🐺 Keystone Species & Trophic Cascade

A keystone species is an organism whose impact on its ecosystem is disproportionately large relative to its abundance; removing it triggers cascading changes throughout the food web, as sea otters do for kelp forests.

Sea otters prey heavily on sea urchins, keeping urchin numbers low enough that urchins cannot graze kelp faster than it regrows; without otters, urchin populations explode and strip the kelp forest bare.

A trophic cascade is a chain reaction that runs down a food web when a predator population changes, altering the abundance of the species it eats and, in turn, the species those prey eat, here otters, then urchins, then kelp.

The otter population is set to zero, removing predation pressure on urchins; over the following seconds urchins boom, graze down the kelp, and the spatial map shifts from lush green kelp forest to a bare grey urchin barren.

Reintroducing a small seed population of otters lets predation resume, gradually suppressing the urchin population and allowing kelp regrowth, though recovery is slower than the original collapse and depends on the otter predation rate, urchin grazing rate and kelp regrowth rate sliders.