for a class that has time to run more than one. Each time you run an ecosystem again, its rows are labeled Run 2, Run 3, and so on. The earlier runs stay in the table, and switching ecosystems keeps every row. To compare the ecosystems, graph Host health by Partner, colored by Ecosystem. To look inside one, try Coral energy against Season, split by Heat pattern. The Collect Data button sends a run that is still going. The same shape turning up in three unrelated places is the argument MS-LS2-2 asks students to make. Have students run all three ecosystems and compare them in that one graph.
Model details
On the savanna, a whistling thorn acacia grows hollow swollen thorns and feeds nectar to the ants that live in them. In return the ants swarm anything that tries to eat the tree. Two trees grow side by side, identical except that one has ant partners and one does not.
In the grassland you look underground. A clover feeds sugar to bacteria that live in pink nodules on its roots. In return, the bacteria turn air into nitrogen food the clover cannot make itself. A second clover of the same kind grows beside it, raised with no such bacteria. It can only use the nitrogen already in the soil.
On the reef, a coral houses golden algae inside its own skin. In return, the algae feed it sugar made from sunlight. A second colony of the same coral grows beside it, raised with no algae. It can only catch its food, so it stays low on energy. Warm water lowers its energy further.
The trees in this savanna run from 1.4 m to 6 m, which is the real range for this species. The two study trees are 3.2 m. Size decides how much an animal can take. An elephant can strip a small tree entirely, but it reaches only the lower crown of a big one.
Everything is drawn to scale except the smallest partners. A real acacia ant is about half a centimeter long, a root nodule a few millimeters, a coral polyp about three millimeters. On screen each would be a pixel, so the close-up views draw them much larger than life. Their numbers still come from the model.
Standards
MS-LS2-2 Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
Design intent
The savanna, the soil and the reef behave the same way for different reasons. Finding that shared pattern is the point.
The surprise is deliberate. Drag browsing pressure down to none and the tree does not simply get better. It improves for a few seasons, then declines. With nothing eating it, the acacia stops making nectar and swollen thorns. Its defending ants starve out. A different ant moves in. That ant does not defend the tree. It nests in tunnels bored by beetle larvae, so it has no reason to drive the beetles off. The wood is damaged.
This is a real result, not a contrivance. Palmer and colleagues fenced large herbivores out of a Kenyan savanna for ten years. The acacias ended up worse off than the ones still being eaten (Science, 2008).
Each world has its own dial and its own breakdown. Take the browsers away and the acacia stops paying its ants. Enrich the soil and the clover stops paying its bacteria. Overheat the reef and the coral expels its algae outright. The first two are slow accounting changes. The reef is a cliff.
Key moments stop the season clock instead of speeding it up, so the class can watch a browse or an ant swarm at real speed and talk about it. Nothing advances on its own. A key moment waits until someone presses Continue.
Discussion prompts
Is being eaten always bad for the tree? Is rich soil always good news for the clover? What does the coral get from its algae while the water is cool, and what does it lose when the water turns hot? If all three hosts stop paying for the same reason, what is that reason?
How do I explore?
Pick a world with the buttons at the top left. Each world has one slider. The savanna has browsing pressure. The grassland has soil nitrogen. The reef has heat. Move it and watch both cards. The numbers tell you what the host is paying and how it is doing.
Click a glowing marker in the scene, or press the button, to fly in close. On the savanna you see inside a swollen thorn. In the other worlds you get close to the partnership itself. The slider still works while you are in there.
What should I do/notice?
Look closely at the ants. The red ones are partners that defend the tree. The black ones do not defend it. The tree without partners fills up with black ants, so it is not an empty tree.
Now and then the seasons stop for a key moment. The camera moves in on something worth watching. The calendar waits for you. Watch as long as you like, then press Continue. Press Skip if you want to move on early.
What about the data?
When a run finishes, you choose to keep it or discard it. A kept run goes to the data tools, where you can graph it. You get one row per season for each side of the comparison. All three ecosystems go into one table. Run an ecosystem twice at two different settings. The rows are labeled Run 1 and Run 2. Plot Host health against the dial you moved, such as Heat, and color by Run. Or plot Host health by Partner, colored by Ecosystem, to compare the three ecosystems. This shows what changed between the runs.