A food web is coupled: every species' fate depends on the others. Here kelp grows, urchins and snails graze it, otters eat the grazers, and an orca sits on top. Left alone they settle into slow predator–prey cycles — grazers rise, kelp dips, otters climb behind the grazers, and around it goes.
Pull the otter out and nothing hunts the grazers. They explode, overgraze the kelp into an "urchin barren," and once the kelp is gone even the grazers starve. One removal, several collapses — a trophic cascade, top-down control made visible.
Pull the orca instead and the web barely flinches — the apex isn't the keystone. The keystone is the species whose loss unravels the most web, out of all proportion to its abundance. That's the real sea otter: a modest animal holding a whole kelp forest together.
Only ~10% of the energy at each level reaches the next — that's why the pyramid narrows so sharply toward the top, and it's wired straight into this model.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.