Rocky Shore Keystone Lab: Starfish, Mussel Takeover, Tidal Zonation and the Paine Removal Experiment

Year-

Diversity: control-

Diversity: no starfish-

Mussels: no starfish-

Scenarios

Pick a scenario; every slider stays editable.

1. Tides

2. Starfish and disturbance

3. Competition

Left wall: control with starfish. Right wall: starfish removed (Paine, 1966). Both start from the same random rock.

Readouts

Diversity through time

Shannon index of cover (mussel, barnacle, algae, bare rock). Starfish removal lets mussels take the rock and diversity collapses.

Zonation: mussel cover vs shore height

Solid lines: the two lattice walls (current snapshot). Dashed: mean-field ODE equilibrium at each height. Vertical line: mean high-water level.

Tide at the shore

Experiments

The science

Keystone predation. Robert Paine (1966) removed the starfish Pisaster from stretches of rocky shore. Within a year or two mussels, the best competitor for space, overgrew barnacles and algae and the number of species fell sharply. A keystone predator keeps the dominant competitor in check so other species can live there. The right wall repeats that experiment.

Tides set the vertical zones. The water level is a sum of the main lunar tide (period 12.42 h) and the solar tide (12 h): h(t) = (R/2) [1 + (cos(2πt/T_M2) + s cos(2πt/T_S2)) / (1 + s)]. Their beat gives spring and neap tides every 14.77 days. The immersion fraction at height z is the share of the cycle when h(t) > z. Low on the shore animals feed and the starfish forage (immersion near 1); high on the shore desiccation kills mussels and algae first while barnacles tolerate it.

Model. Each rock cell is bare, mussel, barnacle or algae. Per year: larvae settle on bare rock (r), neighbours spread (a times the neighbouring fraction), mussels overgrow barnacles and algae (o_MB, o_MA), barnacles overgrow algae slowly, stress and waves clear cells, and starfish eat mussels at p = pred S I³ (they feed only when submerged), barnacles at one fifth of that rate. In the well-mixed option each cell sees the global cover, and the lattice reduces to the mean-field equations dM/dt = ... that are integrated with Runge-Kutta 4 for the dashed lines and used as a numerical check.

Honest simplifications. Three space-occupying groups instead of Paine's 15 species; starfish are an average predation pressure drawn as moving agents (they do not literally hunt in the model); no limpets, drift, larvae dynamics or temperature; parameters are illustrative and were tuned to show the published qualitative result (mussel dominance without the predator), not fitted to field data. Educational use only.