Fig. 2.

Catastrophic shift between kelp beds and sea urchin barrens. (A) Conceptual schematic of discontinuous phase shift (redrawn from ref. 3). If the reef system occurs in the kelp state on the upper path (red) but close to the threshold F₁, a slight increase in sea urchin density may induce a catastrophic “forward-shift” to the alternative and stable sea urchin barrens state. Once barrens have formed, reverting back to the kelp state is difficult because the system demonstrates hysteresis (18), and the “reverse-shift” (blue path) occurs only if sea urchin density is reduced below the return threshold at F₂. The broken gray line indicates the region of instability between the alternative stable states. (B) Macroalgal cover versus Centrostephanus rodgersii density in eastern Tasmania. Bubble size represents relative frequency of particular urchin density and macroalgal cover combinations as measured in 575 individual 5 m² plots at 13 sites spanning the east coast (11). Overlaid arrows and numbers in parentheses indicate magnitude and direction of ecosystem response to removals and additions of C. rodgersii. Removals of C. rodgersii from barrens (blue arrows) in: NSW after 18 months (18, 19) where starting sea urchin densities were 10 and 6 m⁻² respectively; after approximately 5 months (20), with a starting sea urchin density of 4 m⁻²; in Tasmania after 18 months (12) with a starting sea urchin density of 2 m⁻². Additions of C. rodgersii to kelp beds (red arrows) in NSW after approximately 5 months (20), starting sea urchin density 0 m⁻². Dashed lines with arrows represent the theoretical “forward-shift” and “reverse-shift” paths as explained in (A).